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The Variation of Animals and Plants under Domestication by Charles Darwin

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done what is possible at present by a statistical investigation (17/30.
'Journal of Statistical Soc.' June 1875 page 153; and 'Fortnightly Review'
June 1875.), and he has come to the conclusion, from his own researches and
those of Dr. Mitchell, that the evidence as to any evil thus caused is
conflicting, but on the whole points to the evil being very small.


In the case of the FOWL a whole array of authorities could be given against
too close interbreeding. Sir J. Sebright positively asserts that he made many
trials, and that his fowls, when thus treated, became long in the legs, small
in the body, and bad breeders. (17/31. 'The Art of Improving the Breed' page
13.) He produced the famous Sebright Bantams by complicated crosses, and by
breeding in-and-in; and since his time there has been much close interbreeding
with these animals; and they are now notoriously bad breeders. I have seen
Silver Bantams, directly descended from his stock, which had become almost as
barren as hybrids; for not a single chicken had been that year hatched from
two full nests of eggs. Mr. Hewitt says that with these Bantams the sterility
of the male stands, with rare exceptions, in the closest relation with their
loss of certain secondary male characters: he adds, "I have noticed, as a
general rule, that even the slightest deviation from feminine character in the
tail of the male Sebright--say the elongation by only half an inch of the two
principal tail feathers--brings with it improved probability of increased
fertility." (17/32. 'The Poultry Book' by W.B. Tegetmeier 1866 page 245.)

Mr. Wright states (17/33. 'Journal Royal Agricult. Soc.' 1846 volume 7 page
205; see also Ferguson on the Fowl pages 83, 317; see also 'The Poultry Book'
by Tegetmeier 1866 page 135 with respect to the extent to which cock-fighters
found that they could venture to breed in-and-in, viz., occasionally a hen
with her own son; "but they were cautious not to repeat the in-and-in
breeding.") that Mr. Clark, "whose fighting-cocks were so notorious, continued
to breed from his own kind till they lost their disposition to fight, but
stood to be cut up without making any resistance, and were so reduced in size
as to be under those weights required for the best prizes; but on obtaining a
cross from Mr. Leighton, they again resumed their former courage and weight."
It should be borne in mind that game-cocks before they fought were always
weighed, so that nothing was left to the imagination about any reduction or
increase of weight. Mr. Clark does not seem to have bred from brothers and
sisters, which is the most injurious kind of union; and he found, after
repeated trials, that there was a greater reduction in weight in the young
from a father paired with his daughter, than from a mother with her son. I may
add that Mr. Eyton of Eyton, the well-known ornithologist, who is a large
breeder of Grey Dorkings, informs me that they certainly diminish in size, and
become less prolific, unless a cross with another strain is occasionally
obtained. So it is with Malays, according to Mr. Hewitt, as far as size is
concerned. (17/34. 'The Poultry Book' by W.B. Tegetmeier 1866 page 79.)

An experienced writer (17/35. 'The Poultry Chronicle' 1854 volume 1 page 43.)
remarks that the same amateur, as is well known, seldom long maintains the
superiority of his birds; and this, he adds, undoubtedly is due to all his
stock "being of the same blood;" hence it is indispensable that he should
occasionally procure a bird of another strain. But this is not necessary with
those who keep a stock of fowls at different stations. Thus, Mr. Ballance, who
has bred Malays for thirty years, and has won more prizes with these birds
than any other fancier in England, says that breeding in-and-in does not
necessarily cause deterioration; "but all depends upon how this is managed. My
plan has been to keep about five or six distinct runs, and to rear about two
hundred or three hundred chickens each year, and select the best birds from
each run for crossing. I thus secure sufficient crossing to prevent
deterioration." (17/36. 'The Poultry Book' by W.B. Tegetmeier 1866 page 79.)

We thus see that there is almost complete unanimity with poultry-breeders
that, when fowls are kept at the same place, evil quickly follows from
interbreeding carried on to an extent which would be disregarded in the case
of most quadrupeds. Moreover, it is a generally received opinion that cross-
bred chickens are the hardiest and most easily reared. (17/37. 'The Poultry
Chronicle' volume 1 page 89.) Mr. Tegetmeier, who has carefully attended to
poultry of all breeds, says (17/38. 'The Poultry Book' 1866 page 210.) that
Dorking hens, allowed to run with Houdan or Creve-coeur cocks, "produce in the
early spring chickens that for size, hardihood, early maturity, and fitness
for the market, surpass those of any pure breed that we have ever raised." Mr.
Hewitt gives it as a general rule with fowls, that crossing the breed
increases their size. He makes this remark after stating that hybrids from the
pheasant and fowl are considerably larger than either progenitor: so again,
hybrids from the male golden pheasant and female common pheasant "are of far
larger size than either parent-bird." (17/39. Ibid 1866 page 167; and 'Poultry
Chronicle' volume 3 1855 page 15.) To this subject of the increased size of
hybrids I shall presently return.

With PIGEONS, breeders are unanimous, as previously stated, that it is
absolutely indispensable, notwithstanding the trouble and expense thus caused,
occasionally to cross their much-prized birds with individuals of another
strain, but belonging, of course, to the same variety. It deserves notice
that, when size is one of the desired characters, as with pouters (17/40. 'A
Treatise on Fancy Pigeons' by J.M. Eaton page 56.) the evil effects of close
interbreeding are much sooner perceived than when small birds, such as short-
faced tumblers, are valued. The extreme delicacy of the high fancy breeds,
such as these tumblers and improved English carriers, is remarkable; they are
liable to many diseases, and often die in the egg or during the first moult;
and their eggs have generally to be hatched under foster-mothers. Although
these highly-prized birds have invariably been subjected to much close
interbreeding, yet their extreme delicacy of constitution cannot perhaps be
thus fully explained. Mr. Yarrell informed me that Sir J. Sebright continued
closely interbreeding some owl-pigeons, until from their extreme sterility he
as nearly as possible lost the whole family. Mr. Brent (17/41. 'The Pigeon
Book' page 46.) tried to raise a breed of trumpeters, by crossing a common
pigeon, and recrossing the daughter, granddaughter, great-granddaughter, and
great-great-granddaughter, with the same male trumpeter, until he obtained a
bird with 15/16 of trumpeter's blood; but then the experiment failed, for
"breeding so close stopped reproduction." The experienced Neumeister (17/42.
'Das Ganze der Taubenzucht' 1837 s. 18.) also asserts that the offspring from
dovecotes and various other breeds are "generally very fertile and hardy
birds:" so again MM. Boitard and Corbie (17/43. 'Les Pigeons' 1824 page 35.),
after forty-five years' experience, recommend persons to cross their breeds
for amusement; for, if they fail to make interesting birds, they will succeed
under an economical point of view, "as it is found that mongrels are more
fertile than pigeons of pure race."

I will refer only to one other animal, namely, the Hive-bee, because a
distinguished entomologist has advanced this as a case of inevitable close
interbreeding. As the hive is tenanted by a single female, it might have been
thought that her male and female offspring would always have bred together,
more especially as bees of different hives are hostile to each other; a
strange worker being almost always attacked when trying to enter another hive.
But Mr. Tegetmeier has shown (17/44. 'Proc. Entomolog. Soc.' August 6, 1860
page 126.) that this instinct does not apply to drones, which are permitted to
enter any hive; so that there is no a priori improbability of a queen
receiving a foreign drone. The fact of the union invariably and necessarily
taking place on the wing, during the queen's nuptial flight, seems to be a
special provision against continued interbreeding. However this may be,
experience has shown, since the introduction of the yellow-banded Ligurian
race into Germany and England, that bees freely cross: Mr. Woodbury, who
introduced Ligurian bees into Devonshire, found during a single season that
three stocks, at distances of from one to two miles from his hives, were
crossed by his drones. In one case the Ligurian drones must have flown over
the city of Exeter, and over several intermediate hives. On another occasion
several common black queens were crossed by Ligurian drones at a distance of
from one to three and a half miles. (17/45. 'Journal of Horticulture' 1861
pages 39, 77, 158; and 1864 page 206.)


When a single plant of a new species is introduced into any country, if
propagated by seed, many individuals will soon be raised, so that if the
proper insects be present there will be crossing. With newly-introduced trees
or other plants not propagated by seed we are not here concerned. With old-
established plants it is an almost universal practice occasionally to make
exchanges of seed, by which means individuals which have been exposed to
different conditions of life,--and this, as we have seen with animals,
diminishes the evil from close interbreeding,--will occasionally be introduced
into each district.

With respect to individuals belonging to the same sub-variety, Gartner, whose
accuracy and experience exceeded that of all other observers, states (17/46.
'Beitrage zur Kenntniss der Befruchtung' 1844 s. 366.) that he has many times
observed good effects from this step, especially with exotic genera, of which
the fertility is somewhat impaired, such as Passiflora, Lobelia, Fuchsia.
Herbert also says (17/47. 'Amaryllidaceae' page 371.), "I am inclined to think
that I have derived advantage from impregnating the flower from which I wished
to obtain seed with pollen from another individual of the same variety, or at
least from another flower, rather than with its own." Again, Professor Lecoq
ascertained that crossed offspring are more vigorous and robust than their
parents. (17/48. 'De la Fecondation' 2nd edition 1862 page 79.)

General statements of this kind, however, can seldom be fully trusted: I
therefore began a long series of experiments, continued for about ten years,
which will I think conclusively show the good effects of crossing two distinct
plants of the same variety, and the evil effects of long-continued self-
fertilisation. A clear light will thus be thrown on such questions, as why
flowers are almost invariably constructed so as to permit, or favour, or
necessitate the union of two individuals. We shall clearly understand why
monoecious and dioecious,--why dichogamous, dimorphic and trimorphic plants
exist, and many other such cases. I intend soon to publish an account of these
experiments, and I can here give only a few cases in illustration. The plan
which I followed was to grow plants in the same pot, or in pots of the same
size, or close together in the open ground; carefully to exclude insects; and
then to fertilise some of the flowers with pollen from the same flower, and
others on the same plant with pollen from a distinct but adjoining plant. In
many of these experiments, the crossed plants yielded much more seed than the
self-fertilised plants; and I have never seen the reversed case. The self-
fertilised and crossed seeds thus obtained were allowed to germinate in the
same glass vessel on damp sand; and as the seeds germinated, they were planted
in pairs on opposite sides of the same pot, with a superficial partition
between them, and were placed so as to be equally exposed to the light. In
other cases the self-fertilised and crossed seeds were simply sown on opposite
sides of the same small pot. I have, in short, followed different plans, but
in every case have taken all the precautions which I could think of, so that
the two lots should be equally favoured. The growth of the plants raised from
the crossed and self-fertilised seed, were carefully observed from their
germination to maturity, in species belonging to fifty-two genera; and the
difference in their growth, and in withstanding unfavourable conditions, was
in most cases manifest and strongly marked. It is of importance that the two
lots of seed should be sown or planted on opposite sides of the same pot, so
that the seedlings may struggle against each other; for if sown separately in
ample and good soil, there is often but little difference in their growth.

I will briefly describe two of the first cases observed by me. Six crossed and
six self-fertilised seeds of Ipomoea purpurea, from plants treated in the
manner above described, were planted as soon as they had germinated, in pairs
on opposite sides of two pots, and rods of equal thickness were given them to
twine up. Five of the crossed plants grew from the first more quickly than the
opposed self-fertilised plants; the sixth, however, was weakly and was for a
time beaten, but at last its sounder constitution prevailed and it shot ahead
of its antagonist. As soon as each crossed plant reached the top of its seven-
foot rod its fellow was measured, and the result was that, when the crossed
plants were seven feet high the self-fertilised had attained the average
height of only five feet four and a half inches. The crossed plants flowered a
little before, and more profusely than the self-fertilised plants. On opposite
sides of another SMALL pot a large number of crossed and self-fertilised seeds
were sown, so that they had to struggle for bare existence; a single rod was
given to each lot: here again the crossed plants showed from the first their
advantage; they never quite reached the summit of the seven-foot rod, but
relatively to the self-fertilised plants their average height was as seven
feet to five feet two inches. The experiment was repeated during several
succeeding generations, treated in exactly the same manner, and with nearly
the same result. In the second generation, the crossed plants, which were
again crossed, produced 121 seed-capsules, whilst the self-fertilised, again
self-fertilised, produced only 84 capsules.

Some flowers of the Mimulus luteus were fertilised with their own pollen, and
others were crossed with pollen from distinct plants growing in the same pot.
The seeds were thickly sown on opposite sides of a pot. The seedlings were at
first equal in height; but when the young crossed plants were half an inch,
the self-fertilised plants were only a quarter of an inch high. But this
degree of inequality did not last, for, when the crossed plants were four and
a half inches high, the self-fertilised were three inches, and they retained
the same relative difference till their growth was complete. The crossed
plants looked far more vigorous than the uncrossed, and flowered before them;
they produced also a far greater number of capsules. As in the former case,
the experiment was repeated during several succeeding generations. Had I not
watched these plants of Mimulus and Ipomoea during their whole growth, I could
not have believed it possible, that a difference apparently so slight as that
of the pollen being taken from the same flower, or from a distinct plant
growing in the same pot, could have made so wonderful a difference in the
growth and vigour of the plants thus produced. This, under a physiological
point of view, is a most remarkable phenomenon.

With respect to the benefit derived from crossing distinct varieties, plenty
of evidence has been published. Sageret (17/49. 'Memoire sur les
Cucurbitacees' pages 36, 28, 30.) repeatedly speaks in strong terms of the
vigour of melons raised by crossing different varieties, and adds that they
are more easily fertilised than common melons, and produce numerous good seed.
Here follows the evidence of an English gardener (17/50. Loudon's 'Gardener's
Mag.' volume 8 1832 page 52.): "I have this summer met with better success in
my cultivation of melons, in an unprotected state, from the seeds of hybrids
(i.e. mongrels) obtained by cross impregnation, than with old varieties. The
offspring of three different hybridisations (one more especially, of which the
parents were the two most dissimilar varieties I could select) each yielded
more ample and finer produce than any one of between twenty and thirty
established varieties."

Andrew Knight (17/51. 'Transact. Hort. Soc.' volume 1 page 25.) believed that
his seedlings from crossed varieties of the apple exhibited increased vigour
and luxuriance; and M. Chevreul (17/52. 'Annal. des Sc. Nat.' 3rd series, Bot.
tome 6 page 189.) alludes to the extreme vigour of some of the crossed fruit-
trees raised by Sageret.

By crossing reciprocally the tallest and shortest peas, Knight (17/53.
'Philosophical Transactions' 1799 page 200.) says: "I had in this experiment a
striking instance of the stimulative effects of crossing the breeds; for the
smallest variety, whose height rarely exceeded two feet, was increased to six
feet: whilst the height of the large and luxuriant kind was very little
diminished." Mr. Laxton gave me seed-peas produced from crosses between four
distinct kinds; and the plants thus raised were extraordinarily vigorous,
being in each case from one to two or three feet taller than the parent-forms
growing close alongside them.

