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The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom by Charles Darwin

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fertilisation of these plants is generally aided by the stigmas being of
large size or plumose; and in the case of the Coniferae, by the naked
ovules secreting a drop of fluid, as shown by Delpino. Although the
number of anemophilous species is small, as the author just quoted
remarks, the number of individuals is large in comparison with that of
entomophilous species. This holds good especially in cold and temperate
regions, where insects are not so numerous as under a warmer climate,
and where consequently entomophilous plants are less favourably
situated. We see this in our forests of Coniferae and other trees, such
as oaks, beeches, birches, ashes, etc.; and in the Gramineae,
Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all
these trees and plants being fertilised by the wind. As a large quantity
of pollen is wasted by anemophilous plants, it is surprising that so
many vigorous species of this kind abounding with individuals should
still exist in any part of the world; for if they had been rendered
entomophilous, their pollen would have been transported by the aid of
the senses and appetites of insects with incomparably greater safety
than by the wind. That such a conversion is possible can hardly be
doubted, from the remarks lately made on the existence of intermediate
forms; and apparently it has been effected in the group of willows, as
we may infer from the nature of their nearest allies. (10/55. Hermann
Muller 'Die Befruchtung' etc. page 149.)

It seems at first sight a still more surprising fact that plants, after
having been once rendered entomophilous, should ever again have become
anemophilous; but this has occasionally though rarely occurred, for
instance, with the common Poterium sanguisorba, as may be inferred from
its belonging to the Rosaceae. Such cases are, however, intelligible, as
almost all plants require to be occasionally intercrossed; and if any
entomiphilous species ceased to be visited by insects, it would probably
perish unless it were rendered anemophilous. A plant would be neglected
by insects if nectar failed to be secreted, unless indeed a large supply
of attractive pollen was present; and from what we have seen of the
excretion of saccharine fluid from leaves and glands being largely
governed in several cases by climatic influences, and from some few
flowers which do not now secrete nectar still retaining coloured
guiding-marks, the failure of the secretion cannot be considered as a
very improbable event. The same result would follow to a certainty, if
winged insects ceased to exist in any district, or became very rare. Now
there is only a single plant in the great order of the Cruciferae,
namely, Pringlea, which is anemophilous, and this plant is an inhabitant
of Kerguelen Land, where there are hardly any winged insects, owing
probably, as was suggested by me in the case of Madeira, to the risk
which they run of being blown out to sea and destroyed. (10/56. The
Reverend A.E. Eaton in 'Proceedings of the Royal Society' volume 23 1875
page 351.)

A remarkable fact with respect to anemophilous plants is that they are
often diclinous, that is, they are either monoecious with their sexes
separated on the same plant, or dioecious with their sexes on distinct
plants. In the class Monoecia of Linnaeus, Delpino shows that the
species of twenty-eight genera are anemophilous, and of seventeen genera
entomophilous. (10/57. 'Studi sopra un Lignaggio anemofilo delle
Compositae' 1871.) The larger proportion of entomophilous genera in this
latter class is probably the indirect result of insects having the power
of carrying pollen to another and sometimes distant plant much more
securely than the wind. In the above two classes taken together there
are thirty-eight anemophilous and thirty-six entomophilous genera;
whereas in the great mass of hermaphrodite plants the proportion of
anemophilous to entomophilous genera is extremely small. The cause of
this remarkable difference may be attributed to anemophilous plants
having retained in a greater degree than the entomophilous a primordial
condition, in which the sexes were separated and their mutual
fertilisation effected by means of the wind. That the earliest and
lowest members of the vegetable kingdom had their sexes separated, as is
still the case to a large extent, is the opinion of a high authority,
Nageli. (10/58. 'Entstehung und Begriff der Naturhist. Art' 1865 page
22.) It is indeed difficult to avoid this conclusion, if we admit the
view, which seems highly probable, that the conjugation of the Algae and
of some of the simplest animals is the first step towards sexual
reproduction; and if we further bear in mind that a greater and greater
degree of differentiation between the cells which conjugate can be
traced, thus leading apparently to the development of the two sexual
forms. (10/59. See the interesting discussion on this whole subject by
O. Butschli in his 'Studien uber die ersten Entwickelungsvorgange der
Eizelle; etc. 1876 pages 207-219. Also Engelmann "Ueber Entwickelung von
Infusorien" 'Morphol. Jahrbuch' B. 1 page 573. Also Dr. A. Dodel "Die
Kraushaar-Algae" 'Pringsheims Jahrbuch f. Wiss. Bot.' B. 10.) We have
also seen that as plants became more highly developed and affixed to the
ground, they would be compelled to be anemophilous in order to
intercross. Therefore all plants which have not since been greatly
modified, would tend still to be both diclinous and anemophilous; and we
can thus understand the connection between these two states, although
they appear at first sight quite disconnected. If this view is correct,
plants must have been rendered hermaphrodites at a later though still
very early period, and entomophilous at a yet later period, namely,
after the development of winged insects. So that the relationship
between hermaphroditism and fertilisation by means of insects is
likewise to a certain extent intelligible.

Why the descendants of plants which were originally dioecious, and which
therefore profited by always intercrossing with another individual,
should have been converted into hermaphrodites, may perhaps be explained
by the risk which they ran, especially as long as they were
anemophilous, of not being always fertilised, and consequently of not
leaving offspring. This latter evil, the greatest of all to any
organism, would have been much lessened by their becoming
hermaphrodites, though with the contingent disadvantage of frequent
self-fertilisation. By what graduated steps an hermaphrodite condition
was acquired we do not know. But we can see that if a lowly organised
form, in which the two sexes were represented by somewhat different
individuals, were to increase by budding either before or after
conjugation, the two incipient sexes would be capable of appearing by
buds on the same stock, as occasionally occurs with various characters
at the present day. The organism would then be in a monoecious
condition, and this is probably the first step towards hermaphroditism;
for if very simple male and female flowers on the same stock, each
consisting of a single stamen or pistil, were brought close together and
surrounded by a common envelope, in nearly the same manner as with the
florets of the Compositae, we should have an hermaphrodite flower.

There seems to be no limit to the changes which organisms undergo under
changing conditions of life; and some hermaphrodite plants, descended as
we must believe from aboriginally diclinous plants, have had their sexes
again separated. That this has occurred, we may infer from the presence
of rudimentary stamens in the flowers of some individuals, and of
rudimentary pistils in the flowers of other individuals, for example in
Lychnis dioica. But a conversion of this kind will not have occurred
unless cross-fertilisation was already assured, generally by the agency
of insects; but why the production of male and female flowers on
distinct plants should have been advantageous to the species,
cross-fertilisation having been previously assured, is far from obvious.
A plant might indeed produce twice as many seeds as were necessary to
keep up its numbers under new or changed conditions of life; and if it
did not vary by bearing fewer flowers, and did vary in the state of its
reproductive organs (as often occurs under cultivation), a wasteful
expenditure of seeds and pollen would be saved by the flowers becoming

A related point is worth notice. I remarked in my Origin of Species that
in Britain a much larger proportion of trees and bushes than of
herbaceous plants have their sexes separated; and so it is, according to
Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in
the 'London Catalogue of British Plants' that there are thirty-two
indigenous trees and bushes in Great Britain, classed under nine
families; but to err on the safe side, I have counted only six species
of willows. Of the thirty-two trees and bushes, nineteen, or more than
half, have their sexes separated; and this is an enormous proportion
compared with other British plants. New Zealand abounds with diclinous
plants and trees; and Dr. Hooker calculates that out of about 756
phanerogamic plants inhabiting the islands, no less than 108 are trees,
belonging to thirty-five families. Of these 108 trees, fifty-two, or
very nearly half, have their sexes more or less separated. Of bushes
there are 149, of which sixty-one have their sexes in the same state;
whilst of the remaining 500 herbaceous plants only 121, or less than a
fourth, have their sexes separated. Lastly, Professor Asa Gray informs
me that in the United States there are 132 native trees (belonging to
twenty-five families) of which ninety-five (belonging to seventeen
families) "have their sexes more or less separated, for the greater part
decidedly separated.") It is, however, doubtful how far this rule holds
good generally, and it certainly does not do so in Australia. But I have
been assured that the flowers of the prevailing Australian trees,
namely, the Myrtaceae, swarm with insects, and if they are dichogamous
they would be practically diclinous. (10/61. With respect to the
Proteaceae of Australia, Mr. Bentham 'Journal of the Linnean Society
Botany' volume 13 1871 pages 58, 64, remarks on the various contrivances
by which the stigma in the several genera is screened from the action of
the pollen from the same flower. For instance, in Synaphea "the stigma
is held by the eunuch (i.e., one of the stamens which is barren) safe
from all pollution from her brother anthers, and is preserved intact for
any pollen that may be inserted by insects and other agencies.") As far
as anemophilous plants are concerned, we know that they are apt to have
their sexes separated, and we can see that it would be an unfavourable
circumstance for them to bear their flowers very close to the ground, as
their pollen is liable to be blown high up in the air (10/62. Kerner
'Schutzmittel des Pollens' 1873 page 4.); but as the culms of grasses
give sufficient elevation, we cannot thus account for so many trees and
bushes being diclinous. We may infer from our previous discussion that a
tree bearing numerous hermaphrodite flowers would rarely intercross with
another tree, except by means of the pollen of a distinct individual
being prepotent over the plants' own pollen. Now the separation of the
sexes, whether the plant were anemophilous are entomophilous, would most
effectually bar self-fertilisation, and this may be the cause of so many
trees and bushes being diclinous. Or to put the case in another way, a
plant would be better fitted for development into a tree, if the sexes
were separated, than if it were hermaphrodite; for in the former case
its numerous flowers would be less liable to continued
self-fertilisation. But it should also be observed that the long life of
a tree or bush permits of the separation of the sexes, with much less
risk of evil from impregnation occasionally failing and seeds not being
produced, than in the case of short-lived plants. Hence it probably is,
as Lecoq has remarked, that annual plants are rarely dioecious.

Finally, we have seen reason to believe that the higher plants are
descended from extremely low forms which conjugated, and that the
conjugating individuals differed somewhat from one another,--the one
representing the male and the other the female--so that plants were
aboriginally dioecious. At a very early period such lowly organised
dioecious plants probably gave rise by budding to monoecious plants with
the two sexes borne by the same individual; and by a still closer union
of the sexes to hermaphrodite plants, which are now much the commonest
form. (10/63. There is a considerable amount of evidence that all the
higher animals are the descendants of hermaphrodites; and it is a
curious problem whether such hermaphroditism may not have been the
result of the conjugation of two slightly different individuals, which
represented the two incipient sexes. On this view, the higher animals
may now owe their bilateral structure, with all their organs double at
an early embryonic period, to the fusion or conjugation of two
primordial individuals.) As soon as plants became affixed to the ground,
their pollen must have been carried by some means from flower to flower,
at first almost certainly by the wind, then by pollen-devouring, and
afterwards by nectar-seeking insects. During subsequent ages some few
entomophilous plants have been again rendered anemophilous, and some
hermaphrodite plants have had their sexes again separated; and we can
vaguely see the advantages of such recurrent changes under certain

Dioecious plants, however fertilised, have a great advantage over other
plants in their cross-fertilisation being assured. But this advantage is
gained in the case of anemophilous species at the expense of the
production of an enormous superfluity of pollen, with some risk to them
and to entomophilous species of their fertilisation occasionally
failing. Half the individuals, moreover, namely, the males, produce no
seed, and this might possibly be a disadvantage. Delpino remarks that
dioecious plants cannot spread so easily as monoecious and hermaphrodite
species, for a single individual which happened to reach some new site
could not propagate its kind; but it may be doubted whether this is a
serious evil. Monoecious plants can hardly fail to be to a large extent
dioecious in function, owing to the lightness of their pollen and to the
wind blowing laterally, with the great additional advantage of
occasionally or often producing some self-fertilised seeds. When they
are also dichogamous, they are necessarily dioecious in function.
Lastly, hermaphrodite plants can generally produce at least some
self-fertilised seeds, and they are at the same time capable, through
the various means specified in this chapter, of cross-fertilisation.
When their structure absolutely prevents self-fertilisation, they are in
the same relative position to one another as monoecious and dioecious
plants, with what may be an advantage, namely, that every flower is
capable of yielding seeds.