Wiegmann (17/54. 'Ueber die Bastarderzeugung' 1828 s. 32, 33. For Mr.
Chaundy's case see Loudon's 'Gardener's Mag.' volume 7 1831 page 696.) made
many crosses between several varieties of cabbage; and he speaks with
astonishment of the vigour and height of the mongrels, which excited the
amazement of all the gardeners who beheld them. Mr. Chaundy raised a great
number of mongrels by planting together six distinct varieties of cabbage.
These mongrels displayed an infinite diversity of character; "But the most
remarkable circumstance was, that, while all the other cabbages and borecoles
in the nursery were destroyed by a severe winter, these hybrids were little
injured, and supplied the kitchen when there was no other cabbage to be had."

Mr. Maund exhibited before the Royal Agricultural Society (17/55. 'Gardener's
Chronicle' 1846 page 601.) specimens of crossed wheat, together with their
parent varieties; and the editor states that they were intermediate in
character, "united with that greater vigour of growth, which it appears, in
the vegetable as in the animal world, is the result of a first cross." Knight
also crossed several varieties of wheat (17/56. 'Philosoph. Transact.' 1799
page 201.), and he says "that in the years 1795 and 1796, when almost the
whole crop of corn in the island was blighted, the varieties thus obtained,
and these only, escaped in this neighbourhood, though sown in several
different soils and situations."

Here is a remarkable case: M. Clotzsch (17/57. Quoted in 'Bull. Bot. Soc.
France' volume 2 1855 page 327.) crossed Pinus sylvestris and nigricans,
Quercus robur and pedunculata, Alnus glutinosa and incana, Ulmus campestris
and effusa; and the cross-fertilised seeds, as well as seeds of the pure
parent-trees, were all sown at the same time and in the same place. The result
was, that after an interval of eight years, the hybrids were one-third taller
than the pure trees!

The facts above given refer to undoubted varieties, excepting the trees
crossed by Clotzsch, which are ranked by various botanists as strongly-marked
races, sub-species, or species. That true hybrids raised from entirely
distinct species, though they lose in fertility, often gain in size and
constitutional vigour, is certain. It would be superfluous to quote any facts;
for all experimenters, Kolreuter, Gartner, Herbert, Sageret, Lecoq, and
Naudin, have been struck with the wonderful vigour, height, size, tenacity of
life, precocity, and hardiness of their hybrid productions. Gartner (17/58.
Gartner 'Bastarderzeugung' s. 259, 518, 526 et seq.) sums up his conviction on
this head in the strongest terms. Kolreuter (17/59. 'Fortsetzung' 1763 s. 29;
'Dritte Fortsetzung' s. 44, 96; 'Act. Acad. St. Petersburg' 1782 part 2 page
251; 'Nova Acta' 1793 pages 391, 394; 'Nova Acta' 1795 pages 316, 323.) gives
numerous precise measurements of the weight and height of his hybrids in his
comparison with measurements of both parent-forms; and speaks with
astonishment of their "statura portentosa," their "ambitus vastissimus ac
altitudo valde conspicua." Some exceptions to the rule in the case of very
sterile hybrids have, however, been noticed by Gartner and Herbert; but the
most striking exceptions are given by Max Wichura (17/60. 'Die
Bastardbefruchtung' etc. 1865 s. 31, 41, 42.) who found that hybrid willows
were generally tender in constitution, dwarf, and short-lived.

Kolreuter explains the vast increase in the size of the roots, stems, etc., of
his hybrids, as the result of a sort of compensation due to their sterility,
in the same way as many emasculated animals are larger than the perfect males.
This view seems at first sight extremely probable, and has been accepted by
various authors (17/61. Max Wichura fully accepts this view
('Bastardbefruchtung' s. 43), as does the Rev. M.J. Berkeley in 'Journal of
Hort. Soc.' January 1866 page 70.); but Gartner (17/62. 'Bastarderzeugung' s.
394, 526, 528.) has well remarked that there is much difficulty in fully
admitting it; for with many hybrids there is no parallelism between the degree
of their sterility and their increased size and vigour. The most striking
instances of luxuriant growth have been observed with hybrids which were not
sterile in any extreme degree. In the genus Mirabilis, certain hybrids are
unusually fertile, and their extraordinary luxuriance of growth, together with
their enormous roots (17/63. Kolreuter 'Nova Acta' 1795 page 316.) have been
transmitted to their progeny. The result in all cases is probably in part due
to the saving of nutriment and vital force through the sexual organs acting
imperfectly or not at all, but more especially to the general law of good
being derived from a cross. For it deserves especial attention that mongrel
animals and plants, which are so far from being sterile that their fertility
is often actually augmented, have, as previously shown, their size, hardiness,
and constitutional vigour generally increased. It is not a little remarkable
that an accession of vigour and size should thus arise under the opposite
contingencies of increased and diminished fertility.

It is a perfectly well ascertained fact (17/64. Gartner 'Bastarderzeugung' s.
430.) that hybrids invariably breed with either pure parent, and not rarely
with a distinct species, more readily than with one another. Herbert is
inclined to explain even this fact by the advantage derived from a cross; but
Gartner more justly accounts for it by the pollen of the hybrid, and probably
its ovules, being in some degree vitiated, whereas the pollen and ovules of
both pure parents and of any third species are sound. Nevertheless, there are
some well-ascertained and remarkable facts, which, as we shall presently see,
show that a cross by itself undoubtedly tends to increase or re-establish the
fertility of hybrids.

The same law, namely, that the crossed offspring both of varieties and species
are larger than the parent-forms, holds good in the most striking manner with
hybrid animals as well as with mongrels. Mr. Bartlett, who has had such large
experience says, "Among all hybrids of vertebrated animals there is a marked
increase of size." He then enumerates many cases with mammals, including
monkeys, and with various families of birds. (17/65. Quoted by Dr. Murie in
'Proc. Zoolog. Soc.' 1870 page 40.)]


The facts now to be given differ from the foregoing, as self-sterility is not
here the result of long-continued close interbreeding. These facts are,
however, connected with our present subject, because a cross with a distinct
individual is shown to be either necessary or advantageous. Dimorphic and
trimorphic plants, though they are hermaphrodites, must be reciprocally
crossed, one set of forms by the other, in order to be fully fertile, and in
some cases to be fertile in any degree. But I should not have noticed these
plants, had it not been for the following cases given by Dr. Hildebrand
(17/66. 'Botanische Zeitung' January 1864 s. 3.):--

[Primula sinensis is a reciprocally dimorphic species: Dr. Hildebrand
fertilised twenty-eight flowers of both forms, each by pollen of the other
form, and obtained the full number of capsules containing on an average 42.7
seed per capsule; here we have complete and normal fertility. He then
fertilised forty-two flowers of both forms with pollen of the same form, but
taken from a distinct plant, and all produced capsules containing on an
average only 19.6 seed. Lastly, and here we come to our more immediate point,
he fertilised forty-eight flowers of both forms with pollen of the same form
and taken from the same flower, and now he obtained only thirty-two capsules,
and these contained on an average 18.6 seed, or one less per capsule than in
the former case. So that, with these illegitimate unions, the act of
impregnation is less assured, and the fertility slightly less, when the pollen
and ovules belong to the same flower, than when belonging to two distinct
individuals of the same form. Dr. Hildebrand has recently made analogous
experiments on the long-styled form of Oxalis rosea, with the same result.
(17/67. 'Monatsbericht Akad. Wissen.' Berlin 1866 s. 372.)]

It has recently been discovered that certain plants, whilst growing in their
native country under natural conditions, cannot be fertilised with pollen from
the same plant. They are sometimes so utterly self-impotent, that, though they
can readily be fertilised by the pollen of a distinct species or even distinct
genus, yet, wonderful as is the fact, they never produce a single seed by
their own pollen. In some cases, moreover, the plant's own pollen and stigma
mutually act on each other in a deleterious manner. Most of the facts to be
given relate to orchids, but I will commence with a plant belonging to a
widely different family.

[Sixty-three flowers of Corydalis cava, borne on distinct plants, were
fertilised by Dr. Hildebrand (17/68. International Hort. Congress, London
1866.) with pollen from other plants of the same species; and fifty-eight
capsules were obtained, including on an average 4.5 seed in each. He then
fertilised sixteen flowers produced by the same raceme, one with another, but
obtained only three capsules, one of which alone contained any good seeds,
namely, two in number. Lastly, he fertilised twenty-seven flowers, each with
its own pollen; he left also fifty-seven flowers to be spontaneously
fertilised, and this would certainly have ensued if it had been possible, for
the anthers not only touch the stigma, but the pollen-tubes were seen by Dr.
Hildebrand to penetrate it; nevertheless these eighty-four flowers did not
produce a single seed-capsule! This whole case is highly instructive, as it
shows how widely different the action of the same pollen is, according as it
is placed on the stigma of the same flower, or on that of another flower on
the same raceme, or on that of a distinct plant.

With exotic Orchids several analogous cases have been observed, chiefly by Mr.
John Scott. (17/69. 'Proc. Bot. Soc. of Edinburgh' May 1863: these
observations are given in abstract, and others are added, in the 'Journal of
Proc. of Linn. Soc.' volume 8 Bot. 1864 page 162.) Oncidium sphacelatum has
effective pollen, for Mr. Scott fertilised two distinct species with it; the
ovules are likewise capable of impregnation, for they were readily fertilised
by the pollen of O. divaricatum; nevertheless, between one and two hundred
flowers fertilised by their own pollen did not produce a single capsule,
though the stigmas were penetrated by the pollen-tubes. Mr. Robertson Munro,
of the Royal Botanic Gardens of Edinburgh, also informs me (1864) that a
hundred and twenty flowers of this same species were fertilised by him with
their own pollen, and did not produce a capsule, but eight flowers, fertilised
by the pollen of O. divaricatum, produced four fine capsules: again, between
two and three hundred flowers of O. divaricatum, fertilised by their own
pollen, did not set a capsule, but twelve flowers fertilised by O. flexuosum
produced eight fine capsules: so that here we have three utterly self-impotent
species, with their male and female organs perfect, as shown by their mutual
fertilisation. In these cases fertilisation was effected only by the aid of a
distinct species. But, as we shall presently see, distinct plants, raised from
seed, of Oncidium flexuosum, and probably of the other species, would have
been perfectly capable of fertilising each other, for this is the natural
process. Again, Mr. Scott found that the pollen of a plant of O. microchilum
was effective, for with it he fertilised two distinct species; he found its
ovules good, for they could be fertilised by the pollen of one of these
species, and by the pollen of a distinct plant of O. microchilum; but they
could not be fertilised by pollen of the same plant, though the pollen-tubes
penetrated the stigma. An analogous case has been recorded by M. Riviere
(17/70. Prof. Lecoq 'De la Fecondation' 2nd edition 1862 page 76.) with two
plants of O. cavendishianum, which were both self-sterile, but reciprocally
fertilised each other. All these cases refer to the genus Oncidium, but Mr.
Scott found that Maxillaria atro-rubens was "totally insusceptible of
fertilisation with its own pollen," but fertilised, and was fertilised by, a
widely distinct species, viz. M. squalens.

As these orchids had been grown under unnatural conditions in hot-houses, I
concluded that their self-sterility was due to this cause. But Fritz Muller
informs me that at Desterro, in Brazil, he fertilised above one hundred
flowers of the above-mentioned Oncidium flexuosum, which is there endemic,
with its own pollen, and with that taken from distinct plants: all the former
were sterile, whilst those fertilised by pollen from any OTHER PLANT of the
same species were fertile. During the first three days there was no difference
in the action of the two kinds of pollen: that placed on stigma of the same
plant separated in the usual manner into grains, and emitted tubes which
penetrated the column, and the stigmatic chamber shut itself; but only those
flowers which had been fertilised by pollen taken from a distinct plant
produced seed-capsules. On a subsequent occasion these experiments were
repeated on a large scale with the same result. Fritz Muller found that four
other endemic species of Oncidium were in like manner utterly sterile with
their own pollen, but fertile with that from any other plant: some of them
likewise produced seed-capsules when impregnated with pollen of widely
distinct genera, such as Cyrtopodium, and Rodriguezia. Oncidium crispum,
however, differs from the foregoing species in varying much in its self-
sterility; some plants producing fine pods with their own pollen, others
failing to do so in two or three instances, Fritz Muller observed that the
pods produced by pollen taken from a distinct flower on the same plant, were
larger than those produced by the flower's own pollen. In Epidendrum
cinnabarinum, an orchid belonging to another division of the family, fine pods
were produced by the plant's own pollen, but they contained by weight only
about half as much seed as the capsules which had been fertilised by pollen
from a distinct plant, and in one instance from a distinct species; moreover,
a very large proportion, and in some cases nearly all the seeds produced by
the plant's own pollen, were destitute of an embryo. Some self-fertilised
capsules of a Maxillaria were in a similar state.

Another observation made by Fritz Muller is highly remarkable, namely, that
with various orchids the plant's own pollen not only fails to impregnate the
flower, but acts on the stigma, and is acted on, in an injurious or poisonous
manner. This is shown by the surface of the stigma in contact with the pollen,
and by the pollen itself becoming in from three to five days dark brown, and
then decaying. The discoloration and decay are not caused by parasitic
cryptograms, which were observed by Fritz Muller in only a single instance.
These changes are well shown by placing on the same stigma, at the same time,
the plant's own pollen and that from a distinct plant of the same species, or
of another species, or even of another and widely remote genus. Thus, on the
stigma of Oncidium flexuosum, the plant's own pollen and that from a distinct
plant were placed side by side, and in five days' time the latter was
perfectly fresh, whilst the plant's own pollen was brown. On the other hand,
when the pollen of a distinct plant of the Oncidium flexuosum and of the
Epidendrum zebra (nov. spec.?) were placed together on the same stigma, they
behaved in exactly the same manner, the grains separating, emitting tubes, and
penetrating the stigma, so that the two pollen-masses, after an interval of
eleven days, could not be distinguished except by the difference of their
caudicles, which, of course, undergo no change. Fritz Muller has, moreover,
made a large number of crosses between orchids belonging to distinct species
and genera, and he finds that in all cases when the flowers are not fertilised
their footstalks first begin to wither; and the withering slowly spreads
upwards until the germens fall off, after an interval of one or two weeks, and
in one instance of between six and seven weeks; but even in this latter case,
and in most other cases, the pollen and stigma remained in appearance fresh.
Occasionally, however, the pollen becomes brownish, generally on the external
surface, and not in contact with the stigma, as is invariably the case when
the plant's own pollen is applied.

Fritz Muller observed the poisonous action of the plant's own pollen in the
above-mentioned Oncidium flexuosum, O. unicorne, pubes (?), and in two other
unnamed species. Also in two species of Rodriguezia, in two of Notylia, in one
of Burlingtonia, and of a fourth genus in the same group. In all these cases,
except the last, it was proved that the flowers were, as might have been
expected, fertile with pollen from a distinct plant of the same species.
Numerous flowers of one species of Notylia were fertilised with pollen from
the same raceme; in two days' time they all withered, the germens began to
shrink, the pollen-masses became dark brown, and not one pollen-grain emitted
a tube. So that in this orchid the injurious action of the plant's own pollen
is more rapid than with Oncidium flexuosum. Eight other flowers on the same
raceme were fertilised with pollen from a distinct plant of the same species:
two of these were dissected, and their stigmas were found to be penetrated by
numberless pollen-tubes; and the germens of the other six flowers became well
developed. On a subsequent occasion many other flowers were fertilised with
their own pollen, and all fell off dead in a few days; whilst some flowers on
the same raceme which had been left simply unfertilised adhered and long
remained fresh. We have seen that in cross-unions between extremely distinct
orchids the pollen long remains undecayed; but Notylia behaved in this respect
differently; for when its pollen was placed on the stigma of Oncidium
flexuosum, both the stigma and pollen quickly became dark brown, in the same
manner as if the plant's own pollen had been applied.

Fritz Muller suggests that, as in all these cases the plant's own pollen is
not only impotent (thus effectually preventing self-fertilisation), but
likewise prevents, as was ascertained in the case of the Notylia and Oncidium
flexuosum, the action of subsequently applied pollen from a distinct
individual, it would be an advantage to the plant to have its own pollen
rendered more and more deleterious; for the germens would thus quickly be
killed, and dropping off, there would be no further waste in nourishing a part
which ultimately could be of no avail.

The same naturalist found in Brazil three plants of a Bignonia growing near
together. He fertilised twenty-nine flowerets on one of them with their own
pollen, and they did not set a single capsule. Thirty flowers were then
fertilised with pollen from a distinct plant, one of the three, and they
yielded only two capsules. Lastly, five flowers were fertilised with pollen
from a fourth plant growing at a distance, and all five produced capsules.
Fritz Muller thinks that the three plants which grew near one another were
probably seedlings from the same parent, and that from being closely related,
they acted very feebly on one another. This view is extremely probable, for he
has since shown in a remarkable paper (17/71. 'Jenaische Zeitschrift fur
Naturwiss.' b. 7 page 22 1872 and page 441 1873. A large part of this paper
has been translated in the 'American Naturalist' 1874 page 223.), that in the
case of some Brazilian species of Abutilon, which are self-sterile, and
between which he raised some complex hybrids, that these, if near relatives,
were much less fertile inter se, than when not closely related.]

We now come to cases closely analogous with those just given, but different in
so far that only certain individuals of the species are self-sterile. This
self-impotence does not depend on the pollen or ovules being in an unfit state
for fertilisation, for both have been found effective in union with other
plants of the same or of a distinct species. The fact of plants having
acquired so peculiar a constitution, that they can be fertilised more readily
by the pollen of a distinct species than by their own, is exactly the reverse
of what occurs with all ordinary species. For in the latter the two sexual
elements of the same individual plant are of course capable of freely acting
on each other; but are so constituted that they are more or less impotent when
brought into union with the sexual elements of a distinct species, and produce
more or less sterile hybrids.

[Gartner experimented on two plants of Lobelia fulgens, brought from separate
places, and found (17/72. 'Bastarderzeugung' s. 64, 357.) that their pollen
was good, for he fertilised with it L. cardinalis and syphilitica; their
ovules were likewise good, for they were fertilised by the pollen of these
same two species; but these two plants of L. fulgens could not be fertilised
by their own pollen, as can generally be effected with perfect ease with this
species. Again, the pollen of a plant of Verbascum nigrum grown in a pot was
found by Gartner (17/73. Ibid s. 357.) capable of fertilising V. lychnitis and
V. austriacum; the ovules could be fertilised by the pollen of V. thapsus; but
the flowers could not be fertilised by their own pollen. Kolreuter, also
(17/74. 'Zweite Fortsetzung' s. 10; 'Dritte Forts.' s. 40. Mr. Scott likewise
fertilised fifty-four flowers of Verbascum phoeniceum, including two
varieties, with their own pollen, and not a single capsule was produced. Many
of the pollen-grains emitted their tubes, but only a few of them penetrated
the stigmas; some slight effect however was produced, as many of the ovaries
became somewhat developed: 'Journal Asiatic Soc. Bengal' 1867 page 150.),
gives the case of three garden plants of Verbascum phoeniceum, which bore
during two years many flowers; these he fertilised successfully with the
pollen of no less than four distinct species, but they produced not a seed
with their own apparently good pollen; subsequently these same plants, and
others raised from seed, assumed a strangely fluctuating condition, being
temporarily sterile on the male or female side, or on both sides, and
sometimes fertile on both sides; but two of the plants were perfectly fertile
throughout the summer.

With Reseda odorata I have found certain individuals quite sterile with their
own pollen, and so it is with the indigenous Reseda lutea. The self-sterile
plants of both species were perfectly fertile when crossed with pollen from
any other individual of the same species. These observations will hereafter be
published in another work, in which I shall also show that seeds sent to me by
Fritz Muller produced by plants of Eschscholtzia californica which were quite
self-sterile in Brazil, yielded in this country plants which were only
slightly self-sterile.

It appears (17/75. Duvernoy quoted by Gartner 'Bastarderzeugung' s. 334) that
certain flowers on certain plants of Lilium candidum can be fertilised more
freely by pollen from a distinct individual than by their own. So, again, with
the varieties of the potato. Tinzmann (17/76. 'Gardener's Chronicle' 1846 page
183.), who made many trials with this plant, says that pollen from another
variety sometimes "exerts a powerful influence, and I have found sorts of
potatoes which would not bear seed from impregnation with the pollen of their
own flowers would bear it when impregnated with other pollen." It does not,
however, appear to have been proved that the pollen which failed to act on the
flower's own stigma was in itself good.

In the genus Passiflora it has long been known that several species do not
produce fruit, unless fertilised by pollen taken from distinct species: thus,
Mr. Mowbray (17/77. 'Transact. Hort. Soc.' volume 7 1830 page 95.) found that
he could not get fruit from P. alata and racemosa except by reciprocally
fertilising them with each other's pollen; and similar facts have been
observed in Germany and France. (17/78. Prof. Lecoq 'De la Fecondation' 1845
page 70; Gartner 'Bastarderzeugung' s. 64.) I have received two accounts of P.
quadrangularis never producing fruit from its own pollen, but doing so freely
when fertilised in one case with the pollen of P. coerulea, and in another
case with that of P. edulis. But in three other cases this species fruited
freely when fertilised with its own pollen; and the writer in one case
attributed the favourable result to the temperature of the house having been
raised from 5 deg to 10 deg Fahr. above the former temperature, after the
flowers were fertilised. (17/79. 'Gardener's Chronicle' 1868 page 1341.) With
respect to P. laurifolia, a cultivator of much experience has recently
remarked (17/80. 'Gardener's Chronicle' 1866 page 1068.) that the flowers
"must be fertilised with the pollen of P. coerulea, or of some other common
kind, as their own pollen will not fertilise them." But the fullest details on
this subject have been given by Messrs. Scott and Robertson Munro (17/81.
'Journal of Proc. of Linn. Soc.' volume 8 1864 page 1168. Mr. Robertson Munro
in 'Trans. Bot. Soc.' of Edinburgh volume 9 page 399.): plants of Passiflora
racemosa, coerulea, and alata flowered profusely during many years in the
Botanic Gardens of Edinburgh, and, though repeatedly fertilised with their own
pollen, never produced any seed; yet this occurred at once with all three
species when they were crossed together in various ways. In the case of P.
coerulea three plants, two of which grew in the Botanic Gardens, were all
rendered fertile, merely by impregnating each with pollen of one of the
others. The same result was attained in the same manner with P. alata, but
with only one plant out of three. As so many self-sterile species of
Passiflora have been mentioned, it should be stated that the flowers of the
annual P. gracilis are nearly as fertile with their own pollen as with that
from a distinct plant; thus sixteen flowers spontaneously self-fertilised
produced fruit, each containing on an average 21.3 seed, whilst fruit from
fourteen crossed flowers contained 24.1 seed.

Returning to P. alata, I have received (1866) some interesting details from
Mr. Robertson Munro. Three plants, including one in England, have already been
mentioned which were inveterately self-sterile, and Mr. Munro informs me of
several others which, after repeated trials during many years, have been found
in the same predicament. At some other places, however, this species fruits
readily when fertilised with its own pollen. At Taymouth Castle there is a
plant which was formerly grafted by Mr. Donaldson on a distinct species, name
unknown, and ever since the operation it has produced fruit in abundance by
its own pollen; so that this small and unnatural change in the state of this
plant has restored its self-fertility! Some of the seedlings from the Taymouth
Castle plant were found to be not only sterile with their own pollen, but with
each other's pollen, and with the pollen of distinct species. Pollen from the
Taymouth plant failed to fertilise certain plants of the same species, but was
successful on one plant in the Edinburgh Botanic Gardens. Seedlings were
raised from this latter union, and some of their flowers were fertilised by
Mr. Munro with their own pollen; but they were found to be as self-impotent as
the mother-plant had always proved, except when fertilised by the grafted
Taymouth plant, and except, as we shall see, when fertilised by her own
seedlings. For Mr. Munro fertilised eighteen flowers on the self-impotent
mother-plant with pollen from these her own self-impotent seedlings, and
obtained, remarkable as the fact is, eighteen fine capsules full of excellent
seed! I have met with no case in regard to plants which shows so well as this
of P. alata, on what small and mysterious causes complete fertility or
complete sterility depends.]

The facts hitherto given relate to the much-lessened or completely destroyed
fertility of pure species when impregnated with their own pollen, in
comparison with their fertility when impregnated by distinct individuals or
distinct species; but closely analogous facts have been observed with hybrids.

[Herbert states (17/82. 'Amaryllidaceae' 1837 page 371; 'Journal of Hort.
Soc.' volume 2 1847 page 19.) that having in flower at the same time nine
hybrid Hippeastrums, of complicated origin, descended from several species, he
found that "almost every flower touched with pollen from another cross
produced seed abundantly, and those which were touched with their own pollen
either failed entirely, or formed slowly a pod of inferior size, with fewer
seeds." In the 'Horticultural Journal' he adds that "the admission of the
pollen of another cross-bred Hippeastrum (however complicated the cross) to
any one flower of the number, is almost sure to check the fructification of
the others." In a letter written to me in 1839, Dr. Herbert says that he had
already tried these experiments during five consecutive years, and he
subsequently repeated them, with the same invariable result. He was thus led
to make an analogous trial on a pure species, namely, on the Hippeastrum
aulicum, which he had lately imported from Brazil: this bulb produced four
flowers, three of which were fertilised by their own pollen, and the fourth by
the pollen of a triple cross between H. bulbulosum, reginae, and vittatum; the
result was, that "the ovaries of the three first flowers soon ceased to grow,
and after a few days perished entirely: whereas the pod impregnated by the
hybrid made vigorous and rapid progress to maturity, and bore good seed, which
vegetated freely." This is, indeed, as Herbert remarks, "a strange truth," but
not so strange as it then appeared.

As a confirmation of these statements, I may add that Mr. M. Mayes (17/83.
Loudon's 'Gardener's Magazine' volume 11 1835 page 260.) after much experience
in crossing the species of Amaryllis (Hippeastrum), says, "neither the species
nor the hybrids will, we are well aware, produce seed so abundantly from their
own pollen as from that of others." So, again, Mr. Bidwell, in New South Wales
(17/84. 'Gardener's Chronicle' 1850 page 470.) asserts that Amaryllis
belladonna bears many more seeds when fertilised by the pollen of Brunswigia
(Amaryllis of some authors) josephinae or of B. multiflora, than when
fertilised by its own pollen. Mr. Beaton dusted four flowers of a Cyrtanthus
with their own pollen, and four with the pollen of Vallota (Amaryllis)
purpurea; on the seventh day "those which received their own pollen slackened
their growth, and ultimately perished; those which were crossed with the
Vallota held on." (17/85. 'Journal Hort. Soc.' volume 5 page 135. The
seedlings thus raised were given to the Hort. Soc.; but I find, on inquiry,
that they unfortunately died the following winter.) These latter cases,
however, relate to uncrossed species, like those before given with respect to
Passiflora, Orchids, etc., and are here referred to only because the plants
belong to the same group of Amaryllidaceae.

In the experiments on the hybrid Hippeastrums, if Herbert had found that the
pollen of two or three kinds alone had been more efficient on certain kinds
than their own pollen, it might have been argued that these, from their mixed
parentage, had a closer mutual affinity than the others; but this explanation
is inadmissible, for the trials were made reciprocally backwards and forwards
on nine different hybrids; and a cross, whichever way taken, always proved
highly beneficial. I can add a striking and analogous case from experiments
made by the Rev. A. Rawson, of Bromley Common, with some complex hybrids of
Gladiolus. This skilful horticulturist possessed a number of French varieties,
differing from each other only in the colour and size of the flowers, all
descended from Gandavensis, a well-known old hybrid, said to be descended from
G. natalensis by the pollen of G. oppositiflorus. (17/86. Mr. D. Beaton in
'Journal of Hort.' 1861 page 453. Lecoq however ('De la Fecond.' 1862 page
369) states that this hybrid is descended from G. psittacinus and cardinalis;
but this is opposed to Herbert's experience, who found that the former species
could not be crossed.) Mr. Rawson, after repeated trials, found that none of
the varieties would set seed with their own pollen, although taken from
distinct plants of the same variety (which had, of course, been propagated by
bulbs), but that they all seeded freely with pollen from any other variety. To
give two examples: Ophir did not produce a capsule with its own pollen, but
when fertilised with that of Janire, Brenchleyensis, Vulcain and Linne, it
produced ten fine capsules; but the pollen of Ophir was good, for when Linne
was fertilised by it seven capsules were produced. This latter variety, on the
other hand, was utterly barren with its own pollen, which we have seen was
perfectly efficient on Ophir. Altogether, Mr. Rawson, in the year 1861
fertilised twenty-six flowers borne by four varieties with pollen taken from
other varieties, and every single flower produced a fine seed-capsule; whereas
fifty-two flowers on the same plants, fertilised at the same time with their
own pollen, did not yield a single seed-capsule. Mr. Rawson fertilised, in
some cases, the alternate flowers, and in other cases all those down one side
of the spike, with pollen of other varieties, and the remaining flowers with
their own pollen. I saw these plants when the capsules were nearly mature, and
their curious arrangement at once brought full conviction to the mind that an
immense advantage had been derived from crossing these hybrids.