Insects visit the flowers of the same species as long as they can.
Cause of this habit.
Means by which bees recognise the flowers of the same species.
Sudden secretion of nectar.
Nectar of certain flowers unattractive to certain insects.
Industry of bees, and the number of flowers visited within a short time.
Perforation of the corolla by bees.
Skill shown in the operation.
Hive-bees profit by the holes made by humble-bees.
Effects of habit.
The motive for perforating flowers to save time.
Flowers growing in crowded masses chiefly perforated.

Bees and various other insects must be directed by instinct to search
flowers for nectar and pollen, as they act in this manner without
instruction as soon as they emerge from the pupa state. Their instincts,
however, are not of a specialised nature, for they visit many exotic
flowers as readily as the endemic kinds, and they often search for
nectar in flowers which do not secrete any; and they may be seen
attempting to suck it out of nectaries of such length that it cannot be
reached by them. (11/1. See, on this subject Hermann Muller
'Befruchtung' etc. page 427; and Sir J. Lubbock's 'British Wild Flowers'
etc. page 20. Muller 'Bienen Zeitung' June 1876 page 119, assigns good
reasons for his belief that bees and many other Hymenoptera have
inherited from some early nectar-sucking progenitor greater skill in
robbing flowers than that which is displayed by insects belonging to the
other Orders.) All kinds of bees and certain other insects usually visit
the flowers of the same species as long as they can, before going to
another species. This fact was observed by Aristotle with respect to the
hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper
published in 1736 in the Philosophical Transactions. It may be observed
by any one, both with hive and humble-bees, in every flower-garden; not
that the habit is invariably followed. Mr. Bennett watched for several
hours many plants of Lamium album, L. purpureum, and another Labiate
plant, Nepeta glechoma, all growing mingled together on a bank near some
hives; and he found that each bee confined its visits to the same
species. (11/2. 'Nature' 1874 June 4 page 92.) The pollen of these three
plants differs in colour, so that he was able to test his observations
by examining that which adhered to the bodies of the captured bees, and
he found one kind on each bee.

Humble and hive-bees are good botanists, for they know that varieties
may differ widely in the colour of their flowers and yet belong to the
same species. I have repeatedly seen humble-bees flying straight from a
plant of the ordinary red Dictamnus fraxinella to a white variety; from
one to another very differently coloured variety of Delphinium consolida
and of Primula veris; from a dark purple to a bright yellow variety of
Viola tricolor; and with two species of Papaver, from one variety to
another which differed much in colour; but in this latter case some of
the bees flew indifferently to either species, although passing by other
genera, and thus acted as if the two species were merely varieties.
Hermann Muller also has seen hive-bees flying from flower to flower of
Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and
repens; and even from blue hyacinths to blue violets. (11/3. 'Bienen
Zeitung' July 1876 page 183.)

Some species of Diptera or flies keep to the flowers of the same species
with almost as much regularity as do bees; and when captured they are
found covered with pollen. I have seen Rhingia rostrata acting in this
manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici
sepium. Volucella plumosa and Empis cheiroptera flew straight from
flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved
in the same manner with Potentilla tormentilla; and other Diptera with
Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica
hederaefolia and chamoedrys; but some flies visited indifferently the
flowers of these two latter species. I have seen more than once a minute
Thrips, with pollen adhering to its body, fly from one flower to another
of the same kind; and one was observed by me crawling about within a
convolvulus with four grains of pollen adhering to its head, which were
deposited on the stigma.

Fabricius and Sprengel state that when flies have once entered the
flowers of Aristolochia they never escape,--a statement which I could
not believe, as in this case the insects would not aid in the
cross-fertilisation of the plant; and this statement has now been shown
by Hildebrand to be erroneous. As the spathes of Arum maculatum are
furnished with filaments apparently adapted to prevent the exit of
insects, they resemble in this respect the flowers of Aristolochia; and
on examining several spathes, from thirty to sixty minute Diptera
belonging to three species were found in some of them; and many of these
insects were lying dead at the bottom, as if they had been permanently
entrapped. In order to discover whether the living ones could escape and
carry pollen to another plant, I tied in the spring of 1842 a fine
muslin bag tightly round a spathe; and on returning in an hour's time
several little flies were crawling about on the inner surface of the
bag. I then gathered a spathe and breathed hard into it; several flies
soon crawled out, and all without exception were dusted with arum
pollen. These flies quickly flew away, and I distinctly saw three of
them fly to another plant about a yard off; they alighted on the inner
or concave surface of the spathe, and suddenly flew down into the
flower. I then opened this flower, and although not a single anther had
burst, several grains of pollen were lying at the bottom, which must
have been brought from another plant by one of these flies or by some
other insect. In another flower little flies were crawling about, and I
saw them leave pollen on the stigmas.

I do not know whether Lepidoptera generally keep to the flowers of the
same species; but I once observed many minute moths (I believe Lampronia
(Tinea) calthella) apparently eating the pollen of Mercurialis annua,
and they had the whole front of their bodies covered with pollen. I then
went to a female plant some yards off, and saw in the course of fifteen
minutes three of these moths alight on the stigmas. Lepidoptera are
probably often induced to frequent the flowers of the same species,
whenever these are provided with a long and narrow nectary, as in this
case other insects cannot suck the nectar, which will thus be preserved
for those having an elongated proboscis. No doubt the Yucca moth visits
only the flowers whence its name is derived, for a most wonderful
instinct guides this moth to place pollen on the stigma, so that the
ovules may be developed on which the larvae feed. (11/4. Described by
Mr. Riley in the 'American Naturalist' volume 7 October 1873.)With
respect to Coleoptera, I have seen Meligethes covered with pollen flying
from flower to flower of the same species; and this must often occur,
as, according to M. Brisout, 'many of the species affect only one kind
of plant." (11/5. As quoted in 'American Nat.' May 1873 page 270.)

It must not be supposed from these several statements that insects
strictly confine their visits to the same species. They often visit
other species when only a few plants of the same kind grow near
together. In a flower-garden containing some plants of Oenothera, the
pollen of which can easily be recognised, I found not only single grains
but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum,
and Linaria. Other kinds of pollen were likewise detected in these same
flowers. A large number of the stigmas of a plant of Thyme, in which the
anthers were completely aborted, were examined; and these stigmas,
though scarcely larger than a split needle, were covered not only with
pollen of Thyme brought from other plants by the bees, but with several
other kinds of pollen.

That insects should visit the flowers of the same species as long as
they can, is of great importance to the plant, as it favours the
cross-fertilisation of distinct individuals of the same species; but no
one will suppose that insects act in this manner for the good of the
plant. The cause probably lies in insects being thus enabled to work
quicker; they have just learnt how to stand in the best position on the
flower, and how far and in what direction to insert their proboscides.
(11/6. Since these remarks were written, I find that Hermann Muller has
come to almost exactly the same conclusion with respect to the cause of
insects frequenting as long as they can the flowers of the same species:
'Bienen Zeitung' July 1876 page 182.) They act on the same principle as
does an artificer who has to make half-a-dozen engines, and who saves
time by making consecutively each wheel and part for all of them.
Insects, or at least bees, seem much influenced by habit in all their
manifold operations; and we shall presently see that this holds good in
their felonious practice of biting holes through the corolla.

It is a curious question how bees recognise the flowers of the same
species. That the coloured corolla is the chief guide cannot be doubted.
On a fine day, when hive-bees were incessantly visiting the little blue
flowers of Lobelia erinus, I cut off all the petals of some, and only
the lower striped petals of others, and these flowers were not once
again sucked by the bees, although some actually crawled over them. The
removal of the two little upper petals alone made no difference in their
visits. Mr. J. Anderson likewise states that when he removed the
corollas of the Calceolaria, bees never visited the flowers. (11/7.
'Gardeners' Chronicle' 1853 page 534. Kurr cut off the nectaries from a
large number of flowers of several species, and found that the greater
number yielded seeds; but insects probably would not perceive the loss
of the nectary until they had inserted their proboscides into the holes
thus formed, and in doing so would fertilise the flowers. He also
removed the whole corolla from a considerable number of flowers, and
these likewise yielded seeds. Flowers which are self-fertile would
naturally produce seeds under these circumstances; but I am greatly
surprised that Delphinium consolida, as well as another species of
Delphinium, and Viola tricolor, should have produced a fair supply of
seeds when thus treated; but it does not appear that he compared the
number of the seeds thus produced with those yielded by unmutilated
flowers left to the free access of insects: 'Bedeutung der Nektarien'
1833 pages 123-135.) On the other hand, in some large masses of Geranium
phaeum which had escaped out of a garden, I observed the unusual fact of
the flowers continuing to secrete an abundance of nectar after all the
petals had fallen off; and the flowers in this state were still visited
by humble-bees. But the bees might have learnt that these flowers with
all their petals lost were still worth visiting, by finding nectar in
those with only one or two lost. The colour alone of the corolla serves
as an approximate guide: thus I watched for some time humble-bees which
were visiting exclusively plants of the white-flowered Spiranthes
autumnalis, growing on short turf at a considerable distance apart; and
these bees often flew within a few inches of several other plants with
white flowers, and then without further examination passed onwards in
search of the Spiranthes. Again, many hive-bees which confined their
visits to the common ling (Calluna vulgaris), repeatedly flew towards
Erica tetralix, evidently attracted by the nearly similar tint of their
flowers, and then instantly passed on in search of the Calluna.

That the colour of the flower is not the sole guide, is clearly shown by
the six cases above given of bees which repeatedly passed in a direct
line from one variety to another of the same species, although they bore
very differently coloured flowers. I observed also bees flying in a
straight line from one clump of a yellow-flowered Oenothera to every
other clump of the same plant in the garden, without turning an inch
from their course to plants of Eschscholtzia and others with yellow
flowers which lay only a foot or two on either side. In these cases the
bees knew the position of each plant in the garden perfectly well, as we
may infer by the directness of their flight; so that they were guided by
experience and memory. But how did they discover at first that the above
varieties with differently coloured flowers belonged to the same
species? Improbable as it may appear, they seem, at least sometimes, to
recognise plants even from a distance by their general aspect, in the
same manner as we should do. On three occasions I observed humble-bees
flying in a perfectly straight line from a tall larkspur (Delphinium)
which was in full flower to another plant of the same species at the
distance of fifteen yards which had not as yet a single flower open, and
on which the buds showed only a faint tinge of blue. Here neither odour
nor the memory of former visits could have come into play, and the tinge
of blue was so faint that it could hardly have served as a guide. (11/8.
A fact mentioned by Hermann Muller 'Die Befruchtung' etc. page 347,
shows that bees possess acute powers of vision and discrimination; for
those engaged in collecting pollen from Primula elatior invariably
passed by the flowers of the long-styled form, in which the anthers are
seated low down in the tubular corolla. Yet the difference in aspect
between the long-styled and short-styled forms is extremely slight.)