Lastly, I have heard from Dr. E. Bornet, of Antibes, who has made numerous
experiments in crossing the species of Cistus, but has not yet published the
results, that, when any of these hybrids are fertile, they may be said to be,
in regard to function, dioecious; "for the flowers are always sterile when the
pistil is fertilised by pollen taken from the same flower or from flowers on
the same plant. But they are often fertile if pollen be employed from a
distinct individual of the same hybrid nature, or from a hybrid made by a
reciprocal cross."]


That plants should be self-sterile, although both sexual elements are in a fit
state for reproduction, appears at first sight opposed to all analogy. With
respect to the species, all the individuals of which are in this state,
although living under their natural conditions, we may conclude that their
self-sterility has been acquired for the sake of effectually preventing self-
fertilisation. The case is closely analogous with that of dimorphic and
trimorphic or heterostyled plants, which can be fully fertilised only by
plants belonging to a different form, and not, as in the foregoing cases,
indifferently by any other individual of the species. Some of these hetero-
styled plants are completely sterile with pollen taken from the same plant or
from the same form. With respect to species living under their natural
conditions, of which only certain individuals are self-sterile (as with Reseda
lutea), it is probable that these have been rendered self-sterile to ensure
occasional cross-fertilisation, whilst other individuals have remained self-
fertile to ensure the propagation of the species. The case seems to be
parallel with that of plants which produce, as Hermann Muller has discovered,
two forms--one bearing more conspicuous flowers with their structure adapted
for cross-fertilisation by insects, and the other form with less conspicuous
flowers adapted for self-fertilisation. The self-sterility, however, of some
of the foregoing plants is incidental on the conditions to which they have
been subjected, as with the Eschscholtzia, the Verbascum phoeniceum (the
sterility of which varied according to the season), and with the Passiflora
alata, which recovered its self-fertility when grafted on a different stock.

It is interesting to observe in the above several cases the graduated series
from plants which, when fertilised by their own pollen, yield the full number
of seeds, but with the seedlings a little dwarfed in stature--to plants which
when self-fertilised yield few seeds--to those which yield none, but have
their ovaria somewhat developed--and, lastly, to those in which the plant's
own pollen and stigma mutually act on one another like poison. It is also
interesting to observe on how slight a difference in the nature of the pollen
or of the ovules complete self-sterility or complete self-fertility must
depend in some of the above cases. Every individual of the self-sterile
species appears to be capable of producing the full complement of seed when
fertilised by the pollen of any other individual (though judging from the
facts given with respect to Abutilon the nearest kin must be excepted); but
not one individual can be fertilised by its own pollen. As every organism
differs in some slight degree from every other individual of the same species,
so no doubt it is with their pollen and ovules; and in the above cases we must
believe that complete self-sterility and complete self-fertility depend on
such slight differences in the ovules and pollen, and not their having been
differentiated in some special manner in relation to one another; for it is
impossible that the sexual elements of many thousand individuals should have
been specialised in relation to every other individual. In some, however, of
the above cases, as with certain Passifloras, an amount of differentiation
between the pollen and ovules sufficient for fertilisation is gained only by
employing pollen from a distinct species; but this is probably the result of
such plants having been rendered somewhat sterile from the unnatural
conditions to which they have been exposed.

Exotic animals confined in menageries are sometimes in nearly the same state
as the above-described self-impotent plants; for, as we shall see in the
following chapter, certain monkeys, the larger carnivora, several finches,
geese, and pheasants, cross together, quite as freely as, or even more freely
than the individuals of the same species breed together. Cases will, also, be
given of sexual incompatibility between certain, male and female domesticated
animals, which, nevertheless, are fertile when matched with any other
individual of the same kind.

In the early part of this chapter it was shown that the crossing of
individuals belonging to distinct families of the same race, or to different
races or species, gives increased size and constitutional vigour to the
offspring, and, except in the case of crossed species, increased fertility.
The evidence rests on the universal testimony of breeders (for it should be
observed that I am not here speaking of the evil results of close
interbreeding), and is practically exemplified in the higher value of cross-
bred animals for immediate consumption. The good results of crossing have also
been demonstrated with some animals and with numerous plants, by actual weight
and measurement. Although animals of pure blood will obviously be deteriorated
by crossing, as far as their characteristic qualities are concerned, there
seems to be no exception to the rule that advantages of the kind just
mentioned are thus gained, even when there has not been any previous close
interbreeding; and the rule applies to such animals as cattle and sheep, which
can long resist breeding in-and-in between the nearest blood-relations.

In the case of crossed species, although size, vigour, precocity, and
hardiness are, with rare exceptions, gained, fertility, in a greater or less
degree, is lost; but the gain in the above respects can hardly be attributed
to the principle of compensation; for there is no close parallelism between
the increased size and vigour of hybrid offspring and their sterility.
Moreover, it has been clearly proved that mongrels which are perfectly fertile
gain these same advantages as well as sterile hybrids.

With the higher animals no special adaptations for ensuring occasional crosses
between distinct families seem to exist. The eagerness of the males, leading
to severe competition between them, is sufficient; for even with gregarious
animals, the old and dominant males will be dispossessed after a time and it
would be a mere chance if a closely related member of the same family were to
be the victorious successor. The structure of many of the lower animals, when
they are hermaphrodites, is such as to prevent the ovules being fertilised by
the male element of the same individual; so that the concourse of two
individuals is necessary. In other cases the access of the male element of a
distinct individual is at least possible. With plants, which are affixed to
the ground and cannot wander from place to place like animals, the numerous
adaptations for cross-fertilisation are wonderfully perfect, as has been
admitted by every one who has studied the subject.

The evil consequences of long-continued close interbreeding are not so easily
recognised as the good effects from crossing, for the deterioration is
gradual. Nevertheless, it is the general opinion of those who have had most
experience, especially with animals which propagate quickly, that evil does
inevitably follow sooner or later, but at different rates with different
animals. No doubt a false belief may, like a superstition, prevail widely; yet
it is difficult to suppose that so many acute observers have all been deceived
at the expense of much cost and trouble. A male animal may sometimes be paired
with his daughter, granddaughter, and so on, even for seven generations,
without any manifest bad result: but the experiment has never been tried of
matching brothers and sisters, which is considered the closest form of
interbreeding, for an equal number of generations. There is good reason to
believe that by keeping the members of the same family in distinct bodies,
especially if exposed to somewhat different conditions of life, and by
occasionally crossing these families, the evil results of interbreeding may be
much diminished or quite eliminated. These results are loss of constitutional
vigour, size, and fertility; but there is no necessary deterioration in the
general form of the body, or in other good qualities. We have seen that with
pigs first-rate animals have been produced after long-continued close
interbreeding, though they had become extremely infertile when paired with
their near relations. The loss of fertility, when it occurs, seems never to be
absolute, but only relative to animals of the same blood; so that this
sterility is to a certain extent analogous with that of self-impotent plants
which cannot be fertilised by their own pollen, but are perfectly fertile with
pollen of any other individual of the same species. The fact of infertility of
this peculiar nature being one of the results of long-continued interbreeding,
shows that interbreeding does not act merely by combining and augmenting
various morbid tendencies common to both parents; for animals with such
tendencies, if not at the time actually ill, can generally propagate their
kind. Although offspring descended from the nearest blood-relations are not
necessarily deteriorated in structure, yet some authors believe that they are
eminently liable to malformations; and this is not improbable, as everything
which lessens the vital powers acts in this manner. Instances of this kind
have been recorded in the case of pigs, bloodhounds, and some other animals.

Finally, when we consider the various facts now given which plainly show that
good follows from crossing, and less plainly that evil follows from close
interbreeding, and when we bear in mind that with very many organisms
elaborate provisions have been made for the occasional union of distinct
individuals, the existence of a great law of nature is almost proved; namely,
that the crossing of animals and plants which are not closely related to each
other is highly beneficial or even necessary, and that interbreeding prolonged
during many generations is injurious.





In considering whether any facts were known which might throw light on the
conclusion arrived at in the last chapter, namely, that benefits ensue from
crossing, and that it is a law of nature that all organic beings should
occasionally cross, it appeared to me probable that the good derived from
slight changes in the conditions of life, from being an analogous phenomenon,
might serve this purpose. No two individuals, and still less no two varieties,
are absolutely alike in constitution and structure; and when the germ of one
is fertilised by the male element of another, we may believe that it is acted
on in a somewhat similar manner as an individual when exposed to slightly
changed conditions. Now, every one must have observed the remarkable influence
on convalescents of a change of residence, and no medical man doubts the truth
of this fact. Small farmers who hold but little land are convinced that their
cattle derive great benefit from a change of pasture. In the case of plants,
the evidence is strong that a great advantage is derived from exchanging
seeds, tubers, bulbs, and cuttings from one soil or place to another as
different as possible.

[The belief that plants are thus benefited, whether or not well founded, has
been firmly maintained from the time of Columella, who wrote shortly after the
Christian era, to the present day; and it now prevails in England, France, and
Germany. (18/1. For England see below. For Germany see Metzger 'Getreidearten'
1841 s. 63. For France Loiseleur-Deslongchamps ('Consid. sur les Cereales'
1843 page 200) gives numerous references on this subject. For Southern France
see Godron 'Florula Juvenalis' 1854 page 28.) A sagacious observer, Bradley,
writing in 1724 (18/2. 'A General Treatise of Husbandry' volume 3 page 58.),
says, "When we once become Masters of a good Sort of Seed, we should at least
put it into Two or Three Hands, where the Soils and Situations are as
different as possible; and every Year the Parties should change with one
another; by which Means, I find the Goodness of the Seed will be maintained
for several Years. For Want of this Use many Farmers have failed in their
Crops and been great Losers." He then gives his own practical experience on
this head. A modern writer (18/3. 'Gardener's Chronicle and Agricult. Gazette'
1858 page 247; and for the second statement, Ibid 1850 page 702. On this same
subject see also Rev. D. Walker 'Prize Essay of Highland Agricult. Soc.'
volume 2 page 200. Also Marshall 'Minutes of Agriculture' November 1775.)
asserts, "Nothing can be more clearly established in agriculture than that the
continual growth of any one variety in the same district makes it liable to
deterioration either in quality or quantity." Another writer states that he
sowed close together in the same field two lots of wheat-seed, the product of
the same original stock, one of which had been grown on the same land and the
other at a distance, and the difference in favour of the crop from the latter
seed was remarkable. A gentleman in Surrey who has long made it his business
to raise wheat to sell for seed, and who has constantly realised in the market
higher prices than others, assures me that he finds it indispensable
continually to change his seed; and that for this purpose he keeps two farms
differing much in soil and elevation.

With respect to the tubers of the potato, I find that at the present day the
practice of exchanging sets is almost everywhere followed. The great growers
of potatoes in Lancashire formerly used to get tubers from Scotland, but they
found that "a change from the moss-lands, and vice versa, was generally
sufficient." In former times in France the crop of potatoes in the Vosges had
become reduced in the course of fifty or sixty years in the proportion from
120-150 to 30-40 bushels; and the famous Oberlin attributed the surprising
good which he effected in large part to changing the sets. (18/4. Oberlin
'Memoirs' English translation page 73. For Lancashire see Marshall 'Review of
Reports' 1808 page 295.)

A well-known practical gardener, Mr. Robson (18/5. 'Cottage Gardener' 1856
page 186. For Mr. Robson's subsequent statements see 'Journal of Horticulture'
February 18, 1866 page 121. For Mr. Abbey's remarks on grafting etc. Ibid July
18, 1865 page 44.) positively states that he has himself witnessed decided
advantage from obtaining bulbs of the onion, tubers of the potato, and various
seeds, all of the same kind, from different soils and distant parts of
England. He further states that with plants propagated by cuttings, as with
the Pelargonium, and especially the Dahlia, manifest advantage is derived from
getting plants of the same variety, which have been cultivated in another
place; or, "where the extent of the place allows, to take cuttings from one
description of soil to plant on another, so as to afford the change that seems
so necessary to the well-being of the plants." He maintains that after a time
an exchange of this nature is "forced on the grower, whether he be prepared
for it or not." Similar remarks have been made by another excellent gardener,
Mr. Fish, namely, that cuttings of the same variety of Calceolaria, which he
obtained from a neighbour, "showed much greater vigour than some of his own
that were "treated in exactly the same manner," and he attributed this solely
to his own plants having become "to a certain extent worn out or tired of
their quarters." Something of this kind apparently occurs in grafting and
budding fruit-trees; for, according to Mr. Abbey, grafts or buds generally
take with greater facility on a distinct variety or even species, or on a
stock previously grafted, than on stocks raised from seeds of the variety
which is to be grafted; and he believes this cannot be altogether explained by
the stocks in question being better adapted to the soil and climate of the
place. It should, however, be added, that varieties grafted or budded on very
distinct kinds, though they may take more readily and grow at first more
vigorously than when grafted on closely allied stocks, afterwards often become

I have studied M. Tessier's careful and elaborate experiments (18/6. 'Mem. de
l'Acad. des Sciences' 1790 page 209.) made to disprove the common belief that
good is derived from a change of seed; and he certainly shows that the same
seed may with care be cultivated on the same farm (it is not stated whether on
exactly the same soil) for ten consecutive years without loss. Another
excellent observer, Colonel Le Couteur (18/7. 'On the Varieties of Wheat' page
52.) has come to the same conclusion; but then he expressly adds, if the same
seed be used, "that which is grown on land manured from the mixen one year
becomes seed for land prepared with lime, and that again becomes seed for land
dressed with ashes, then for land dressed with mixed manure, and so on." But
this in effect is a systematic exchange of seed, within the limits of the same

On the whole the belief, which has long been held by many cultivators, that
good follows from exchanging seed, tubers, etc., seems to be fairly well
founded. It seems hardly credible that the advantage thus derived can be due
to the seeds, especially if very small ones, obtaining in one soil some
chemical element deficient in the other and in sufficient quantity to
influence the whole after-growth of the plant. As plants after once
germinating are fixed to the same spot, it might have been anticipated that
they would show the good effects of a change more plainly than do animals
which continually wander about; and this apparently is the case. Life
depending on, or consisting in, an incessant play of the most complex forces,
it would appear that their action is in some way stimulated by slight changes
in the circumstances to which each organism is exposed. All forces throughout
nature, as Mr. Herbert Spencer (18/8. Mr. Spencer has fully and ably discussed
this whole subject in his 'Principles of Biology' 1864 volume 2 chapter 10. In
the first edition of my 'Origin of Species' 1859 page 267, I spoke of the good
effects from slight changes in the conditions of life and from cross-breeding,
and of the evil effects from great changes in the conditions and from crossing
widely distinct forms, as a series of facts "connected together by some common
but unknown bond, which is essentially related to the principle of life.)
remarks, tend towards an equilibrium, and for the life of each organism it is
necessary that this tendency should be checked. These views and the foregoing
facts probably throw light, on the one hand, on the good effects of crossing
the breed, for the germ will be thus slightly modified or acted on by new
forces; and on the other hand, on the evil effects of close interbreeding
prolonged during many generations, during which the germ will be acted on by a
male having almost identically the same constitution.


I will now attempt to show that animals and plants, when removed from their
natural conditions, are often rendered in some degree infertile or completely
barren; and this occurs even when the conditions have not been greatly
changed. This conclusion is not necessarily opposed to that at which we have
just arrived, namely, that lesser changes of other kinds are advantageous to
organic beings. Our present subject is of some importance, from having an
intimate connection with the causes of variability. Indirectly it perhaps
bears on the sterility of species when crossed: for as, on the one hand,
slight changes in the conditions of life are favourable to plants and animals,
and the crossing of varieties adds to the size, vigour, and fertility of their
offspring; so, on the other hand, certain other changes in the conditions of
life cause sterility; and as this likewise ensues from crossing much-modified
forms or species, we have a parallel and double series of facts, which
apparently stand in close relation to each other.

It is notorious that many animals, though perfectly tamed, refuse to breed in
captivity. Isidore Geoffroy St.-Hilaire (18/9. 'Essais de Zoologie Generale'
1841 page 256. ) consequently has drawn a broad distinction between tamed
animals which will not breed under captivity, and truly domesticated animals
which breed freely--generally more freely, as shown in the sixteenth chapter,
than in a state of nature. It is possible and generally easy to tame most
animals; but experience has shown that it is difficult to get them to breed
regularly, or even at all. I shall discuss this subject in detail; but will
give only those cases which seem most illustrative. My materials are derived
from notices scattered through various works, and especially from a Report,
kindly drawn up for me by the officers of the Zoological Society of London,
which has especial value, as it records all the cases, during nine years from
1838-46, in which the animals were seen to couple but produced no offspring,
as well as the cases in which they never, as far as known, coupled. This MS.
Report I have corrected by the annual Reports subsequently published up to the
year 1865. (18/10. Since the appearance of the first edition of this work, Mr.
Sclater has published ('Proc. Zoolog. Soc.' 1868 page 623) a list of the
species of mammals which have bred in the gardens from 1848 to 1867 inclusive.
Of the Artiodactyla 85 species have been kept, and of these 1 species in 1.9
have bred at least once during the 20 years; of 28 Marsupialia, 1 in 2.5 have
bred; of 74 Carnivora, 1 in 3.0 have bred; of 52 Rodentia, 1 in 4.7 have bred;
and of Quadrumana 75 species have been kept, and 1 in 6.2 have bred.) Many
facts are given on the breeding of the animals in that magnificent work,
'Gleanings from the Menageries of Knowsley Hall' by Dr. Gray. I made, also,
particular inquiries from the experienced keeper of the birds in the old
Surrey Zoological Gardens. I should premise that a slight change in the
treatment of animals sometimes makes a great difference in their fertility;
and it is probable that the results observed in different menageries would
differ. Indeed, some animals in our Zoological Gardens have become more
productive since the year 1846. It is, also, manifest from F. Cuvier's account
of the Jardin des Plantes (18/11. Du Rut 'Annales du Museum' 1807 tome 9 page
120.) that the animals formerly bred much less freely there than with us; for
instance, in the Duck tribe, which is highly prolific, only one species had at
that period produced young.

[The most remarkable cases, however, are afforded by animals kept in their
native country, which, though perfectly tamed, quite healthy, and allowed some
freedom, are absolutely incapable of breeding. Rengger (18/12. 'Saugethiere
von Paraguay' 1830 s. 49, 106, 118, 124, 201, 208, 249, 265, 327.), who in
Paraguay particularly attended to this subject, specifies six quadrupeds in
this condition; and he mentions two or three others which most rarely breed.
Mr. Bates, in his admirable work on the Amazons, strongly insists on similar
cases (18/13. 'The Naturalist on the Amazons' 1863 volume 1 pages 99, 193;
volume 2 page 113.); and he remarks, that the fact of thoroughly tamed native
mammals and birds not breeding when kept by the Indians, cannot be wholly
accounted for by their negligence or indifference, for the turkey and fowl are
kept and bred by various remote tribes. In almost every part of the world--for
instance, in the interior of Africa, and in several of the Polynesian islands
--the natives are extremely fond of taming the indigenous quadrupeds and
birds; but they rarely or never succeed in getting them to breed.

The most notorious case of an animal not breeding in captivity is that of the
elephant. Elephants are kept in large numbers in their native Indian home,
live to old age, and are vigorous enough for the severest labour; yet, with a
very few exceptions, they have never been known even to couple, though both
males and females have their proper periodical seasons. If, however, we
proceed a little eastward to Ava, we hear from Mr. Crawfurd (18/14. 'Embassy
to the Court of Ava' volume 1 page 534.) that their "breeding in the domestic
state, or at least in the half-domestic state in which the female elephants
are generally kept, is of everyday occurrence;" and Mr. Crawfurd informs me
that he believes that the difference must be attributed solely to the females
being allowed to roam the forest with some degree of freedom. The captive
rhinoceros, on the other hand, seems from Bishop Heber's account (18/15.
'Journal' volume 1 page 213.) to breed in India far more readily than the
elephant. Four wild species of the horse genus have bred in Europe, though
here exposed to a great change in their natural habits of life; but the
species have generally been crossed one with another. Most of the members of
the pig family breed readily in our menageries; even the Red River hog
(Potamochoerus penicillatus), from the sweltering plains of West Africa, has
bred twice in the Zoological Gardens. Here also the Peccary (Dicotyles
torquatus) has bred several times; but another species, the D. labiatus,
though rendered so tame as to be half-domesticated, is said to breed so rarely
in its native country of Paraguay, that according to Rengger (18/16.
'Saugethiere' s. 327.) the fact requires confirmation. Mr. Bates remarks that
the tapir, though often kept tame in Amazonia by the Indians, never breeds.

Ruminants generally breed quite freely in England, though brought from widely
different climates, as may be seen in the Annual Reports of the Zoological
Gardens, and in the Gleanings from Lord Derby's menagerie.

The Carnivora, with the exception of the Plantigrade division, breed (though
with capricious exceptions) about half as freely as ruminants. Many species of
Felidae have bred in various menageries, although imported from diverse
climates and closely confined. Mr. Bartlett, the present superintendent of the
Zoological Gardens (18/17. On the Breeding of the Larger Felidae 'Proc.
Zoolog. Soc.' 1861 page 140.) remarks that the lion appears to breed more
frequently and to bring forth more young at a birth than any other species of
the family. He adds that the tiger has rarely bred; "but there are several
well-authenticated instances of the female tiger breeding with the lion."
Strange as the fact may appear, many animals under confinement unite with
distinct species and produce hybrids quite as freely as, or even more freely
than, with their own species. On inquiring from Dr. Falconer and others, it
appears that the tiger when confined in India does not breed, though it has
been known to couple. The chetah (Felis jubata) has never been known by Mr.
Bartlett to breed in England, but it has bred at Frankfort; nor does it breed
in India, where it is kept in large numbers for hunting; but no pains would be
taken to make them breed, as only those animals which have hunted for
themselves in a state of nature are serviceable and worth training. (18/18.
Sleeman's 'Rambles in India' volume 2 page 10.) According to Rengger, two
species of wild cats in Paraguay, though thoroughly tamed, have never bred.
Although so many of the Felidae breed readily in the Zoological Gardens, yet
conception by no means always follows union: in the nine-year Report, various
species are specified which were observed to couple seventy-three times, and
no doubt this must have passed many times unnoticed; yet from the seventy-
three unions only fifteen births ensued. The Carnivora in the Zoological
Gardens were formerly less freely exposed to the air and cold than at present,
and this change of treatment, as I was assured by the former superintendent,
Mr. Miller, greatly increased their fertility. Mr. Bartlett, and there cannot
be a more capable judge, says, "it is remarkable that lions breed more freely
in travelling collections than in the Zoological Gardens; probably the
constant excitement and irritation produced by moving from place to place, or
change of air, may have considerable influence in the matter."

Many members of the Dog family breed readily when confined. The Dhole is one
of the most untamable animals in India, yet a pair kept there by Dr. Falconer
produced young. Foxes, on the other hand, rarely breed, and I have never heard
of such an occurrence with the European fox: the silver fox of North America
(Canis argentatus), however, has bred several times in the Zoological Gardens.
Even the otter has bred there. Every one knows how readily the semi-
domesticated ferret breeds, though shut up in miserably small cages; but other
species of Viverra and Paradoxurus absolutely refuse to breed in the
Zoological Gardens. The Genetta has bred both here and in the Jardin des
Plantes, and produced hybrids. The Herpestes fasciatus has likewise bred; but
I was formerly assured that the H. griseus, though many were kept in the
Gardens, never bred.

The Plantigrade Carnivora breed under confinement much less freely than other
Carnivora, although no reason can be assigned for this fact. In the nine-year
Report it is stated that the bears had been seen in the Zoological Gardens to
couple freely, but previously to 1848 had most rarely conceived. In the
Reports published since this date three species have produced young (hybrids
in one case), and, wonderful to relate, the white Polar bear has produced
young. The badger (Meles taxus) has bred several times in the Gardens; but I
have not heard of this occurring elsewhere in England, and the event must be
very rare, for an instance in Germany has been thought worth recording.
(18/19. Wiegmann 'Archiv. fur Naturgesch.' 1837 s. 162.) In Paraguay the
native Nasua, though kept in pairs during many years and perfectly tamed, has
never been known, according to Rengger, to breed or show any sexual passion;
nor, as I hear from Mr. Bates, does this animal, or the Cercoleptes, breed in
Amazonia. Two other plantigrade genera, Procyon and Gulo, though often kept
tame in Paraguay, never breed there. In the Zoological Gardens species of
Nasua and Procyon have been seen to couple; but they did not produce young.

As domesticated rabbits, guinea-pigs, and white mice breed so abundantly when
closely confined under various climates, it might have been thought that most
other members of the Rodent order would have bred in captivity, but this is
not the case. It deserves notice, as showing how the capacity to breed
sometimes goes by affinity, that the one native rodent of Paraguay, which
there breeds FREELY and has yielded successive generations, is the Cavia
aperea; and this animal is so closely allied to the guinea-pig, that it has
been erroneously thought to be the parent form. (18/20. Rengger 'Saugethiere'
etc. s. 276. On the parentage of the guinea-pig, see also Isid. Geoffroy St.-
Hilaire 'Hist. Nat. Gen.' I sent to Mr. H. Denny of Leeds the lice which I
collected from the wild aperea in La Plata, and he informs me that they belong
to a genus distinct from those found on the guinea-pig. This is important
evidence that the aperea is not the parent of the guinea-pig; and is worth
giving, as some authors erroneously suppose that the guinea-pig since being
domesticated has become sterile when crossed with the aperea.) In the
Zoological Gardens, some rodents have coupled, but have never produced young;
some have neither coupled nor bred; but a few have bred, as the porcupine more
than once, the Barbary mouse, lemming, chinchilla, and agouti (Dasyprocta
aguti) several times. This latter animal has also produced young in Paraguay,
though they were born dead and ill-formed; but in Amazonia, according to Mr.
Bates, it never breeds, though often kept tame about the houses. Nor does the
paca (Coelogenys paca) breed there. The common hare when confined has, I
believe, never bred in Europe; though, according to a recent statement, it has
crossed with the rabbit. (18/21. Although the existence of the Leporides, as
described by Dr. Broca ('Journal de Phys.' tome 2 page 370), has been
positively denied, yet Dr. Pigeaux ('Annals and Mag. of Nat. Hist.' volume 20
1867 page 75) affirms that the hare and rabbit have produced hybrids.) I have
never heard of the dormouse breeding in confinement. But squirrels offer a
more curious case: with one exception, no species has bred in the Zoological
Gardens, yet as many as fourteen individuals of S. palmarum were kept together
during several years. The S. cinera has been seen to couple, but it did not
produce young; nor has this species, when rendered extremely tame in its
native country, North America, been ever known to breed. (18/22. 'Quadrupeds
of North America' by Audubon and Bachman 1846 page 268.) At Lord Derby's
menagerie squirrels of many kinds were kept in numbers, but Mr. Thompson, the
superintendent, told me that none had ever bred there, or elsewhere as far as
he knew. I have never heard of the English squirrel breeding in confinement.
But the species which has bred more than once in the Zoological Gardens is the
one which perhaps might have been least expected, namely, the flying squirrel
(Sciuropterus volucella): it has, also, bred several times near Birmingham;
but the female never produced more than two young at a birth, whereas in its
native American home she bears from three to six young. (18/23. Loudon's 'Mag.
of Nat. Hist.' volume 9 1836 page 571; Audubon and Bachman 'Quadrupeds of
North America' page 221.)

Monkeys, in the nine-year Report from the Zoological Gardens, are stated to
unite most freely, but during this period, though many individuals were kept,
there were only seven births. I have heard of only one American monkey, the
Ouistiti, breeding in Europe. (18/24. Flourens 'De l'Instinct' etc. 1845 page
88.) A Macacus, according to Flourens, bred in Paris; and more than one
species of this genus has produced young in London, especially the Macacus
rhesus, which everywhere shows a special capacity to breed under confinement.
Hybrids have been produced both in Paris and London from this same genus. The
Arabian baboon, or Cynocephalus hamadryas (18/25. See 'Annual Reports Zoolog.
Soc.' 1855, 1858, 1863, 1864; 'Times' newspaper August 10, 1847; Flourens 'De
l'Instinct' page 85.), and a Cercopithecus have bred in the Zoological
Gardens, and the latter species at the Duke of Northumberland's. Several
members of the family of Lemurs have produced hybrids in the Zoological
Gardens. It is much more remarkable that monkeys very rarely breed when
confined in their native country; thus the Cay (Cebus azara) is frequently and
completely tamed in Paraguay, but Rengger (18/26. 'Saugethiere' etc. s. 34,
49.) says that it breeds so rarely, that he never saw more than two females
which had produced young. A similar observation has been made with respect to
the monkeys which are frequently tamed by the aborigines in Brazil. (18/27.
Art. Brazil 'Penny Cyclop.' page 363.) In Amazonia, these animals are so often
kept in a tame state, that Mr. Bates in walking through the streets of Para
counted thirteen species; but, as he asserts, they have never been known to
breed in captivity. (18/28. 'The Naturalist on the Amazons' volume 1 page 99.)