The conspicuousness of the corolla does not suffice to induce repeated
visits from insects, unless nectar is at the same time secreted,
together perhaps with some odour emitted. I watched for a fortnight many
times daily a wall covered with Linaria cymbalaria in full flower, and
never saw a bee even looking at one. There was then a very hot day, and
suddenly many bees were industriously at work on the flowers. It appears
that a certain degree of heat is necessary for the secretion of nectar;
for I observed with Lobelia erinus that if the sun ceased to shine for
only half an hour, the visits of the bees slackened and soon ceased. An
analogous fact with respect to the sweet excretion from the stipules of
Vicia sativa has been already given. As in the case of the Linaria, so
with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some
species of Trifolium, I have watched the flowers day after day without
seeing a bee at work, and then suddenly all the flowers were visited by
many bees. Now how did so many bees discover at once that the flowers
were secreting nectar? I presume that it must have been by their odour;
and that as soon as a few bees began to suck the flowers, others of the
same and of different kinds observed the fact and profited by it. We
shall presently see, when we treat of the perforation of the corolla,
that bees are fully capable of profiting by the labour of other species.
Memory also comes into play, for, as already remarked, bees know the
position of each clump of flowers in a garden. I have repeatedly seen
them passing round a corner, but otherwise in as straight a line as
possible, from one plant of Fraxinella and of Linaria to another and
distant one of the same species; although, owing to the intervention of
other plants, the two were not in sight of each other.

It would appear that either the taste or the odour of the nectar of
certain flowers is unattractive to hive or to humble-bees, or to both;
for there seems no other reason why certain open flowers which secrete
nectar are not visited by them. The small quantity of nectar secreted by
some of these flowers can hardly be the cause of their neglect, as
hive-bees search eagerly for the minute drops on the glands on the
leaves of the Prunus laurocerasus. Even the bees from different hives
sometimes visit different kinds of flowers, as is said to be the case by
Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9.
'Gardeners' Chronicle' 1844 page 374.) I have known humble-bees to visit
the flowers of Lobelia fulgens in one garden and not in another at the
distance of only a few miles. The cupful of nectar in the labellum of
Epipactis latifolia is never touched by hive- or humble-bees, although I
have seen them flying close by; and yet the nectar has a pleasant taste
to us, and is habitually consumed by the common wasp. As far as I have
seen, wasps seek for nectar in this country only from the flowers of
this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10.
The same fact apparently holds good in Italy, for Delpino says that the
flowers of these three plants are alone visited by wasps: 'Nettarii
Estranuziali, Bulletino Entomologico' anno 6.), and Tritoma; the two
former plants being endemic, and the two latter exotic. As wasps are so
fond of sugar and of any sweet fluid, and as they do not disdain the
minute drops on the glands of Prunus laurocerasus, it is a strange fact
that they do not suck the nectar of many open flowers, which they could
do without the aid of a proboscis. Hive-bees visit the flowers of the
Symphoricarpus and Tritoma, and this makes it all the stranger that they
do not visit the flowers of the Epipactis, or, as far as I have seen,
those of the Scrophularia aquatica; although they do visit the flowers
of Scrophularia nodosa, at least in North America. (11/11. 'Silliman's
American Journal of Science' August 1871.)

The extraordinary industry of bees and the number of flowers which they
visit within a short time, so that each flower is visited repeatedly,
must greatly increase the chance of each receiving pollen from a
distinct plant. When the nectar is in any way hidden, bees cannot tell
without inserting their proboscides whether it has lately been exhausted
by other bees, and this, as remarked in a former chapter, forces them to
visit many more flowers than they otherwise would. But they endeavour to
lose as little time as they can; thus in flowers having several
nectaries, if they find one dry they do not try the others, but as I
have often observed, pass on to another flower. They work so
industriously and effectually, that even in the case of social plants,
of which hundreds of thousands grow together, as with the several kinds
of heath, every single flower is visited, of which evidence will
presently be given. They lose no time and fly very quickly from plant to
plant, but I do not know the rate at which hive-bees fly. Humble-bees
fly at the rate of ten miles an hour, as I was able to ascertain in the
case of the males from their curious habit of calling at certain fixed
points, which made it easy to measure the time taken in passing from one
place to another.

With respect to the number of flowers which bees visit in a given time,
I observed that in exactly one minute a humble-bee visited twenty-four
of the closed flowers of the Linaria cymbalaria; another bee visited in
the same time twenty-two flowers of the Symphoricarpus racemosa; and
another seventeen flowers on two plants of a Delphinium. In the course
of fifteen minutes a single flower on the summit of a plant of Oenothera
was visited eight times by several humble-bees, and I followed the last
of these bees, whilst it visited in the course of a few additional
minutes every plant of the same species in a large flower-garden. In
nineteen minutes every flower on a small plant of Nemophila insignis was
visited twice. In one minute six flowers of a Campanula were entered by
a pollen-collecting hive-bee; and bees when thus employed work slower
than when sucking nectar. Lastly, seven flower-stalks on a plant of
Dictamnus fraxinella were observed on the 15th of June 1841 during ten
minutes; they were visited by thirteen humble-bees each of which entered
many flowers. On the 22nd the same flower-stalks were visited within the
same time by eleven humble-bees. This plant bore altogether 280 flowers,
and from the above data, taking into consideration how late in the
evening humble-bees work, each flower must have been visited at least
thirty times daily, and the same flower keeps open during several days.
The frequency of the visits of bees is also sometimes shown by the
manner in which the petals are scratched by their hooked tarsi; I have
seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of
their flowers thus sadly defaced.


I have already alluded to bees biting holes in flowers for the sake of
obtaining the nectar. They often act in this manner, both with endemic
and exotic species, in many parts of Europe, in the United States, and
in the Himalaya; and therefore probably in all parts of the world. The
plants, the fertilisation of which actually depends on insects entering
the flowers, will fail to produce seed when their nectar is stolen from
the outside; and even with those species which are capable of
fertilising themselves without any aid, there can be no
cross-fertilisation, and this, as we know, is a serious evil in most
cases. The extent to which humble-bees carry on the practice of biting
holes is surprising: a remarkable case was observed by me near
Bournemouth, where there were formerly extensive heaths. I took a long
walk, and every now and then gathered a twig of Erica tetralix, and when
I had got a handful all the flowers were examined through a lens. This
process was repeated many times; but though many hundreds were examined,
I did not succeed in finding a single flower which had not been
perforated. Humble-bees were at the time sucking the flowers through
these perforations. On the following day a large number of flowers were
examined on another heath with the same result, but here hive-bees were
sucking through the holes. This case is all the more remarkable, as the
innumerable holes had been made within a fortnight, for before that time
I saw the bees everywhere sucking in the proper manner at the mouths of
the corolla. In an extensive flower-garden some large beds of Salvia
grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower
perforated, and many scores were examined. I have seen whole fields of
red clover (Trifolium pratense) in the same state. Dr. Ogle found that
90 per cent of the flowers of Salvia glutinosa had been bitten. In the
United States Mr. Bailey says it is difficult to find a blossom of the
native Gerardia pedicularia without a hole in it; and Mr. Gentry, in
speaking of the introduced Wistaria sinensis, says "that nearly every
flower had been perforated." (11/12. Dr. Ogle 'Pop. Science Review' July
1869 page 267. Bailey 'American Naturalist' November 1873 page 690.
Gentry ibid May 1875 page 264.)

As far as I have seen, it is always humble-bees which first bite the
holes, and they are well fitted for the work by possessing powerful
mandibles; but hive-bees afterwards profit by the holes thus made. Dr.
Hermann Muller, however, writes to me that hive-bees sometimes bite
holes through the flowers of Erica tetralix. No insects except bees,
with the single exception of wasps in the case of Tritoma, have sense
enough, as far as I have observed, to profit by the holes already made.
Even humble-bees do not always discover that it would be advantageous to
them to perforate certain flowers. There is an abundant supply of nectar
in the nectary of Tropaeolum tricolor, yet I have found this plant
untouched in more than one garden, while the flowers of other plants had
been extensively perforated; but a few years ago Sir J. Lubbock's
gardener assured me that he had seen humble-bees boring through the
nectary of this Tropaeolum. Muller has observed humble-bees trying to
suck at the mouths of the flowers of Primula elatior and of an
Aquilegia, and, failing in their attempts, they made holes through the
corolla; but they often bite holes, although they could with very little
more trouble obtain the nectar in a legitimate manner by the mouth of
the corolla.

Dr. W. Ogle has communicated to me a curious case. He gathered in
Switzerland 100 flower-stems of the common blue variety of the monkshood
(Aconitum napellus), and not a single flower was perforated; he then
gathered 100 stems of a white variety growing close by, and every one of
the open flowers had been perforated. (11/13. Dr. Ogle 'Popular Science
Review' July 1869 page 267. Bailey 'American Naturalist' November 1873
page 690. Gentry ibid May 1875 page 264.) This surprising difference in
the state of the flowers may be attributed with much probability to the
blue variety being distasteful to bees, from the presence of the acrid
matter which is so general in the Ranunculaceae, and to its absence in
the white variety in correlation with the loss of the blue tint.
According to Sprengel, this plant is strongly proterandrous (11/14. 'Das
Entdeckte' etc. page 278.); it would therefore be more or less sterile
unless bees carried pollen from the younger to the older flowers.
Consequently the white variety, the flowers of which were always bitten
instead of being properly entered by the bees, would fail to yield the
full number of seeds and would be a comparatively rare plant, as Dr.
Ogle informs me was the case.

Bees show much skill in their manner of working, for they always make
their holes from the outside close to the spot where the nectar lies
hidden within the corolla. All the flowers in a large bed of Stachys
coccinea had either one or two slits made on the upper side of the
corolla near the base. The flowers of a Mirabilis and of Salvia coccinea
were perforated in the same manner; whilst those of Salvia grahami, in
which the calyx is much elongated, had both the calyx and the corolla
invariably perforated. The flowers of Pentstemon argutus are broader
than those of the plants just named, and two holes alongside each other
had here always been made just above the calyx. In these several cases
the perforations were on the upper side, but in Antirrhinum majus one or
two holes had been made on the lower side, close to the little
protuberance which represents the nectary, and therefore directly in
front of and close to the spot where the nectar is secreted.

But the most remarkable case of skill and judgment known to me, is that
of the perforation of the flowers of Lathyrus sylvestris, as described
by my son Francis. (11/15. 'Nature' January 8, 1874 page 189.) The
nectar in this plant is enclosed within a tube, formed by the united
stamens, which surround the pistil so closely that a bee is forced to
insert its proboscis outside the tube; but two natural rounded passages
or orifices are left in the tube near the base, in order that the nectar
may be reached by the bees. Now my son found in sixteen out of
twenty-four flowers on this plant, and in eleven out of sixteen of those
on the cultivated everlasting pea, which is either a variety of the same
species or a closely allied one, that the left passage was larger than
the right one. And here comes the remarkable point,--the humble-bees
bite holes through the standard-petal, and they always operated on the
left side over the passage, which is generally the larger of the two. My
son remarks: "It is difficult to say how the bees could have acquired
this habit. Whether they discovered the inequality in the size of the
nectar-holes in sucking the flowers in the proper way, and then utilised
this knowledge in determining where to gnaw the hole; or whether they
found out the best situation by biting through the standard at various
points, and afterwards remembered its situation in visiting other
flowers. But in either case they show a remarkable power of making use
of what they have learnt by experience." It seems probable that bees owe
their skill in biting holes through flowers of all kinds to their having
long practised the instinct of moulding cells and pots of wax, or of
enlarging their old cocoons with tubes of wax; for they are thus
compelled to work on the inside and outside of the same object.