Birds offer in some respects better evidence than quadrupeds, from their
breeding more rapidly and being kept in greater numbers. (18/29. A list of the
species of birds which have bred in the Zoological Gardens from 1848 to 1867
inclusive has been published by Mr. Sclater in 'Proc. Zoolog. Soc.' 1869 page
626, since the first edition of this work appeared. Of Columbae 51 species
have been kept, and of Anseres 80 species, and in both these families 1
species in 2.6 have bred at least once in the 20 years. Of Gallinae 83 species
have been kept and 1 in 27 have bred; of 57 Grallae 1 in 9 have bred; of 110
Prehensores 1 in 22 have bred; of 178 Passeres 1 in 25.4 have bred; of 94
Accipitres 1 in 47 have bred; of 25 Picariae and of 35 Herodiones not one
species in either group has bred.) We have seen that carnivorous animals are
more fertile under confinement than most other mammals. The reverse holds good
with carnivorous birds. It is said (18/30. 'Encyclop. of Rural Sports' page
691.) that as many as eighteen species have been used in Europe for hawking,
and several others in Persia and India (18/31. According to Sir A. Burnes
'Cabool' etc. page 51, eight species are used for hawking in Sinde.); they
have been kept in their native country in the finest condition, and have been
flown during six, eight, or nine years (18/32. Loudon's 'Mag. of Nat. Hist.'
volume 6 1833 page 110.); yet there is no record of their having ever produced
young. As these birds were formerly caught whilst young, at great expense,
being imported from Iceland, Norway, and Sweden, there can be little doubt
that, if possible, they would have been propagated. In the Jardin des Plantes,
no bird of prey has been known to couple. (18/33. F. Cuvier 'Annal. du Museum'
tome 9 page 128.) No hawk, vulture, or owl has ever produced fertile eggs in
the Zoological Gardens, or in the old Surrey Gardens, with the exception, in
the former place on one occasion, of a condor and a kite (Milvus niger). Yet
several species, namely, the Aquila fusca, Haliaetus leucocephalus, Falco
tinnunculus, F. subbuteo, and Buteo vulgaris, have been seen to couple in the
Zoological Gardens. Mr. Morris (18/34. 'The Zoologist' volume 7-8 1849-50 page
2648.) mentions as a unique fact that a kestrel (Falco tinnunculus) bred in an
aviary. The one kind of owl which has been known to couple in the Zoological
Gardens was the Eagle Owl (Bubo maximus); and this species shows a special
inclination to breed in captivity; for a pair at Arundel Castle, kept more
nearly in a state of nature "than ever fell to the lot of an animal deprived
of its liberty" (18/35. Knox 'Ornithological Rambles in Sussex' page 91.),
actually reared their young. Mr. Gurney has given another instance of this
same owl breeding in confinement; and he records the case of a second species
of owl, the Strix passerina, breeding in captivity. (18/36. 'The Zoologist'
volume 7-8 1849-50 page 2566; volume 9-10 1851-2 page 3207.)

Of the smaller graminivorous birds, many kinds have been kept tame in their
native countries, and have lived long; yet, as the highest authority on cage-
birds (18/37. Bechstein 'Naturgesch. der Stubenvogel' 1840 s. 20.) remarks,
their propagation is "uncommonly difficult." The canary-bird shows that there
is no inherent difficulty in these birds breeding freely in confinement; and
Audubon says (18/38. 'Ornithological Biography' volume 5 page 517.) that the
Fringilla (Spiza) ciris of North America breeds as perfectly as the canary.
The difficulty with the many finches which have been kept in confinement is
all the more remarkable as more than a dozen species could be named which have
yielded hybrids with the canary; but hardly any of these, with the exception
of the siskin (Fringilla spinus), have reproduced their own kind. Even the
bullfinch (Loxia pyrrhula) has bred as frequently with the canary, though
belonging to a distinct genus, as with its own species. (18/39. A case is
recorded in 'The Zoologist' volume 1-2 1843-45 page 453. For the siskin
breeding, volume 3-4 1845-46 page 1075. Bechstein 'Stubenvogel' s. 139 speaks
of bullfinches making nests, but rarely producing young.) With respect to the
skylark (Alauda arvensis), I have heard of birds living for seven years in an
aviary, which never produced young; and a great London bird-fancier assured me
that he had never known an instance of their breeding; nevertheless one case
has been recorded. (18/40. Yarrell 'Hist. British Birds' 1839 volume 1 page
412.) In the nine-year Report from the Zoological Society, twenty-four
insessorial species are enumerated which had not bred, and of these only four
were known to have coupled.

Parrots are singularly long-lived birds; and Humboldt mentions the curious
fact of a parrot in South America, which spoke the language of an extinct
Indian tribe, so that this bird preserved the sole relic of a lost language.
Even in this country there is reason to believe (18/41. Loudon's 'Mag. of Nat.
History' volume 19 1836 page 347.) that parrots have lived to the age of
nearly one hundred years; yet they breed so rarely, though many have been kept
in Europe, that the event has been thought worth recording in the gravest
publications. (18/42. 'Memoires du Museum d'Hist. Nat.' tome 10 page 314: five
cases of parrots breeding in France are here recorded. See also 'Report Brit.
Assoc. Zoolog.' 1843.) Nevertheless, when Mr. Buxton turned out a large number
of parrots in Norfolk, three pairs bred and reared ten young birds in the
course of two seasons; and this success may be attributed to their free life.
(18/43. 'Annals and Mag. of Nat. Hist.' November 1868 page 311.) According to
Bechstein (18/44. 'Stubenvogel' s. 105, 83.) the African Psittacus erithacus
breeds oftener than any other species in Germany: the P. macoa occasionally
lays fertile eggs, but rarely succeeds in hatching them; this bird, however,
has the instinct of incubation sometimes so strongly developed, that it will
hatch the eggs of fowls or pigeons. In the Zoological Gardens and in the old
Surrey Gardens some few species have coupled, but, with the exception of three
species of parakeets, none have bred. It is a much more remarkable fact that
in Guiana parrots of two kinds, as I am informed by Sir R. Schomburgk, are
often taken from the nests by the Indians and reared in large numbers; they
are so tame that they fly freely about the houses, and come when called to be
fed, like pigeons; yet he has never heard of a single instance of their
breeding. (18/45. Dr. Hancock remarks ('Charlesworth's Mag. of Nat. Hist.'
volume 2 1838 page 492) "it is singular that, amongst the numerous useful
birds that are indigenous to Guiana, none are found to propagate among the
Indians; yet the common fowl is reared in abundance throughout the country.")
In Jamaica, a resident naturalist, Mr. R. Hill (18/46. 'A Week at Pert Royal'
1855 page 7.), says, "no birds more readily submit to human dependence than
the parrot-tribe, but no instance of a parrot breeding in this tame life has
been known yet." Mr. Hill specifies a number of other native birds kept tame
in the West Indies, which never breed in this state.

The great pigeon family offers a striking contrast with the parrots: in the
nine-year Report thirteen species are recorded as having bred, and, what is
more noticeable, only two were seen to couple without any result. Since the
above date every annual Report gives many cases of various pigeons breeding.
The two magnificent crowned pigeons (Goura coronata and victoriae) produced
hybrids; nevertheless, of the former species more than a dozen birds were
kept, as I am informed by Mr. Crawfurd, in a park at Penang, under a perfectly
well-adapted climate, but never once bred. The Columba migratoria in its
native country, North America, invariably lays two eggs, but in Lord Derby's
menagerie never more than one. The same fact has been observed with the C.
leucocephala. (18/47. Audubon 'American Ornithology' volume 5 pages 552, 557.)

Gallinaceous birds of many genera likewise show an eminent capacity for
breeding under captivity. This is particularly the case with pheasants, yet
our English species seldom lays more than ten eggs in confinement; whilst from
eighteen to twenty is the usual number in the wild state. (18/48. Mowbray on
'Poultry' 7th edition page 133.) With the Gallinaceae, as with all other
orders, there are marked and inexplicable exceptions in regard to the
fertility of certain species and genera under confinement. Although many
trials have been made with the common partridge, it has rarely bred, even when
reared in large aviaries; and the hen will never hatch her own eggs. (18/49.
Temminck 'Hist. Nat. Gen. des Pigeons' etc. 1813 tome 3 pages 288, 382;
'Annals and Mag. of Nat. Hist.' volume 12 1843 page 453. Other species of
partridge have occasionally bred; as the red-legged (P. rubra), when kept in a
large court in France (see Journal de Physique' tome 25 page 294), and in the
Zoological Gardens in 1856.) The American tribe of Guans or Cracidae are tamed
with remarkable ease, but are very shy breeders in this country (18/50. Rev.
E.S. Dixon 'The Dovecote' 1851 pages 243-252.); but with care various species
were formerly made to breed rather freely in Holland. (18/51. Temminck 'Hist.
Nat. Gen. des Pigeons' etc. tome 2 pages 456, 458; tome 3 pages 2, 13, 47.)
Birds of this tribe are often kept in a perfectly tamed condition in their
native country by the Indians, but they never breed. (18/52. Bates 'The
Naturalist on the Amazons' volume 1 page 193; volume 2 page 112.) It might
have been expected that grouse from their habits of life would not have bred
in captivity, more especially as they are said soon to languish and die.
(18/53. Temminck 'Hist. Nat. Gen.' etc. tome 2 page 125. For Tetrao urogallus
see L. Lloyd 'Field Sports of North of Europe' volume 1 pages 287, 314; and
Bull. de la Soc. d'Acclimat.' tome 7 1860 page 600. For T. scoticus Thompson
'Nat. Hist. of Ireland' volume 2 1850 page 49. For T. cupido 'Boston Journal
of Nat. Hist.' volume 3 page 199.) But many cases are recorded of their
breeding: the capercailzie (Tetrao urogallus) has bred in the Zoological
Gardens; it breeds without much difficulty when confined in Norway, and in
Russia five successive generations have been reared: Tetrao tetrix has
likewise bred in Norway; T. scoticus in Ireland; T. umbellus at Lord Derby's;
and T. cupido in North America.

It is scarcely possible to imagine a greater change in habits than that which
the members of the ostrich family must suffer, when cooped up in small
enclosures under a temperate climate, after freely roaming over desert and
tropical plains or entangled forests; yet almost all the kinds have frequently
produced young in the various European menageries, even the mooruk (Casuarius
bennetii) from New Ireland. The African ostrich, though perfectly healthy and
living long in the South of France, never lays more than from twelve to
fifteen eggs, though in its native country it lays from twenty-five to thirty.
(18/54. Marcel de Serres 'Annales des Sc. Nat.' 2nd series Zoolog. tome 13
page 175.) Here we have another instance of fertility impaired, but not lost,
under confinement, as with the flying squirrel, the hen-pheasant, and two
species of American pigeons.

Most Waders can be tamed, as the Rev. E.S. Dixon informs me, with remarkable
facility; but several of them are short-lived under confinement, so that their
sterility in this state is not surprising. The cranes breed more readily than
other genera: Grus montigresia has bred several times in Paris and in the
Zoological Gardens, as has G. cinerea at the latter place, and G. antigone at
Calcutta. Of other members of this great order, Tetrapteryx paradisea has bred
at Knowsley, a Porphyrio in Sicily, and the Gallinula chloropus in the
Zoological Gardens. On the other hand, several birds belonging to this order
will not breed in their native country, Jamaica; and the Psophia, though often
kept by the Indians of Guiana about their houses, "is seldom or never known to
breed." (18/55. Dr. Hancock in 'Charlesworth's Mag. of Nat. Hist.' volume 2
1838 page 491; R. Hill 'A Week at Port Royal' page 8; 'Guide to the Zoological
Gardens' by P.L. Sclater 1859 pages 11, 12; 'The Knowsley Menagerie' by D.
Gray 1846 p1. 14; E. Blyth 'Report Asiatic Soc. of Bengal' May 1855.)

The members of the great Duck family breed as readily in confinement as do the
Columbae and Gallinae and this, considering their aquatic and wandering
habits, and the nature of their food, could not have been anticipated. Even
some time ago above two dozen species had bred in the Zoological Gardens; and
M. Selys-Longchamps has recorded the production of hybrids from forty-four
different members of the family; and to these Professor Newton has added a few
more cases. (18/56. Prof. Newton in 'Proc. Zoolog. Soc.' 1860 page 336.)
"There is not," says Mr. Dixon (18/57. 'The Dovecote and Aviary' page 428.),
"in the wide world, a goose which is not in the strict sense of the word
domesticable;" that is, capable of breeding under confinement; but this
statement is probably too bold. The capacity to breed sometimes varies in
individuals of the same species; thus Audubon (18/58. 'Ornithological
Biography' volume 3 page 9.) kept for more than eight years some wild geese
(Anser canadensis), but they would not mate; whilst other individuals of the
same species produced young during the second year. I know of but one instance
in the whole family of a species which absolutely refuses to breed in
captivity, namely, the Dendrocygna viduata, although, according to Sir R.
Schomburgk (18/59. 'Geograph. Journal' volume 13 1844 page 32.), it is easily
tamed, and is frequently kept by the Indians of Guiana. Lastly, with respect
to Gulls, though many have been kept in the Zoological Gardens and in the old
Surrey Gardens, no instance was known before the year 1848 of their coupling
or breeding; but since that period the herring gull (Larus argentatus) has
bred many times in the Zoological Gardens and at Knowsley.

There is reason to believe that insects are affected by confinement like the
higher animals. It is well known that the Sphingidae rarely breed when thus
treated. An entomologist (18/60. Loudon's 'Mag. of Nat. Hist.' volume 5 1832
page 153.) in Paris kept twenty-five specimens of Saturnia pyri, but did not
succeed in getting a single fertile egg. A number of females of Orthosia munda
and of Mamestra suasa reared in confinement were unattractive to the males.
(18/61. 'Zoologist' volumes 5-6 1847-48 page 1660.) Mr. Newport kept nearly a
hundred individuals of two species of Vanessa, but not one paired; this,
however, might have been due to their habit of coupling on the wing. (18/62.
'Transact. Entomolog. Soc.' volume 4 1845 page 60.) Mr. Atkinson could never
succeed in India in making the Tarroo silk-moth breed in confinement. (18/63.
'Transact. Linn. Soc.' volume 7 page 40.) It appears that a number of moths,
especially the Sphingidae, when hatched in the autumn out of their proper
season, are completely barren; but this latter case is still involved in some
obscurity. (18/64. See an interesting paper by Mr. Newman in the 'Zoologist'
1857 page 5764; and Dr. Wallace in 'Proc. Entomolog. Soc.' June 4, 1860 page

Independently of the fact of many animals under confinement not coupling, or,
if they couple, not producing young, there is evidence of another kind that
their sexual functions are disturbed. For many cases have been recorded of the
loss by male birds when confined of their characteristic plumage. Thus the
common linnet (Linota cannabina) when caged does not acquire the fine crimson
colour on its breast, and one of the buntings (Emberiza passerina) loses the
black on its head. A Pyrrhula and an Oriolus have been observed to assume the
quiet plumage of the hen-bird; and the Falco albidus returned to the dress of
an earlier age. (18/65. Yarrell 'British Birds' volume 1 page 506; Bechstein
'Stubenvogel' s. 185; 'Philosoph. Transact.' 1772 page 271. Bronn 'Geschichte
der Natur' b. 2 s. 96 has collected a number of cases. For the case of the
deer see 'Penny Cyclop.' volume 8 page 350.) Mr. Thompson, the superintendent
of the Knowsley menagerie, informed me that he had often observed analogous
facts. The horns of a male deer (Cervus canadensis) during the voyage from
America were badly developed; but subsequently in Paris perfect horns were

When conception takes place under confinement, the young are often born dead,
or die soon, or are ill-formed. This frequently occurs in the Zoological
Gardens, and, according to Rengger, with native animals confined in Paraguay.
The mother's milk often fails. We may also attribute to the disturbance of the
sexual functions the frequent occurrence of that monstrous instinct which
leads the mother to devour her own offspring,--a mysterious case of
perversion, as it at first appears.