In the early part of the summer of 1857 I was led to observe during some
weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus),
whilst attending to the fertilisation of this plant, and daily saw
humble- and hive-bees sucking at the mouths of the flowers. But one day
I found several humble-bees employed in cutting holes in flower after
flower; and on the next day every single hive-bee, without exception,
instead of alighting on the left wing-petal and sucking the flower in
the proper manner, flew straight without the least hesitation to the
calyx, and sucked through the holes which had been made only the day
before by the humble-bees; and they continued this habit for many
following days. (11/16. 'Gardeners' Chronicle' 1857 page 725.) Mr. Belt
has communicated to me (July 28th, 1874) a similar case, with the sole
difference that less than half of the flowers had been perforated by the
humble-bees; nevertheless, all the hive-bees gave up sucking at the
mouths of the flowers and visited exclusively the bitten ones. Now how
did the hive-bees find out so quickly that holes had been made? Instinct
seems to be out of the question, as the plant is an exotic. The holes
cannot be seen by bees whilst standing on the wing-petals, where they
had always previously alighted. From the ease with which bees were
deceived when the petals of Lobelia erinus were cut off, it was clear
that in this case they were not guided to the nectar by its smell; and
it may be doubted whether they were attracted to the holes in the
flowers of the Phaseolus by the odour emitted from them. Did they
perceive the holes by the sense of touch in their proboscides, whilst
sucking the flowers in the proper manner, and then reason that it would
save them time to alight on the outside of the flowers and use the
holes? This seems almost too abstruse an act of reason for bees; and it
is more probable that they saw the humble-bees at work, and
understanding what they were about, imitated them and took advantage of
the shorter path to the nectar. Even with animals high in the scale,
such as monkeys, we should be surprised at hearing that all the
individuals of one species within the space of twenty-four hours
understood an act performed by a distinct species, and profited by it.

I have repeatedly observed with various kinds of flowers that all the
hive and humble-bees which were sucking through the perforations, flew
to them, whether on the upper or under side of the corolla, without the
least hesitation; and this shows how quickly all the individuals within
the district had acquired the same knowledge. Yet habit comes into play
to a certain extent, as in so many of the other operations of bees. Dr.
Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus
multiflorus that certain individuals went exclusively to the
perforations, while others of the same species visited only the mouths
of the flowers. (11/17. Dr. Ogle 'Pop. Science Review' April 1870 page
167. Mr. Farrer 'Annals and Magazine of Natural History' 4th series
volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861
exactly the same fact with Trifolium pratense. So persistent is the
force of habit, that when a bee which is visiting perforated flowers
comes to one which has not been bitten, it does not go to the mouth, but
instantly flies away in search of another bitten flower. Nevertheless, I
once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it
entered the mouths of some flowers and cut holes into the others. Dr.
Hermann Muller informs me that in the same district he has seen some
individuals of Bombus mastrucatus boring through the calyx and corolla
of Rhinanthus alecterolophus, and others through the corolla alone.
Different species of bees may, however, sometimes be observed acting
differently at the same time on the same plant. I have seen hive-bees
sucking at the mouths of the flowers of the common bean; humble-bees of
one kind sucking through holes bitten in the calyx, and humble-bees of
another kind sucking the little drops of fluid excreted by the stipules.
Mr. Beal of Michigan informs me that the flowers of the Missouri currant
(Ribes aureum) abound with nectar, so that children often suck them; and
he saw hive-bees sucking through holes made by a bird, the oriole, and
at the same time humble-bees sucking in the proper manner at the mouths
of the flowers. (11/18. The flowers of the Ribes are however sometimes
perforated by humble-bees, and Mr. Bundy says that they were able to
bite through and rob seven flowers of their honey in a minute: 'American
Naturalist' 1876 page 238.) This statement about the oriole calls to
mind what I have before said of certain species of humming-birds boring
holes through the flowers of the Brugmansia, whilst other species
entered by the mouth.

The motive which impels bees to gnaw holes through the corolla seems to
be the saving of time, for they lose much time in climbing into and out
of large flowers, and in forcing their heads into closed ones. They were
able to visit nearly twice as many flowers, as far as I could judge, of
a Stachys and Pentstemon by alighting on the upper surface of the
corolla and sucking through the cut holes, than by entering in the
proper way. Nevertheless each bee before it has had much practice, must
lose some time in making each new perforation, especially when the
perforation has to be made through both calyx and corolla. This action
therefore implies foresight, of which faculty we have abundant evidence
in their building operations; and may we not further believe that some
trace of their social instinct, that is, of working for the good of
other members of the community, may here likewise play a part?

Many years ago I was struck with the fact that humble-bees as a general
rule perforate flowers only when these grow in large numbers near
together. In a garden where there were some very large beds of Stachys
coccinea and of Pentstemon argutus, every single flower was perforated,
but I found two plants of the former species growing quite separate with
their petals much scratched, showing that they had been frequently
visited by bees, and yet not a single flower was perforated. I found
also a separate plant of the Pentstemon, and saw bees entering the mouth
of the corolla, and not a single flower had been perforated. In the
following year (1842) I visited the same garden several times: on the
19th of July humble-bees were sucking the flowers of Stachys coccinea
and Salvia grahami in the proper manner, and none of the corollas were
perforated. On the 7th of August all the flowers were perforated, even
those on some few plants of the Salvia which grew at a little distance
from the great bed. On the 21st of August only a few flowers on the
summits of the spikes of both species remained fresh, and not one of
these was now bored. Again, in my own garden every plant in several rows
of the common bean had many flowers perforated; but I found three plants
in separate parts of the garden which had sprung up accidentally, and
these had not a single flower perforated. General Strachey formerly saw
many perforated flowers in a garden in the Himalaya, and he wrote to the
owner to inquire whether this relation between the plants growing
crowded and their perforation by the bees there held good, and was
answered in the affirmative. Hence it follows that the red clover
(Trifolium pratense) and the common bean when cultivated in great masses
in fields,--that Erica tetralix growing in large numbers on
heaths,--rows of the scarlet kidney-bean in the kitchen-garden,--and
masses of any species in the flower-garden,--are all eminently liable to
be perforated.

The explanation of this fact is not difficult. Flowers growing in large
numbers afford a rich booty to the bees, and are conspicuous from a
distance. They are consequently visited by crowds of these insects, and
I once counted between twenty and thirty bees flying about a bed of
Pentstemon. They are thus stimulated to work quickly by rivalry, and,
what is much more important, they find a large proportion of the
flowers, as suggested by my son, with their nectaries sucked dry.
(11/19. 'Nature' January 8, 1874 page 189.) They thus waste much time in
searching many empty flowers, and are led to bite the holes, so as to
find out as quickly as possible whether there is any nectar present, and
if so, to obtain it.

Flowers which are partially or wholly sterile unless visited by insects
in the proper manner, such as those of most species of Salvia, of
Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less
completely to produce seeds if the bees confine their visits to the
perforations. The perforated flowers of those species, which are capable
of fertilising themselves, will yield only self-fertilised seeds, and
the seedlings will in consequence be less vigorous. Therefore all plants
must suffer in some degree when bees obtain their nectar in a felonious
manner by biting holes through the corolla; and many species, it might
be thought, would thus be exterminated. But here, as is so general
throughout nature, there is a tendency towards a restored equilibrium.
If a plant suffers from being perforated, fewer individuals will be
reared, and if its nectar is highly important to the bees, these in
their turn will suffer and decrease in number; but, what is much more
effective, as soon as the plant becomes somewhat rare so as not to grow
in crowded masses, the bees will no longer be stimulated to gnaw holes
in the flowers, but will enter them in a legitimate manner. More seed
will then be produced, and the seedlings being the product of
cross-fertilisation will be vigorous, so that the species will tend to
increase in number, to be again checked, as soon as the plant again
grows in crowded masses.



Cross-fertilisation proved to be beneficial, and self-fertilisation
Allied species differ greatly in the means by which cross-fertilisation
is favoured and self-fertilisation avoided.
The benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.
The evil effects not due to the combination of morbid tendencies in the
Nature of the conditions to which plants are subjected when growing near
together in a state of nature or under culture, and the effects of such
Theoretical considerations with respect to the interaction of
differentiated sexual elements.
Practical lessons.
Genesis of the two sexes.
Close correspondence between the effects of cross-fertilisation and
self-fertilisation, and of the legitimate and illegitimate unions of
heterostyled plants, in comparison with hybrid unions.

The first and most important of the conclusions which may be drawn from
the observations given in this volume, is that cross-fertilisation is
generally beneficial, and self-fertilisation injurious. This is shown by
the difference in height, weight, constitutional vigour, and fertility
of the offspring from crossed and self-fertilised flowers, and in the
number of seeds produced by the parent-plants. With respect to the
second of these two propositions, namely, that self-fertilisation is
generally injurious, we have abundant evidence. The structure of the
flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc.,
renders the aid of insects almost indispensable for their fertilisation;
and bearing in mind the prepotency of pollen from a distinct individual
over that from the same individual, such plants will almost certainly
have been crossed during many or all previous generations. So it must
be, owing merely to the prepotency of foreign pollen, with cabbages and
various other plants, the varieties of which almost invariably
intercross when grown together. The same inference may be drawn still
more surely with respect to those plants, such as Reseda and
Eschscholtzia, which are sterile with their own pollen, but fertile with
that from any other individual. These several plants must therefore have
been crossed during a long series of previous generations, and the
artificial crosses in my experiments cannot have increased the vigour of
the offspring beyond that of their progenitors. Therefore the difference
between the self-fertilised and crossed plants raised by me cannot be
attributed to the superiority of the crossed, but to the inferiority of
the self-fertilised seedlings, due to the injurious effects of

With respect to the first proposition, namely, that cross-fertilisation
is generally beneficial, we likewise have excellent evidence. Plants of
Ipomoea were intercrossed for nine successive generations; they were
then again intercrossed, and at the same time crossed with a plant of a
fresh stock, that is, one brought from another garden; and the offspring
of this latter cross were to the intercrossed plants in height as 100 to
78, and in fertility as 100 to 51. An analogous experiment with
Eschscholtzia gave a similar result, as far as fertility was concerned.
In neither of these cases were any of the plants the product of
self-fertilisation. Plants of Dianthus were self-fertilised for three
generations, and this no doubt was injurious; but when these plants were
fertilised by a fresh stock and by intercrossed plants of the same
stock, there was a great difference in fertility between the two sets of
seedlings, and some difference in their height. Petunia offers a nearly
parallel case. With various other plants, the wonderful effects of a
cross with a fresh stock may be seen in Table 7/C. Several accounts have
also been published of the extraordinary growth of seedlings from a
cross between two varieties of the same species, some of which are known
never to fertilise themselves; so that here neither self-fertilisation
nor relationship even in a remote degree can have come into play. (12/1.
See 'Variation under Domestication' chapter 19 2nd edition volume 2 page
159.) We may therefore conclude that the above two propositions are
true,--that cross-fertilisation is generally beneficial and
self-fertilisation injurious to the offspring.

That certain plants, for instance, Viola tricolor, Digitalis purpurea,
Sarothamnus scoparius, Cyclamen persicum, etc., which have been
naturally cross-fertilised for many or all previous generations, should
suffer to an extreme degree from a single act of self-fertilisation is a
most surprising fact. Nothing of the kkind has been observed in our
domestic animals; but then we must remember that the closest possible
interbreeding with such animals, that is, between brothers and sisters,
cannot be considered as nearly so close a union as that between the
pollen and ovules of the same flower. Whether the evil from
self-fertilisation goes on increasing during successive generations is
not as yet known; but we may infer from my experiments that the increase
if any is far from rapid. After plants have been propagated by
self-fertilisation for several generations, a single cross with a fresh
stock restores their pristine vigour; and we have a strictly analogous
result with our domestic animals. (12/2. Ibid chapter 19 2nd edition
volume 2 page 159.) The good effects of cross-fertilisation are
transmitted by plants to the next generation; and judging from the
varieties of the common pea, to many succeeding generations. But this
may merely be that crossed plants of the first generation are extremely
vigorous, and transmit their vigour, like any other character, to their

Notwithstanding the evil which many plants suffer from
self-fertilisation, they can be thus propagated under favourable
conditions for many generations, as shown by some of my experiments, and
more especially by the survival during at least half a century of the
same varieties of the common pea and sweet-pea. The same conclusion
probably holds good with several other exotic plants, which are never or
most rarely cross-fertilised in this country. But all these plants, as
far as they have been tried, profit greatly by a cross with a fresh
stock. Some few plants, for instance, Ophrys apifera, have almost
certainly been propagated in a state of nature for thousands of
generations without having been once intercrossed; and whether they
would profit by a cross with a fresh stock is not known. But such cases
ought not to make us doubt that as a general rule crossing is
beneficial, any more than the existence of plants which, in a state of
nature, are propagated exclusively by rhizomes, stolons, etc. (their
flowers never producing seeds), (12/3. I have given several cases in my
'Variation under Domestication' chapter 18 2nd edition volume 2 page
152.) (their flowers never producing seeds), should make us doubt that
seminal generation must have some great advantage, as it is the common
plan followed by nature. Whether any species has been reproduced
asexually from a very remote period cannot, of course, be ascertained.
Our sole means for forming any judgment on this head is the duration of
the varieties of our fruit trees which have been long propagated by
grafts or buds. Andrew Knight formerly maintained that under these
circumstances they always become weakly, but this conclusion has been
warmly disputed by others. A recent and competent judge, Professor Asa
Gray, leans to the side of Andrew Knight, which seems to me, from such
evidence as I have been able to collect, the more probable view,
notwithstanding many opposed facts. (12/4. 'Darwiniana: Essays and
Reviews pertaining to Darwinism' 1876 page 338.)

The means for favouring cross-fertilisation and preventing
self-fertilisation, or conversely for favouring self-fertilisation and
preventing to a certain extent cross-fertilisation, are wonderfully
diversified; and it is remarkable that these differ widely in closely
allied plants,--in the species of the same genus, and sometimes in the
individuals of the same species. (12/5. Hildebrand has insisted strongly
to this effect in his valuable observations on the fertilisation of the
Gramineae: 'Monatsbericht K. Akad. Berlin' October 1872 page 763.) It is
not rare to find hermaphrodite plants and others with separated sexes
within the same genus; and it is common to find some of the species
dichogamous and others maturing their sexual elements simultaneously.
The dichogamous genus Saxifraga contains proterandrous and proterogynous
species. (12/6. Dr. Engler 'Botanische Zeitung' 1868 page 833.) Several
genera include both heterostyled (dimorphic or trimorphic forms) and
homostyled species. Ophrys offers a remarkable instance of one species
having its structure manifestly adapted for self-fertilisation, and
other species as manifestly adapted for cross-fertilisation. Some
con-generic species are quite sterile and others quite fertile with
their own pollen. From these several causes we often find within the
same genus species which do not produce seeds, while others produce an
abundance, when insects are excluded. Some species bear cleistogene
flowers which cannot be crossed, as well as perfect flowers, whilst
others in the same genus never produce cleistogene flowers. Some species
exist under two forms, the one bearing conspicuous flowers adapted for
cross-fertilisation, the other bearing inconspicuous flowers adapted for
self-fertilisation, whilst other species in the same genus present only
a single form. Even with the individuals of the same species, the degree
of self-sterility varies greatly, as in Reseda. With polygamous plants,
the distribution of the sexes differs in the individuals of the same
species. The relative period at which the sexual elements in the same
flower are mature, differs in the varieties of Pelargonium; and Carriere
gives several cases, showing that the period varies according to the
temperature to which the plants are exposed. (12/7. 'Des Varieties' 1865
page 30.)

This extraordinary diversity in the means for favouring or preventing
cross- and self-fertilisation in closely allied forms, probably depends
on the results of both processes being highly beneficial to the species,
but directly opposed in many ways to one another and dependent on
variable conditions. Self-fertilisation assures the production of a
large supply of seeds; and the necessity or advantage of this will be
determined by the average length of life of the plant, which largely
depends on the amount of destruction suffered by the seeds and
seedlings. This destruction follows from the most various and variable
causes, such as the presence of animals of several kinds, and the growth
of surrounding plants. The possibility of cross-fertilisation depends
mainly on the presence and number of certain insects, often of insects
belonging to special groups, and on the degree to which they are
attracted to the flowers of any particular species in preference to
other flowers,--all circumstances likely to change. Moreover, the
advantages which follow from cross-fertilisation differ much in
different plants, so that it is probable that allied plants would often
profit in different degrees by cross-fertilisation. Under these
extremely complex and fluctuating conditions, with two somewhat opposed
ends to be gained, namely, the safe propagation of the species and the
production of cross-fertilised, vigorous offspring, it is not surprising
that allied forms should exhibit an extreme diversity in the means which
favour either end. If, as there is reason to suspect, self-fertilisation
is in some respects beneficial, although more than counterbalanced by
the advantages derived from a cross with a fresh stock, the problem
becomes still more complicated.

As I only twice experimented on more than a single species in a genus, I
cannot say whether the crossed offspring of the several species within
the same genus differ in their degree of superiority over their
self-fertilised brethren; but I should expect that this would often
prove to be the case from what was observed with the two species of
Lobelia and with the individuals of the same species of Nicotiana. The
species belonging to distinct genera in the same family certainly differ
in this respect. The effects of cross- and self-fertilisation may be
confined either to the growth or to the fertility of the offspring, but
generally extends to both qualities. There does not seem to exist any
close correspondence between the degree to which their offspring profit
by this process; but we may easily err on this head, as there are two
means for ensuring cross-fertilisation which are not externally
perceptible, namely, self-sterility and the prepotent fertilising
influence of pollen from another individual. Lastly, it has been shown
in a former chapter that the effect produced by cross and
self-fertilisation on the fertility of the parent-plants does not always
correspond with that produced on the height, vigour, and fertility of
their offspring. The same remark applies to crossed and self-fertilised
seedlings when these are used as the parent-plants. This want of
correspondence probably depends, at least in part, on the number of
seeds produced being chiefly determined by the number of the
pollen-tubes which reach the ovules, and this will be governed by the
reaction between the pollen and the stigmatic secretion or tissues;
whereas the growth and constitutional vigour of the offspring will be
chiefly determined, not only by the number of pollen-tubes reaching the
ovules, but by the nature of the reaction between the contents of the
pollen-grains and ovules.

There are two other important conclusions which may be deduced from my
observations: firstly, that the advantages of cross-fertilisation do not
follow from some mysterious virtue in the mere union of two distinct
individuals, but from such individuals having been subjected during
previous generations to different conditions, or to their having varied
in a manner commonly called spontaneous, so that in either case their
sexual elements have been in some degree differentiated. And secondly,
that the injury from self-fertilisation follows from the want of such
differentiation in the sexual elements. These two propositions are fully
established by my experiments. Thus, when plants of the Ipomoea and of
the Mimulus, which had been self-fertilised for the seven previous
generations and had been kept all the time under the same conditions,
were intercrossed one with another, the offspring did not profit in the
least by the cross. Mimulus offers another instructive case, showing
that the benefit of a cross depends on the previous treatment of the
progenitors: plants which had been self-fertilised for the eight
previous generations were crossed with plants which had been
intercrossed for the same number of generations, all having been kept
under the same conditions as far as possible; seedlings from this cross
were grown in competition with others derived from the same
self-fertilised mother-plant crossed by a fresh stock; and the latter
seedlings were to the former in height as 100 to 52, and in fertility as
100 to 4. An exactly parallel experiment was tried on Dianthus, with
this difference, that the plants had been self-fertilised only for the
three previous generations, and the result was similar though not so
strongly marked. The foregoing two cases of the offspring of Ipomoea and
Eschscholtzia, derived from a cross with a fresh stock, being as much
superior to the intercrossed plants of the old stock, as these latter
were to the self-fertilised offspring, strongly supports the same
conclusion. A cross with a fresh stock or with another variety seems to
be always highly beneficial, whether or not the mother-plants have been
intercrossed or self-fertilised for several previous generations. The
fact that a cross between two flowers on the same plant does no good or
very little good, is likewise a strong corroboration of our conclusion;
for the sexual elements in the flowers on the same plant can rarely have
been differentiated, though this is possible, as flower-buds are in one
sense distinct individuals, sometimes varying and differing from one
another in structure or constitution. Thus the proposition that the
benefit from cross-fertilisation depends on the plants which are crossed
having been subjected during previous generations to somewhat different
conditions, or to their having varied from some unknown cause as if they
had been thus subjected, is securely fortified on all sides.

Before proceeding any further, the view which has been maintained by
several physiologists must be noticed, namely, that all the evils from
breeding animals too closely, and no doubt, as they would say, from the
self-fertilisation of plants, is the result of the increase of some
morbid tendency or weakness of constitution common to the closely
related parents, or to the two sexes of hermaphrodite plants.
Undoubtedly injury has often thus resulted; but it is a vain attempt to
extend this view to the numerous cases given in my Tables. It should be
remembered that the same mother-plant was both self-fertilised and
crossed, so that if she had been unhealthy she would have transmitted
half her morbid tendencies to her crossed offspring. But plants
appearing perfectly healthy, some of them growing wild, or the immediate
offspring of wild plants, or vigorous common garden-plants, were
selected for experiment. Considering the number of species which were
tried, it is nothing less than absurd to suppose that in all these cases
the mother-plants, though not appearing in any way diseased, were weak
or unhealthy in so peculiar a manner that their self-fertilised
seedlings, many hundreds in number, were rendered inferior in height,
weight, constitutional vigour and fertility to their crossed offspring.
Moreover, this belief cannot be extended to the strongly marked
advantages which invariably follow, as far as my experience serves, from
intercrossing the individuals of the same variety or of distinct
varieties, if these have been subjected during some generations to
different conditions.

It is obvious that the exposure of two sets of plants during several
generations to different conditions can lead to no beneficial results,
as far as crossing is concerned, unless their sexual elements are thus
affected. That every organism is acted on to a certain extent by a
change in its environment, will not, I presume, be disputed. It is
hardly necessary to advance evidence on this head; we can perceive the
difference between individual plants of the same species which have
grown in somewhat more shady or sunny, dry or damp places. Plants which
have been propagated for some generations under different climates or at
different seasons of the year transmit different constitutions to their
seedlings. Under such circumstances, the chemical constitution of their
fluids and the nature of their tissues are often modified. (12/8.
Numerous cases together with references are given in my 'Variation under
Domestication' chapter 23 2nd edition volume 2 page 264. With respect to
animals, Mr. Brackenridge 'A Contribution to the Theory of Diathesis'
Edinburgh 1869, has well shown that the different organs of animals are
excited into different degrees of activity by differences of temperature
and food, and become to a certain extent adapted to them.) Many other
such facts could be adduced. In short, every alteration in the function
of a part is probably connected with some corresponding, though often
quite imperceptible change in structure or composition.