Sufficient evidence has now been advanced to prove that animals when first
confined are eminently liable to suffer in their reproductive systems. We feel
at first naturally inclined to attribute the result to loss of health, or at
least to loss of vigour; but this view can hardly be admitted when we reflect
how healthy, long-lived, and vigorous many animals are under captivity, such
as parrots, and hawks when used for hawking, cheetahs when used for hunting,
and elephants. The reproductive organs themselves are not diseased; and the
diseases, from which animals in menageries usually perish, are not those which
in any way affect their fertility. No domestic animal is more subject to
disease than the sheep, yet it is remarkably prolific. The failure of animals
to breed under confinement has been sometimes attributed exclusively to a
failure in their sexual instincts: this may occasionally come into play, but
there is no obvious reason why this instinct should be especially liable to be
affected with perfectly tamed animals, except, indeed, indirectly through the
reproductive system itself being disturbed. Moreover, numerous cases have been
given of various animals which couple freely under confinement, but never
conceive; or, if they conceive and produce young, these are fewer in number
than is natural to the species. In the vegetable kingdom instinct of course
can play no part; and we shall presently see that plants when removed from
their natural conditions are affected in nearly the same manner as animals.
Change of climate cannot be the cause of the loss of fertility, for, whilst
many animals imported into Europe from extremely different climates breed
freely, many others when confined in their native land are completely sterile.
Change of food cannot be the chief cause; for ostriches, ducks, and many other
animals, which must have undergone a great change in this respect, breed
freely. Carnivorous birds when confined are extremely sterile, whilst most
carnivorous mammals, except plantigrades, are moderately fertile. Nor can the
amount of food be the cause; for a sufficient supply will certainly be given
to valuable animals; and there is no reason to suppose that much more food
would be given to them than to our choice domestic productions which retain
their full fertility. Lastly, we may infer from the case of the elephant,
cheetah, various hawks, and of many animals which are allowed to lead an
almost free life in their native land, that want of exercise is not the sole

It would appear that any change in the habits of life, whatever these habits
may be, if great enough, tends to affect in an inexplicable manner the powers
of reproduction. The result depends more on the constitution of the species
than on the nature of the change; for certain whole groups are affected more
than others; but exceptions always occur, for some species in the most fertile
groups refuse to breed, and some in the most sterile groups breed freely.
Those animals which usually breed freely under confinement, rarely breed, as I
was assured, in the Zoological Gardens, within a year or two after their first
importation. When an animal which is generally sterile under confinement
happens to breed, the young apparently do not inherit this power: for had this
been the case, various quadrupeds and birds, which are valuable for
exhibition, would have become common. Dr. Broca even affirms (18/66. 'Journal
de Physiologie' tome 2 page 347.) that many animals in the Jardin des Plantes,
after having produced young for three or four successive generations, become
sterile; but this may be the result of too close interbreeding. It is a
remarkable circumstance that many mammals and birds have produced hybrids
under confinement quite as readily as, or even more readily than, they have
procreated their own kind. Of this fact many instances have been given (18/67.
For additional evidence on this subject see F. Cuvier in 'Annales du Museum'
tome 12 page 119.); and we are thus reminded of those plants which when
cultivated refuse to be fertilised by their own pollen, but can easily be
fertilised by that of a distinct species. Finally, we must conclude, limited
as the conclusion is, that changed conditions of life have an especial power
of acting injuriously on the reproductive system. The whole case is quite
peculiar, for these organs, though not diseased, are thus rendered incapable
of performing their proper functions, or perform them imperfectly.


With respect to domesticated animals, as their domestication mainly depends on
the accident of their breeding freely under captivity, we ought not to expect
that their reproductive system would be affected by any moderate degree of
change. Those orders of quadrupeds and birds, of which the wild species breed
most readily in our menageries, have afforded us the greatest number of
domesticated productions. Savages in most parts of the world are fond of
taming animals (18/68. Numerous instances could be given. Thus Livingstone
('Travels' page 217) states that the King of the Barotse, an inland tribe
which never had any communication with white men, was extremely fond of taming
animals, and every young antelope was brought to him. Mr. Galton informs me
that the Damaras are likewise fond of keeping pets. The Indians of South
America follow the same habit. Capt. Wilkes states that the Polynesians of the
Samoan Islands tamed pigeons; and the New Zealanders, as Mr. Mantell informs
me, kept various kinds of birds.); and if any of these regularly produced
young, and were at the same time useful, they would be at once domesticated.
If, when their masters migrated into other countries, they were in addition
found capable of withstanding various climates, they would be still more
valuable; and it appears that the animals which breed readily in captivity can
generally withstand different climates. Some few domesticated animals, such as
the reindeer and camel, offer an exception to this rule. Many of our
domesticated animals can bear with undiminished fertility the most unnatural
conditions; for instance, rabbits, guinea-pigs, and ferrets breed in miserably
confined hutches. Few European dogs of any kind withstand the climate of India
without degenerating, but as long as they survive, they retain, as I hear from
Dr. Falconer, their fertility; so it is, according to Dr. Daniell, with
English dogs taken to Sierra Leone. The fowl, a native of the hot jungles of
India, becomes more fertile than its parent-stock in every quarter of the
world, until we advance as far north as Greenland and Northern Siberia, where
this bird will not breed. Both fowls and pigeons, which I received during the
autumn direct from Sierra Leone, were at once ready to couple. (18/69. For
analogous cases with the fowl see Reaumur 'L'Art de faire Eclore' etc. 1749
page 243; and Col. Sykes in 'Proc. Zoolog. Soc.' 1832 etc. With respect to the
fowl not breeding in northern regions see Latham 'Hist. of Birds' volume 8
1823 page 169.) I have, also, seen pigeons breeding as freely as the common
kinds within a year after their importation from the upper Nile. The guinea-
fowl, an aboriginal of the hot and dry deserts of Africa, whilst living under
our damp and cool climate, produces a large supply of eggs.

Nevertheless, our domesticated animals under new conditions occasionally show
signs of lessened fertility. Roulin asserts that in the hot valleys of the
equatorial Cordillera sheep are not fully fecund (18/70. "Mem. par divers
Savans" 'Acad. des Sciences' tome 6 1835 page 347.); and according to Lord
Somerville (18/71. 'Youatt on Sheep' page 181.) the merino-sheep which he
imported from Spain were not at first perfectly fertile, it is said (18/72. J.
Mills 'Treatise on Cattle' 1776 page 72.) that mares brought up on dry food in
the stable, and turned out to grass, do not at first breed. The peahen, as we
have seen, is said not to lay so many eggs in England as in India. It was long
before the canary-bird was fully fertile, and even now first-rate breeding
birds are not common. (18/73. Bechstein 'Stubenvogel' s. 242.) In the hot and
dry province of Delhi, as I hear from Dr. Falconer, the eggs of the turkey,
though placed under a hen, are extremely liable to fail. According to Roulin,
geese taken to the lofty plateau of Bogota, at first laid seldom, and then
only a few eggs; of these scarcely a fourth were hatched, and half the young
birds died; in the second generation they were more fertile; and when Roulin
wrote they were becoming as fertile as our geese in Europe. With respect to
the valley of Quito, Mr. Orton says (18/74. 'The Andes and the Amazon' 1870
page 107.) "the only geese in the valley are a few imported from Europe, and
these refuse to propagate." In the Philippine Archipelago the goose, it is
asserted, will not breed or even lay eggs. (18/75. Crawford 'Descriptive Dict.
of the Indian Islands' 1856 page 145.) A more curious case is that of the
fowl, which, according to Roulin, when first introduced would not breed at
Cusco in Bolivia, but subsequently became quite fertile; and the English Game
fowl, lately introduced, had not as yet arrived at its full fertility, for to
raise two or three chickens from a nest of eggs was thought fortunate. In
Europe close confinement has a marked effect on the fertility of the fowl: it
has been found in France that with fowls allowed considerable freedom only
twenty per cent of the eggs failed; when allowed less freedom forty per cent
failed; and in close confinement sixty out of the hundred were not hatched.
(18/76. 'Bull. de la Soc. d'Acclimat.' tome 9 1862 pages 380, 384.) So we see
that unnatural and changed conditions of life produce some effect on the
fertility of our most thoroughly domesticated animals, in the same manner,
though in a far less degree, as with captive wild animals.

It is by no means rare to find certain males and females which will not breed
together, though both are known to be perfectly fertile with other males and
females. We have no reason to suppose that this is caused by these animals
having been subjected to any change in their habits of life; therefore such
cases are hardly related to our present subject. The cause apparently lies in
an innate sexual incompatibility of the pair which are matched. Several
instances have been communicated to me by Mr. W.C. Spooner (well known for his
essay on Cross-breeding), by Mr. Eyton of Eyton, by Mr. Wicksted and other
breeders, and especially by Mr. Waring of Chelsfield, in relation to horses,
cattle, pigs, foxhounds, other dogs, and pigeons. (18/77. For pigeons see Dr.
Chapuis 'Le Pigeon Voyageur Belge' 1865 page 66.) In these cases, females,
which either previously or subsequently were proved to be fertile, failed to
breed with certain males, with whom it was particularly desired to match them.
A change in the constitution of the female may sometimes have occurred before
she was put to the second male; but in other cases this explanation is hardly
tenable, for a female, known not to be barren, has been unsuccessfully paired
seven or eight times with the same male likewise known to be perfectly
fertile. With cart-mares, which sometimes will not breed with stallions of
pure blood, but subsequently have bred with cart-stallions, Mr. Spooner is
inclined to attribute the failure to the lesser sexual power of the racehorse.
But I have heard from the greatest breeder of racehorses at the present day,
through Mr. Waring, that "it frequently occurs with a mare to be put several
times during one or two seasons to a particular stallion of acknowledged
power, and yet prove barren; the mare afterwards breeding at once with some
other horse." These facts are worth recording, as they show, like so many
previous facts, on what slight constitutional differences the fertility of an
animal often depends.]


In the vegetable kingdom cases of sterility frequently occur, analogous with
those previously given in the animal kingdom. But the subject is obscured by
several circumstances, presently to be discussed, namely, the contabescence of
the anthers, as Gartner has named a certain affection--monstrosities--
doubleness of the flower--much-enlarged fruit--and long-continued or excessive
propagation by buds.

[It is notorious that many plants in our gardens and hot-houses, though
preserved in the most perfect health, rarely or never produce seed. I do not
allude to plants which run to leaves, from being kept too damp, or too warm,
or too much manured; for these do not flower, and the case may be wholly
different. Nor do I allude to fruit not ripening from want of heat or rotting
from too much moisture. But many exotic plants, with their ovules and pollen
appearing perfectly sound, will not set any seed. The sterility in many cases,
as I know from my own observation, is simply due to the absence of the proper
insects for carrying the pollen to the stigma. But after excluding the several
cases just specified, there are many plants in which the reproductive system
has been seriously affected by the altered conditions of life to which they
have been subjected.

It would be tedious to enter on many details. Linnaeus long ago observed
(18/78. 'Swedish Acts' volume 1 1739 page 3. Pallas makes the same remark in
his 'Travels' English translation volume 1 page 292.) that Alpine plants,
although naturally loaded with seed, produce either few or none when
cultivated in gardens. But exceptions often occur: the Draba sylvestris, one
of our most thoroughly Alpine plants, multiplies itself by seed in Mr. H.C.
Watson's garden, near London; and Kerner, who has particularly attended to the
cultivation of Alpine plants, found that various kinds, when cultivated,
spontaneously sowed themselves. (18/79. A. Kerner 'Die Cultur der
Alpenpflanzen' 1864 s. 139; Watson 'Cybele Britannica' volume 1 page 131; Mr.
D. Cameron also has written on the culture of Alpine plants in 'Gard.
Chronicle' 1848 pages 253, 268, and mentions a few which seed.) Many plants
which naturally grow in peat-earth are entirely sterile in our gardens. I have
noticed the same fact with several liliaceous plants, which nevertheless grew

Too much manure renders some kinds utterly sterile, as I have myself observed.
The tendency to sterility from this cause runs in families; thus, according to
Gartner (18/80. 'Beitrage zur Kenntniss der Befruchtung' 1844 s. 333.), it is
hardly possible to give too much manure to most Gramineae, Cruciferae, and
Leguminosae, whilst succulent and bulbous-rooted plants are easily affected.
Extreme poverty of soil is less apt to induce sterility; but dwarfed plants of
Trifolium minus and repens, growing on a lawn often mown and never manured,
were found by me not to produce any seed. The temperature of the soil, and the
season at which plants are watered, often have a marked effect on their
fertility, as was observed by Kolreuter in the case of Mirabilis. (18/81.
'Nova Acta Petrop.' 1793 page 391.) Mr. Scott, in the Botanic Gardens of
Edinburgh, observed that Oncidium divaricatum would not set seed when grown in
a basket in which it throve, but was capable of fertilisation in a pot where
it was a little damper. Pelargonium fulgidum, for many years after its
introduction, seeded freely; it then became sterile; now it is fertile (18/82.
'Cottage Gardener' 1856 pages 44, 109.) if kept in a dry stove during the
winter. Other varieties of pelargonium are sterile and others fertile without
our being able to assign any cause. Very slight changes in the position of a
plant, whether planted on a bank or at its base, sometimes make all the
difference in its producing seed. Temperature apparently has a much more
powerful influence on the fertility of plants than on that of animals.
Nevertheless it is wonderful what changes some few plants will withstand with
undiminished fertility: thus the Zephyranthes candida, a native of the
moderately warm banks of the Plata, sows itself in the hot dry country near
Lima, and in Yorkshire resists the severest frosts, and I have seen seeds
gathered from pods which had been covered with snow during three weeks.
(18/83. Dr. Herbert 'Amaryllidaceae' page 176.) Berberis wallichii, from the
hot Khasia range in India, is uninjured by our sharpest frosts, and ripens its
fruit under our cool summers. Nevertheless, I presume we must attribute to
change of climate the sterility of many foreign plants; thus, the Persian and
Chinese lilacs (Syringa persica and chinensis), though perfectly hardy here,
never produce a seed; the common lilac (S. vulgaris) seeds with us moderately
well, but in parts of Germany the capsules never contain seed. (18/84. Gartner
'Beitrage zur Kenntniss' etc. s. 560, 564.) Some few of the cases, given in
the last chapter, of self-impotent plants, might have been here introduced, as
their state seems due to the conditions to which they have been subjected.