Whatever affects an organism in any way, likewise tends to act on its
sexual elements. We see this in the inheritance of newly acquired
modifications, such as those from the increased use or disuse of a part,
and even from mutilations if followed by disease. (12/9. 'Variation
under Domestication' chapter 12 2nd edition volume 1 page 466.) We have
abundant evidence how susceptible the reproductive system is to changed
conditions, in the many instances of animals rendered sterile by
confinement; so that they will not unite, or if they unite do not
produce offspring, though the confinement may be far from close; and of
plants rendered sterile by cultivation. But hardly any cases afford more
striking evidence how powerfully a change in the conditions of life acts
on the sexual elements, than those already given, of plants which are
completely self-sterile in one country, and when brought to another,
yield, even in the first generation, a fair supply of self-fertilised

But it may be said, granting that changed conditions act on the sexual
elements, how can two or more plants growing close together, either in
their native country or in a garden, be differently acted on, inasmuch
as they appear to be exposed to exactly the same conditions? Although
this question has been already considered, it deserves further
consideration under several points of view. In my experiments with
Digitalis purpurea, some flowers on a wild plant were self-fertilised,
and others were crossed with pollen from another plant growing within
two or three feet's distance. The crossed and self-fertilised plants
raised from the seeds thus obtained, produced flower-stems in number as
100 to 47, and in average height as 100 to 70. Therefore the cross
between these two plants was highly beneficial; but how could their
sexual elements have been differentiated by exposure to different
conditions? If the progenitors of the two plants had lived on the same
spot during the last score of generations, and had never been crossed
with any plant beyond the distance of a few feet, in all probability
their offspring would have been reduced to the same state as some of the
plants in my experiments,--such as the intercrossed plants of the ninth
generation of Ipomoea,--or the self-fertilised plants of the eighth
generation of Mimulus,--or the offspring from flowers on the same
plant,--and in this case a cross between the two plants of Digitalis
would have done no good. But seeds are often widely dispersed by natural
means, and one of the above two plants or one of their ancestors may
have come from a distance, from a more shady or sunny, dry or moist
place, or from a different kind of soil containing other organic or
inorganic matter. We know from the admirable researches of Messrs. Lawes
and Gilbert that different plants require and consume very different
amounts of inorganic matter. (12/10. 'Journal of the Royal Agricultural
Society of England' volume 24 part 1.) But the amount in the soil would
probably not make so great a difference to the several individuals of
any particular species as might at first be expected; for the
surrounding species with different requirements would tend, from
existing in greater or lesser numbers, to keep each species in a sort of
equilibrium, with respect to what it could obtain from the soil. So it
would be even with respect to moisture during dry seasons; and how
powerful is the influence of a little more or less moisture in the soil
on the presence and distribution of plants, is often well shown in old
pasture fields which still retain traces of former ridges and furrows.
Nevertheless, as the proportional numbers of the surrounding plants in
two neighbouring places is rarely exactly the same, the individuals of
the same species will be subjected to somewhat different conditions with
respect to what they can absorb from the soil. It is surprising how the
free growth of one set of plants affects others growing mingled with
them; I allowed the plants on rather more than a square yard of turf
which had been closely mown for several years, to grow up; and nine
species out of twenty were thus exterminated; but whether this was
altogether due to the kinds which grew up robbing the others of
nutriment, I do not know.

Seeds often lie dormant for several years in the ground, and germinate
when brought near the surface by any means, as by burrowing animals.
They would probably be affected by the mere circumstance of having long
lain dormant; for gardeners believe that the production of double
flowers and of fruit is thus influenced. Seeds, moreover, which were
matured during different seasons, will have been subjected during the
whole course of their development to different degrees of heat and

It was shown in the last chapter that pollen is often carried by insects
to a considerable distance from plant to plant. Therefore one of the
parents or ancestors of our two plants of Digitalis may have been
crossed by a distant plant growing under somewhat different conditions.
Plants thus crossed often produce an unusually large number of seeds; a
striking instance of this fact is afforded by the Bignonia, previously
mentioned, which was fertilised by Fritz Muller with pollen from some
adjoining plants and set hardly any seed, but when fertilised with
pollen from a distant plant, was highly fertile. Seedlings from a cross
of this kind grow with great vigour, and transmit their vigour to their
descendants. These, therefore, in the struggle for life, will generally
beat and exterminate the seedlings from plants which have long grown
near together under the same conditions, and will thus tend to spread.

When two varieties which present well-marked differences are crossed,
their descendants in the later generations differ greatly from one
another in external characters; and this is due to the augmentation or
obliteration of some of these characters, and to the reappearance of
former ones through reversion; and so it will be, as we may feel almost
sure, with any slight differences in the constitution of their sexual
elements. Anyhow, my experiments indicate that crossing plants which
have been long subjected to almost though not quite the same conditions,
is the most powerful of all the means for retaining some degree of
differentiation in the sexual elements, as shown by the superiority in
the later generations of the intercrossed over the self-fertilised
seedlings. Nevertheless, the continued intercrossing of plants thus
treated does tend to obliterate such differentiation, as may be inferred
from the lessened benefit derived from intercrossing such plants, in
comparison with that from a cross with a fresh stock. It seems probable,
as I may add, that seeds have acquired their endless curious adaptations
for wide dissemination, not only that the seedlings would thus be
enabled to find new and fitting homes, but that the individuals which
have been long subjected to the same conditions should occasionally
intercross with a fresh stock. (12/11. See Professor Hildebrand's
excellent treatise 'Verbreitungsmittel der Pflanzen' 1873.)

From the foregoing several considerations we may, I think, conclude that
in the above case of the Digitalis, and even in that of plants which
have grown for thousands of generations in the same district, as must
often have occurred with species having a much restricted range, we are
apt to over-estimate the degree to which the individuals have been
subjected to absolutely the same conditions. There is at least no
difficulty in believing that such plants have been subjected to
sufficiently distinct conditions to differentiate their sexual elements;
for we know that a plant propagated for some generations in another
garden in the same district serves as a fresh stock and has high
fertilising powers. The curious cases of plants which can fertilise and
be fertilised by any other individual of the same species, but are
altogether sterile with their own pollen, become intelligible, if the
view here propounded is correct, namely, that the individuals of the
same species growing in a state of nature near together, have not really
been subjected during several previous generations to quite the same

Some naturalists assume that there is an innate tendency in all beings
to vary and to advance in organisation, independently of external
agencies; and they would, I presume, thus explain the slight differences
which distinguish all the individuals of the same species both in
external characters and in constitution, as well as the greater
differences in both respects between nearly allied varieties. No two
individuals can be found quite alike; thus if we sow a number of seeds
from the same capsule under as nearly as possible the same conditions,
they germinate at different rates and grow more or less vigorously. They
resist cold and other unfavourable conditions differently. They would in
all probability, as we know to be the case with animals of the same
species, be somewhat differently acted on by the same poison, or by the
same disease. They have different powers of transmitting their
characters to their offspring; and many analogous facts could be given.
(12/12. Vilmorin as quoted by Verlot 'Des Varieties' pages 32, 38, 39.)
Now, if it were true that plants growing near together in a state of
nature had been subjected during many previous generations to absolutely
the same conditions, such differences as those just specified would be
quite inexplicable; but they are to a certain extent intelligible in
accordance with the views just advanced.

As most of the plants on which I experimented were grown in my garden or
in pots under glass, a few words must be added on the conditions to
which they were exposed, as well as on the effects of cultivation. When
a species is first brought under culture, it may or may not be subjected
to a change of climate, but it is always grown in ground broken up, and
more or less manured; it is also saved from competition with other
plants. The paramount importance of this latter circumstance is proved
by the multitude of species which flourish and multiply in a garden, but
cannot exist unless they are protected from other plants. When thus
saved from competition they are able to get whatever they require from
the soil, probably often in excess; and they are thus subjected to a
great change of conditions. It is probably in chief part owing to this
cause that all plants with rare exceptions vary after being cultivated
for some generations. The individuals which have already begun to vary
will intercross one with another by the aid of insects; and this
accounts for the extreme diversity of character which many of our long
cultivated plants exhibit. But it should be observed that the result
will be largely determined by the degree of their variability and by the
frequency of the intercrosses; for if a plant varies very little, like
most species in a state of nature, frequent intercrosses tend to give
uniformity of character to it.

I have attempted to show that with plants growing naturally in the same
district, except in the unusual case of each individual being surrounded
by exactly the same proportional numbers of other species having certain
powers of absorption, each will be subjected to slightly different
conditions. This does not apply to the individuals of the same species
when cultivated in cleared ground in the same garden. But if their
flowers are visited by insects, they will intercross; and this will give
to their sexual elements during a considerable number of generations a
sufficient amount of differentiation for a cross to be beneficial.
Moreover, seeds are frequently exchanged or procured from other gardens
having a different kind of soil; and the individuals of the same
cultivated species will thus be subjected to a change of conditions. If
the flowers are not visited by our native insects, or very rarely so, as
in the case of the common and sweet pea, and apparently in that of the
tobacco when kept in a hothouse, any differentiation in the sexual
elements caused by intercrosses will tend to disappear. This appears to
have occurred with the plants just mentioned, for they were not
benefited by being crossed one with another, though they were greatly
benefited by a cross with a fresh stock.

I have been led to the views just advanced with respect to the causes of
the differentiation of the sexual elements and of the variability of our
garden plants, by the results of my various experiments, and more
especially by the four cases in which extremely inconstant species,
after having been self-fertilised and grown under closely similar
conditions for several generations, produced flowers of a uniform and
constant tint. These conditions were nearly the same as those to which
plants, growing in a garden clear of weeds, are subjected, if they are
propagated by self-fertilised seeds on the same spot. The plants in pots
were, however, exposed to less severe fluctuations of climate than those
out of doors; but their conditions, though closely uniform for all the
individuals of the same generation, differed somewhat in the successive
generations. Now, under these circumstances, the sexual elements of the
plants which were intercrossed in each generation retained sufficient
differentiation during several years for their offspring to be superior
to the self-fertilised, but this superiority gradually and manifestly
decreased, as was shown by the difference in the result between a cross
with one of the intercrossed plants and with a fresh stock. These
intercrossed plants tended also in a few cases to become somewhat more
uniform in some of their external characters than they were at first.
With respect to the plants which were self-fertilised in each
generation, their sexual elements apparently lost, after some years, all
differentiation, for a cross between them did no more good than a cross
between the flowers on the same plant. But it is a still more remarkable
fact, that although the seedlings of Mimulus, Ipomoea, Dianthus, and
Petunia which were first raised, varied excessively in the colour of
their flowers, their offspring, after being self-fertilised and grown
under uniform conditions for some generations, bore flowers almost as
uniform in tint as those on a natural species. In one case also the
plants themselves became remarkably uniform in height.

The conclusion that the advantages of a cross depend altogether on the
differentiation of the sexual elements, harmonises perfectly with the
fact that an occasional and slight change in the conditions of life is
beneficial to all plants and animals. (12/13. I have given sufficient
evidence on this head in my 'Variation under Domestication' chapter 18
volume 2 2nd edition page 127.) But the offspring from a cross between
organisms which have been exposed to different conditions, profit in an
incomparably higher degree than do young or old beings from a mere
change in the conditions. In this latter case we never see anything like
the effect which generally follows from a cross with another individual,
especially from a cross with a fresh stock. This might, perhaps, have
been expected, for the blending together of the sexual elements of two
differentiated beings will affect the whole constitution at a very early
period of life, whilst the organisation is highly flexible. We have,
moreover, reason to believe that changed conditions generally act
differently on the several parts or organs of the same individual
(12/14. See, for instance, Brackenridge 'Theory of Diathesis' Edinburgh
1869.); and if we may further believe that these now slightly
differentiated parts react on one another, the harmony between the
beneficial effects on the individual due to changed conditions, and
those due to the interaction of differentiated sexual elements, becomes
still closer.

That wonderfully accurate observer, Sprengel, who first showed how
important a part insects play in the fertilisation of flowers, called
his book 'The Secret of Nature Displayed;' yet he only occasionally saw
that the object for which so many curious and beautiful adaptations have
been acquired, was the cross-fertilisation of distinct plants; and he
knew nothing of the benefits which the offspring thus receive in growth,
vigour, and fertility. But the veil of secrecy is as yet far from
lifted; nor will it be, until we can say why it is beneficial that the
sexual elements should be differentiated to a certain extent, and why,
if the differentiation be carried still further, injury follows. It is
an extraordinary fact that with many species, flowers fertilised with
their own pollen are either absolutely or in some degree sterile; if
fertilised with pollen from another flower on the same plant, they are
sometimes, though rarely, a little more fertile; if fertilised with
pollen from another individual or variety of the same species, they are
fully fertile; but if with pollen from a distinct species, they are
sterile in all possible degrees, until utter sterility is reached. We
thus have a long series with absolute sterility at the two ends;--at one
end due to the sexual elements not having been sufficiently
differentiated, and at the other end to their having been differentiated
in too great a degree, or in some peculiar manner.

The fertilisation of one of the higher plants depends, in the first
place, on the mutual action of the pollen-grains and the stigmatic
secretion or tissues, and afterwards on the mutual action of the
contents of the pollen-grains and ovules. Both actions, judging from the
increased fertility of the parent-plants and from the increased powers
of growth in the offspring, are favoured by some degree of
differentiation in the elements which interact and unite so as to form a
new being. Here we have some analogy with chemical affinity or
attraction, which comes into play only between atoms or molecules of a
different nature. As Professor Miller remarks: "Generally speaking, the
greater the difference in the properties of two bodies, the more intense
is their tendency to mutual chemical action...But between bodies of a
similar character the tendency to unite is feeble." (12/15. 'Elements of
Chemistry' 4th edition 1867 part 1 page 11. Dr. Frankland informs me
that similar views with respect to chemical affinity are generally
accepted by chemists.) This latter proposition accords well with the
feeble effects of a plant's own pollen on the fertility of the
mother-plant and on the growth of the offspring; and the former
proposition accords well with the powerful influence in both ways of
pollen from an individual which has been differentiated by exposure to
changed conditions, or by so-called spontaneous variation. But the
analogy fails when we turn to the negative or weak effects of pollen
from one species on a distinct species; for although some substances
which are extremely dissimilar, for instance, carbon and chlorine, have
a very feeble affinity for each other, yet it cannot be said that the
weakness of the affinity depends in such cases on the extent to which
the substances differ. It is not known why a certain amount of
differentiation is necessary or favourable for the chemical affinity or
union of two substances, any more than for the fertilisation or union of
two organisms.

Mr. Herbert Spencer has discussed this whole subject at great length,
and after stating that all the forces throughout nature tend towards an
equilibrium, remarks, "that the need of this union of sperm-cell and
germ-ccell is the need for overthrowing this equilibrium and
re-establishing active molecular change in the detached germ--a result
which is probably effected by mixing the slightly-different
physiological units of slightly-different individuals." (12/16.
'Principles of Biology' volume 1 page 274 1864. In my 'Origin of
Species' published in 1859, I spoke of the good effects from slight
changes in the condition of life and from cross-fertilisation, and of
the evil effects from great changes in the conditions and from crossing
widely distinct forms (i.e., species), as a series of facts "connected
together by some common but unknown bond, which is essentially related
to the principle of life.") But we must not allow this highly
generalised view, or the analogy of chemical affinity, to conceal from
us our ignorance. We do not know what is the nature or degree of the
differentiation in the sexual elements which is favourable for union,
and what is injurious for union, as in the case of distinct species. We
cannot say why the individuals of certain species profit greatly, and
others very little by being crossed. There are some few species which
have been self-fertilised for a vast number of generations, and yet are
vigorous enough to compete successfully with a host of surrounding
plants. We can form no conception why the advantage from a cross is
sometimes directed exclusively to the vegetative system, and sometimes
to the reproductive system, but commonly to both. It is equally
inconceivable why some individuals of the same species should be
sterile, whilst others are fully fertile with their own pollen; why a
change of climate should either lessen or increase the sterility of
self-sterile species; and why the individuals of some species should be
even more fertile with pollen from a distinct species than with their
own pollen. And so it is with many other facts, which are so obscure
that we stand in awe before the mystery of life.

Under a practical point of view, agriculturists and horticulturists may
learn something from the conclusions at which we have arrived. Firstly,
we see that the injury from the close breeding of animals and from the
self-fertilisation of plants, does not necessarily depend on any
tendency to disease or weakness of constitution common to the related
parents, and only indirectly on their relationship, in so far as they
are apt to resemble each other in all respects, including their sexual
nature. And, secondly, that the advantages of cross-fertilisation depend
on the sexual elements of the parents having become in some degree
differentiated by the exposure of their progenitors to different
conditions, or from their having intercrossed with individuals thus
exposed, or, lastly, from what we call in our ignorance spontaneous
variation. He therefore who wishes to pair closely related animals ought
to keep them under conditions as different as possible. Some few
breeders, guided by their keen powers of observation, have acted on this
principle, and have kept stocks of the same animals at two or more
distant and differently situated farms. They have then coupled the
individuals from these farms with excellent results. (12/17. 'Variation
of Animals and Plants under Domestication' chapter 17 2nd edition volume
2 pages 98, 105.) This same plan is also unconsciously followed whenever
the males, reared in one place, are let out for propagation to breeders
in other places. As some kinds of plants suffer much more from
self-fertilisation than do others, so it probably is with animals from
too close interbreeding. The effects of close interbreeding on animals,
judging again from plants, would be deterioration in general vigour,
including fertility, with no necessary loss of excellence of form; and
this seems to be the usual result.

It is a common practice with horticulturists to obtain seeds from
another place having a very different soil, so as to avoid raising
plants for a long succession of generations under the same conditions;
but with all the species which freely intercross by aid of insects or
the wind, it would be an incomparably better plan to obtain seeds of the
required variety, which had been raised for some generations under as
different conditions as possible, and sow them in alternate rows with
seeds matured in the old garden. The two stocks would then intercross,
with a thorough blending of their whole organisations, and with no loss
of purity to the variety; and this would yield far more favourable
results than a mere exchange of seeds. We have seen in my experiments
how wonderfully the offspring profited in height, weight, hardiness, and
fertility, by crosses of this kind. For instance, plants of Ipomoea thus
crossed were to the intercrossed plants of the same stock, with which
they grew in competition, as 100 to 78 in height, and as 100 to 51 in
fertility; and plants of Eschscholtzia similarly compared were as 100 to
45 in fertility. In comparison with self-fertilised plants the results
are still more striking; thus cabbages derived from a cross with a fresh
stock were to the self-fertilised as 100 to 22 in weight.

Florists may learn from the four cases which have been fully described,
that they have the power of fixing each fleeting variety of colour, if
they will fertilise the flowers of the desired kind with their own
pollen for half-a-dozen generations, and grow the seedlings under the
same conditions. But a cross with any other individual of the same
variety must be carefully prevented, as each has its own peculiar
constitution. After a dozen generations of self-fertilisation, it is
probable that the new variety would remain constant even if grown under
somewhat different conditions; and there would no longer be any
necessity to guard against intercrosses between the individuals of the
same variety.

With respect to mankind, my son George has endeavoured to discover by a
statistical investigation whether the marriages of first cousins are at
all injurious, although this is a degree of relationship which would not
be objected to in our domestic animals; 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 its being very small. From the facts given in this volume we
may infer that with mankind the marriages of nearly related persons,
some of whose parents and ancestors had lived under very different
conditions, would be much less injurious than that of persons who had
always lived in the same place and followed the same habits of life. Nor
can I see reason to doubt that the widely different habits of life of
men and women in civilised nations, especially amongst the upper
classes, would tend to counterbalance any evil from marriages between
healthy and somewhat closely related persons.

Under a theoretical point of view it is some gain to science to know
that numberless structures in hermaphrodite plants, and probably in
hermaphrodite animals, are special adaptations for securing an
occasional cross between two individuals; and that the advantages from
such a cross depend altogether on the beings which are united, or their
progenitors, having had their sexual elements somewhat differentiated,
so that the embryo is benefited in the same manner as is a mature plant
or animal by a slight change in its conditions of life, although in a
much higher degree.

Another and more important result may be deduced from my observations.
Eggs and seeds are highly serviceable as a means of dissemination, but
we now know that fertile eggs can be produced without the aid of the
male. There are also many other methods by which organisms can be
propagated asexually. Why then have the two sexes been developed, and
why do males exist which cannot themselves produce offspring? The answer
lies, as I can hardly doubt, in the great good which is derived from the
fusion of two somewhat differentiated individuals; and with the
exception of the lowest organisms this is possible only by means of the
sexual elements, these consisting of cells separated from the body,
containing the germs of every part, and capable of being fused
completely together.

It has been shown in the present volume that the offspring from the
union of two distinct individuals, especially if their progenitors have
been subjected to very different conditions, have an immense advantage
in height, weight, constitutional vigour and fertility over the
self-fertilised offspring from one of the same parents. And this fact is
amply sufficient to account for the development of the sexual elements,
that is, for the genesis of the two sexes.

It is a different question why the two sexes are sometimes combined in
the same individual and are sometimes separated. As with many of the
lowest plants and animals the conjugation of two individuals which are
either quite similar or in some degree different, is a common
phenomenon, it seems probable, as remarked in the last chapter, that the
sexes were primordially separate. The individual which receives the
contents of the other, may be called the female; and the other, which is
often smaller and more locomotive, may be called the male; though these
sexual names ought hardly to be applied as long as the whole contents of
the two forms are blended into one. The object gained by the two sexes
becoming united in the same hermaphrodite form probably is to allow of
occasional or frequent self-fertilisation, so as to ensure the
propagation of the species, more especially in the case of organisms
affixed for life to the same spot. There does not seem to be any great
difficulty in understanding how an organism, formed by the conjugation
of two individuals which represented the two incipient sexes, might have
given rise by budding first to a monoecious and then to an hermaphrodite
form; and in the case of animals even without budding to an
hermaphrodite form, for the bilateral structure of animals perhaps
indicates that they were aboriginally formed by the fusion of two

It is a more difficult problem why some plants and apparently all the
higher animals, after becoming hermaphrodites, have since had their
sexes re-separated. This separation has been attributed by some
naturalists to the advantages which follow from a division of
physiological labour. The principle is intelligible when the same organ
has to perform at the same time diverse functions; but it is not obvious
why the male and female glands when placed in different parts of the
same compound or simple individual, should not perform their functions
equally well as when placed in two distinct individuals. In some
instances the sexes may have been re-separated for the sake of
preventing too frequent self-fertilisation; but this explanation does
not seem probable, as the same end might have been gained by other and
simpler means, for instance dichogamy. It may be that the production of
the male and female reproductive elements and the maturation of the
ovules was too great a strain and expenditure of vital force for a
single individual to withstand, if endowed with a highly complex
organisation; and that at the same time there was no need for all the
individuals to produce young, and consequently that no injury, on the
contrary, good resulted from half of them, or the males, failing to
produce offspring.