The liability of plants to be affected in their fertility by slightly changed
conditions is the more remarkable, as the pollen when once in process of
formation is not easily injured; a plant may be transplanted, or a branch with
flower-buds be cut off and placed in water, and the pollen will be matured.
Pollen, also, when once mature, may be kept for weeks or even months. (18/85.
'Gardener's Chronicle' 1844 page 215; 1850 page 470. Faivre gives a good
resume on this subject in his 'La Variabilite des Especes' 1868 page 155.) The
female organs are more sensitive, for Gartner (18/86. 'Beitrage zur Kenntniss'
etc. s. 252, 338.) found that dicotyledonous plants, when carefully removed so
that they did not in the least flag, could seldom be fertilised; this occurred
even with potted plants if the roots had grown out of the hole at the bottom.
In some few cases, however, as with Digitalis, transplantation did not prevent
fertilisation; and according to the testimony of Mawz, Brassica rapa, when
pulled up by its roots and placed in water, ripened its seed. Flower-stems of
several monocotyledonous plants when cut off and placed in water likewise
produce seed. But in these cases I presume that the flowers had been already
fertilised, for Herbert (18/87. 'Journal of Hort. Soc.' volume 2 1847 page
83.) found with the Crocus that the plants might be removed or mutilated after
the act of fertilisation, and would still perfect their seeds; but that, if
transplanted before being fertilised, the application of pollen was powerless.

Plants which have been long cultivated can generally endure with undiminished
fertility various and great changes; but not in most cases so great a change
of climate as domesticated animals. It is remarkable that many plants under
these circumstances are so much affected that the proportion and the nature of
their chemical ingredients are modified, yet their fertility is unimpaired.
Thus, as Dr. Falconer informs me, there is a great difference in the character
of the fibre in hemp, in the quantity of oil in the seed of the Linum, in the
proportion of narcotin to morphine in the poppy, in gluten to starch in wheat,
when these plants are cultivated on the plains and on the mountains of India;
nevertheless, they all remain fully fertile.


Gartner has designated by this term a peculiar condition of the anthers in
certain plants, in which they are shrivelled, or become brown and tough, and
contain no good pollen. When in this state they exactly resemble the anthers
of the most sterile hybrids. Gartner (18/88. 'Beitrage zur Kenntniss' etc. s.
117 et seq.; Kolreuter 'Zweite Fortsetzung' s. 10, 121; 'Dritte Fortsetzung'
s. 57. Herbert 'Amaryllidaceae' page 355. Wiegmann 'Ueber die
Bastarderzeugung' s. 27.), in his discussion on this subject, has shown that
plants of many orders are occasionally thus affected; but the Caryophyllaceae
and Liliaceae suffer most, and to these orders, I think, the Ericaceae may be
added. Contabescence varies in degree, but on the same plant all the flowers
are generally affected to nearly the same extent. The anthers are affected at
a very early period in the flower-bud, and remain in the same state (with one
recorded exception) during the life of the plant. The affection cannot be
cured by any change of treatment, and is propagated by layers, cuttings, etc.,
and perhaps even by seed. In contabescent plants the female organs are seldom
affected, or merely become precocious in their development. The cause of this
affection is doubtful, and is different in different cases. Until I read
Gartner's discussion I attributed it, as apparently did Herbert, to the
unnatural treatment of the plants; but its permanence under changed
conditions, and the female organs not being affected, seem incompatible with
this view. The fact of several endemic plants becoming contabescent in our
gardens seems, at first sight, equally incompatible with this view; but
Kolreuter believes that this is the result of their transplantation. The
contabescent plants of Dianthus and Verbascum, found wild by Wiegmann, grew on
a dry and sterile bank. The fact that exotic plants are eminently liable to
this affection also seems to show that it is in some manner caused by their
unnatural treatment. In some instances, as with Silene, Gartner's view seems
the most probable, namely, that it is caused by an inherent tendency in the
species to become dioecious. I can add another cause, namely, the illegitimate
unions of heterostyled plants, for I have observed seedlings of three species
of Primula and of Lythrum salicaria, which had been raised from plants
illegitimately fertilised by their own-form pollen, with some or all their
anthers in a contabescent state. There is perhaps an additional cause, namely,
self-fertilisation; for many plants of Dianthus and Lobelia, which had been
raised from self-fertilised seeds, had their anthers in this state; but these
instances are not conclusive, as both genera are liable from other causes to
this affection.

Cases of an opposite nature likewise occur, namely, plants with the female
organs struck with sterility, whilst the male organs remain perfect. Dianthus
japonicus, a Passiflora, and Nicotiana, have been described by Gartner (18/89.
'Bastarderzengung' s. 356.) as being in this unusual condition.


Great deviations of structure, even when the reproductive organs themselves
are not seriously affected, sometimes cause plants to become sterile. But in
other cases plants may become monstrous to an extreme degree and yet retain
their full fertility. Gallesio, who certainly had great experience (18/90.
'Teoria della Riproduzione' 1816 page 84; 'Traite du Citrus' 1811 page 67.),
often attributes sterility to this cause; but it may be suspected that in some
of his cases sterility was the cause, and not the result, of the monstrous
growths. The curious St. Valery apple, although it bears fruit, rarely
produces seed. The wonderfully anomalous flowers of Begonia frigida, formerly
described, though they appear fit for fructification, are sterile. (18/91. Mr.
C.W. Crocker in 'Gardener's Chronicle' 1861 page 1092.) Species of Primula in
which the calyx is brightly coloured are said (18/92. Verlot 'Des Varietes'
1865 page 80.) to be often sterile, though I have known them to be fertile. On
the other hand, Verlot gives several cases of proliferous flowers which can be
propagated by seed. This was the case with a poppy, which had become
monopetalous by the union of its petals. (18/93. Verlot ibid page 88.) Another
extraordinary poppy, with the stamens replaced by numerous small supplementary
capsules, likewise reproduces itself by seed. This has also occurred with a
plant of Saxifraga geum, in which a series of adventitious carpels, bearing
ovules on their margins, had been developed between the stamens and the normal
carpels (18/94. Prof. Allman, Brit. Assoc., quoted in the 'Phytologist' volume
2 page 483. Prof. Harvey, on the authority of Mr. Andrews, who discovered the
plant, informed me that this monstrosity could be propagated by seed. With
respect to the poppy see Prof. Goeppert as quoted in 'Journal of Horticulture'
July 1, 1863 page 171.) Lastly, with respect to peloric flowers, which depart
wonderfully from the natural structure,--those of Linaria vulgaris seem
generally to be more or less sterile, whilst those before described of
Antirrhinum majus, when artificially fertilised with their own pollen, are
perfectly fertile, though sterile when left to themselves, for bees are unable
to crawl into the narrow tubular flower. The peloric flowers of Corydalis
solida, according to Godron (18/95. 'Comptes Rendus' December 19, 1864 page
1039.), are sometimes barren and sometimes fertile; whilst those of Gloxinia
are well known to yield plenty of seed. In our greenhouse Pelargoniums, the
central flower of the truss is often peloric, and Mr. Masters informs me that
he tried in vain during several years to get seed from these flowers. I
likewise made many vain attempts, but sometimes succeeded in fertilising them
with pollen from a normal flower of another variety; and conversely I several
times fertilised ordinary flowers with peloric pollen. Only once I succeeded
in raising a plant from a peloric flower fertilised by pollen from a peloric
flower borne by another variety; but the plant, it may be added, presented
nothing particular in its structure. Hence we may conclude that no general
rule can be laid down; but any great deviation from the normal structure, even
when the reproductive organs themselves are not seriously affected, certainly
often leads to sexual impotence.


When the stamens are converted into petals, the plant becomes on the male side
sterile; when both stamens and pistils are thus changed, the plant becomes
completely barren. Symmetrical flowers having numerous stamens and petals are
the most liable to become double, as perhaps follows from all multiple organs
being the most subject to variability. But flowers furnished with only a few
stamens, and others which are asymmetrical in structure, sometimes become
double, as we see with the double gorse or Ulex, and Antirrhinum. The
Compositae bear what are called double flowers by the abnormal development of
the corolla of their central florets. Doubleness is sometimes connected with
prolification (18/96. 'Gardener's Chronicle' 1866 page 681.), or the continued
growth of the axis of the flower. Doubleness is strongly inherited. No one has
produced, as Lindley remarks (18/97. 'Theory of Horticulture' page 333.),
double flowers by promoting the perfect health of the plant. On the contrary,
unnatural conditions of life favour their production. There is some reason to
believe that seeds kept during many years, and seeds believed to be
imperfectly fertilised, yield double flowers more freely than fresh and
perfectly fertilised seed. (18/98. Mr. Fairweather 'Transact. Hort. Soc.'
volume 3 page 406: Bosse quoted by Bronn 'Geschichte der Natur' b. 2 s. 77. On
the effects of the removal of the anthers see Mr. Leitner in Silliman's 'North
American Journ. of Science' volume 23 page 47; and Verlot 'Des Varietes' 1865
page 84.) Long-continued cultivation in rich soil seems to be the commonest
exciting cause. A double narcissus and a double Anthemis nobilis, transplanted
into very poor soil, has been observed to become single (18/99. Lindley's
'Theory of Horticulture' page 3?3.); and I have seen a completely double white
primrose rendered permanently single by being divided and transplanted whilst
in full flower. It has been observed by Professor E. Morren that doubleness of
the flowers and variegation of the leaves are antagonistic states; but so many
exceptions to the rule have lately been recorded (18/100. 'Gardener's
Chronicle' 1865 page 626; 1866 pages 290, 730; and Verlot 'Des Varietes' page
75.), that, though general, it cannot be looked at as invariable. Variegation
seems generally to result from a feeble or atrophied condition of the plant,
and a large proportion of the seedlings raised from parents, if both are
variegated, usually perish at an early age; hence we may perhaps infer that
doubleness, which is the antagonistic state, commonly arises from a plethoric
condition. On the other hand, extremely poor soil sometimes, though rarely,
appears to cause doubleness: I formerly described (18/101. 'Gardener's
Chronicle' 1843 page 628. In this article I suggested the theory above given
on the doubleness of flowers. This view is adopted by Carriere 'Production et
Fix. des Varietes' 1865 page 67.) some completely double, bud-like, flowers
produced in large numbers by stunted wild plants of Gentiana amarella growing
on a poor chalky bank. I have also noticed a distinct tendency to doubleness
in the flowers of a Ranunculus, Horse-chestnut, and Bladder-nut (Ranunculus
repens, Aesculus pavia, and Staphylea), growing under very unfavourable
conditions. Professor Lehmann (18/102. Quoted by Gartner 'Bastarderzeugung' s.
567.) found several wild plants growing near a hot spring with double flowers.
With respect to the cause of doubleness, which arises, as we see, under widely
different circumstances, I shall presently attempt to show that the most
probable view is that unnatural conditions first give a tendency to sterility,
and that then, on the principle of compensation, as the reproductive organs do
not perform their proper functions, they either become developed into petals,
or additional petals are formed. This view has lately been supported by Mr.
Laxton (18/103. 'Gardener's Chronicle' 1866 page 901.) who advances the case
of some common peas, which, after long-continued heavy rain, flowered a second
time, and produced double flowers.


Many of our most valuable fruits, although consisting in a homological sense
of widely different organs, are either quite sterile, or produce extremely few
seeds. This is notoriously the case with our best pears, grapes, and figs,
with the pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms,
and some members of the orange-tribe. Poorer varieties of these same fruits
either habitually or occasionally yield seed. (18/104. Lindley 'Theory of
Horticulture' pages 175-179; Godron 'De l'Espece' tome 2 page 106; Pickering
'Races of Man;' Gallesio 'Teoria della Riproduzione' l816 pages 101-110. Meyen
'Reise um Erde' Th. 2 s. 214 states that at Manilla one variety of the banana
is full of seeds: and Chamisso (Hooker's 'Bot. Misc.' volume 1 page 310)
describes a variety of the bread-fruit in the Mariana Islands with small
fruit, containing seeds which are frequently perfect. Burnes in his 'Travels
in Bokhara' remarks on the pomegranate seeding in Mazenderan, as a remarkable
peculiarity.) Most horticulturists look at the great size and anomalous
development of the fruit as the cause, and sterility as the result; but the
opposite view, as we shall presently see, is more probable.


Plants which from any cause grow too luxuriantly, and produce leaves, stems,
runners, suckers, tubers, bulbs, etc., in excess, sometimes do not flower, or
if they flower do not yield seed. To make European vegetables under the hot
climate of India yield seed, it is necessary to check their growth; and, when
one-third grown, they are taken up, and their stems and tap-roots are cut or
mutilated. (18/105. Ingledew in 'Transact. of Agricult. and Hort. Soc. of
India' volume 2.) So it is with hybrids; for instance, Prof. Lecoq (18/106.
'De la Fecondation' 1862 page 308.) had three plants of Mirabilis, which,
though they grew luxuriantly and flowered, were quite sterile; but after
beating one with a stick until a few branches alone were left, these at once
yielded good seed. The sugar-cane, which grows vigorously and produces a large
supply of succulent stems, never, according to various observers, bears seed
in the West Indies, Malaga, India, Cochin China, Mauritius, or the Malay
Archipelago. (18/107. Hooker 'Bot. Misc.' volume 1 page 99; Gallesio 'Teoria
della Riproduzione' page 110. Dr. J. de Cordemoy in 'Transact. of the R. Soc.
of Mauritius' new series volume 6 1873 pages 60-67, gives a large number of
cases of plants which never seed, including several species indigenous in
Mauritius.) Plants which produce a large number of tubers are apt to be
sterile, as occurs, to a certain extent, with the common potato; and Mr.
Fortune informs me that the sweet potato (Convolvulus batatas) in China never,
as far as he has seen, yields seed. Dr. Royle remarks (18/108. 'Transact.
Linn. Soc.' volume 17 page 563.) that in India the Agave vivipara, when grown
in rich soil, invariably produces bulbs, but no seeds; whilst a poor soil and
dry climate lead to an opposite result. In China, according to Mr. Fortune, an
extraordinary number of little bulbs are developed in the axils of the leaves
of the yam, and this plant does not bear seed. Whether in these cases, as in
those of double flowers and seedless fruit, sexual sterility from changed
conditions of life is the primary cause which leads to the excessive
development of the organs of vegetation, is doubtful; though some evidence
might be advanced in favour of this view. It is perhaps a more probable view
that plants which propagate themselves largely by one method, namely by buds,
have not sufficient vital power or organised matter for the other method of
sexual generation.

Several distinguished botanists and good practical judges believe that long-
continued propagation by cuttings, runners, tubers, bulbs, etc., independently

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