There is another subject on which some light is thrown by the facts
given in this volume, namely, hybridisation. It is notorious that when
distinct species of plants are crossed, they produce with the rarest
exceptions fewer seeds than the normal number. This unproductiveness
varies in different species up to sterility so complete that not even an
empty capsule is formed; and all experimentalists have found that it is
much influenced by the conditions to which the crossed species are
subjected. The pollen of each species is strongly prepotent over that of
any other species, so that if a plant's own pollen is placed on the
stigma some time after foreign pollen has been applied to it, any effect
from the latter is quite obliterated. It is also notorious that not only
the parent species, but the hybrids raised from them are more or less
sterile; and that their pollen is often in a more or less aborted
condition. The degree of sterility of various hybrids does not always
strictly correspond with the degree of difficulty in uniting the parent
forms. When hybrids are capable of breeding inter se, their descendants
are more or less sterile, and they often become still more sterile in
the later generations; but then close interbreeding has hitherto been
practised in all such cases. The more sterile hybrids are sometimes much
dwarfed in stature, and have a feeble constitution. Other facts could be
given, but these will suffice for us. Naturalists formerly attributed
all these results to the difference between species being fundamentally
distinct from that between the varieties of the same species; and this
is still the verdict of some naturalists.

The results of my experiments in self-fertilising and cross-fertilising
the individuals or the varieties of the same species, are strikingly
analogous with those just given, though in a reversed manner. With the
majority of species flowers fertilised with their own pollen yield
fewer, sometimes much fewer seeds, than those fertilised with pollen
from another individual or variety. Some self-fertilised flowers are
absolutely sterile; but the degree of their sterility is largely
determined by the conditions to which the parent plants have been
exposed, as was well exemplified in the case of Eschscholtzia and
Abutilon. The effects of pollen from the same plant are obliterated by
the prepotent influence of pollen from another individual or variety,
although the latter may have been placed on the stigma some hours
afterwards. The offspring from self-fertilised flowers are themselves
more or less sterile, sometimes highly sterile, and their pollen is
sometimes in an imperfect condition; but I have not met with any case of
complete sterility in self-fertilised seedlings, as is so common with
hybrids. The degree of their sterility does not correspond with that of
the parent-plants when first self-fertilised. The offspring of
self-fertilised plants suffer in stature, weight, and constitutional
vigour more frequently and in a greater degree than do the hybrid
offspring of the greater number of crossed species. Decreased height is
transmitted to the next generation, but I did not ascertain whether this
applies to decreased fertility.

I have elsewhere shown that by uniting in various ways dimorphic or
trimorphic heterostyled plants, which belong to the same undoubted
species, we get another series of results exactly parallel with those
from crossing distinct species. (12/18. 'Journal of the Linnean Society
Botany' volume 10 1867 page 393.) Plants illegitimately fertilised with
pollen from a distinct plant belonging to the same form, yield fewer,
often much fewer seeds, than they do when legitimately fertilised with
pollen from a plant belonging to a distinct form. They sometimes yield
no seed, not even an empty capsule, like a species fertilised with
pollen from a distinct genus. The degree of sterility is much affected
by the conditions to which the plants have been subjected. (12/19.
'Journal of the Linnean Society Botany' volume 8 1864 page 180.) The
pollen from a distinct form is strongly prepotent over that from the
same form, although the former may have been placed on the stigma many
hours afterwards. The offspring from a union between plants of the same
form are more or less sterile, like hybrids, and have their pollen in a
more or less aborted condition; and some of the seedlings are as barren
and as dwarfed as the most barren hybrid. They also resemble hybrids in
several other respects, which need not here be specified in
detail,--such as their sterility not corresponding in degree with that
of the parent plants,--the unequal sterility of the latter, when
reciprocally united,--and the varying sterility of the seedlings raised
from the same seed-capsule.

We thus have two grand classes of cases giving results which correspond
in the most striking manner with those which follow from the crossing of
so-called true and distinct species. With respect to the difference
between seedlings raised from cross and self-fertilised flowers, there
is good evidence that this depends altogether on whether the sexual
elements of the parents have been sufficiently differentiated, by
exposure to different conditions or by spontaneous variation. It is
probable that nearly the same conclusion may be extended to heterostyled
plants; but this is not the proper place for discussing the origin of
the long-styled, short-styled and mid-styled forms, which all belong to
the same species as certainly as do the two sexes of the same species.
We have therefore no right to maintain that the sterility of species
when first crossed and of their hybrid offspring, is determined by some
cause fundamentally different from that which determines the sterility
of the individuals both of ordinary and of heterostyled plants when
united in various ways. Nevertheless, I am aware that it will take many
years to remove this prejudice.

There is hardly anything more wonderful in nature than the sensitiveness
of the sexual elements to external influences, and the delicacy of their
affinities. We see this in slight changes in the conditions of life
being favourable to the fertility and vigour of the parents, while
certain other and not great changes cause them to be quite sterile
without any apparent injury to their health. We see how sensitive the
sexual elements of those plants must be, which are completely sterile
with their own pollen, but are fertile with that of any other individual
of the same species. Such plants become either more or less self-sterile
if subjected to changed conditions, although the change may be far from
great. The ovules of a heterostyled trimorphic plant are affected very
differently by pollen from the three sets of stamens belonging to the
same species. With ordinary plants the pollen of another variety or
merely of another individual of the same variety is often strongly
prepotent over its own pollen, when both are placed at the same time on
the same stigma. In those great families of plants containing many
thousand allied species, the stigma of each distinguishes with unerring
certainty its own pollen from that of every other species.

There can be no doubt that the sterility of distinct species when first
crossed, and of their hybrid offspring, depends exclusively on the
nature or affinities of their sexual elements. We see this in the want
of any close correspondence between the degree of sterility and the
amount of external difference in the species which are crossed; and
still more clearly in the wide difference in the results of crossing
reciprocally the same two species;--that is, when species A is crossed
with pollen from B, and then B is crossed with pollen from A. Bearing in
mind what has just been said on the extreme sensitiveness and delicate
affinities of the reproductive system, why should we feel any surprise
at the sexual elements of those forms, which we call species, having
been differentiated in such a manner that they are incapable or only
feebly capable of acting on one another? We know that species have
generally lived under the same conditions, and have retained their own
proper characters, for a much longer period than varieties.
Long-continued domestication eliminates, as I have shown in my
'Variation under Domestication,' the mutual sterility which distinct
species lately taken from a state of nature almost always exhibit when
intercrossed; and we can thus understand the fact that the most
different domestic races of animals are not mutually sterile. But
whether this holds good with cultivated varieties of plants is not
known, though some facts indicate that it does. The elimination of
sterility through long-continued domestication may probably be
attributed to the varying conditions to which our domestic animals have
been subjected; and no doubt it is owing to this same cause that they
withstand great and sudden changes in their conditions of life with far
less loss of fertility than do natural species. From these several
considerations it appears probable that the difference in the affinities
of the sexual elements of distinct species, on which their mutual
incapacity for breeding together depends, is caused by their having been
habituated for a very long period each to its own conditions, and to the
sexual elements having thus acquired firmly fixed affinities. However
this may be, with the two great classes of cases before us, namely,
those relating to the self-fertilisation and cross-fertilisation of the
individuals of the same species, and those relating to the illegitimate
and legitimate unions of heterostyled plants, it is quite unjustifiable
to assume that the sterility of species when first crossed and of their
hybrid offspring, indicates that they differ in some fundamental manner
from the varieties or individuals of the same species.


Abutilon darwinii, self-sterile in Brazil.
moderately self-fertile in England.
fertilised by birds.

Acacia sphaerocephala.


Aconitum napellus.

Adlumia cirrhosa.

Adonis aestivalis.
relative heights of crossed and self-fertilised plants.

Ajuga reptans.

Allium cepa (blood-red var.)

Anagallis collina (var. grandiflora).

Anderson, J., on the Calceolaria.
removing the corollas.


Anemophilous plants.
often diclinous.

Antirrhinum majus (red var.)
perforated corolla.
--(white var.).
--(peloric var.).

Apium petroselinum.
result of experiments.

Argemone ochroleuca.


Aristotle on bees frequenting flowers of the same species.

Arum maculatum.

Bailey, Mr., perforation of corolla.

Bartonia aurea.
result of experiments.

Bartsia odontites.

Beal, W.J., sterility of Kalmia latifolia.
on nectar in Ribes aureum.

Bean, the common.

Bees distinguish colours.
frequent the flowers of the same species.
guided by coloured corolla.
powers of vision and discrimination.
unattracted by odour of certain flowers.
profit by the corolla perforated by humble-bees.
skill in working.

Bees, humble, recognise varieties as of one species.
colour not the sole guide.
rate of flying.
number of flowers visited.
corolla perforated by.
skill and judgment.

Belt, Mr., the hairs of Digitalis purpurea.
Phaseolus multiflorus.
not visited by bees in Nicaragua.
humming-birds carrying pollen.
secretion of nectar.
in Acacia sphaerocephalus and passion-flower.
perforation of corolla.

Bennett, A.W., on Viola tricolor.
structure of Impatiens fulva.
plants flowering in winter.
bees frequenting flowers of same species.

Bentham, on protection of the stigma in Synaphea.

Beta vulgaris.
crossed not exceeded by self-fertilised.
prepotency of other pollen.


Birds, means of fertilisation.

Blackley, Mr., on anthers of rye.
pollen carried by wind, experiments with a kite.


Borago officinalis.
early flowering of crossed.
partially self-sterile.

Brackenridge, Mr., organism of animals affected by temperature and food.
different effect of changed conditions.

Brassica oleracea.
remarks on experiments.
superiority of crossed.
period of flowering.

Brisout, M., insects frequenting flowers of same species.


humming-birds boring the flower.

Bulrush, weight of pollen produced by one plant.

Bundy, Mr., Ribes perforated by bees.

Butschli, O., sexual relations.

affected by pollen of purple bastard.
prepotency of other pollen.
--, Ragged Jack.


Calluna vulgaris.

Campanula carpathica.


Candolle, A. de, on ascending a mountain the flowers of the same species
disappear abruptly.

Canna warscewiczi.
result of crossed and self-fertilised.
period of flowering.
highly self-fertile.


Carduus arctioides.


Carriere, relative period of the maturity of the sexual elements on same


Caspary, Professor, on Corydalis cava.
Euryale ferox.

Cecropia, food-bodies of.

Centradenia floribunda.

Cereals, grains of.

Cheeseman, Mr., on Orchids in New Zealand.



Clarkia elegans.
early flowering of self-fertilised.

Cleistogene flowers.

Coe, Mr., crossing Phaseolus vulgaris.

Colgate, R., red clover never sucked by hive-bees in New Zealand.

Colour, uniform, of flowers on plants self-fertilised and grown under
similar conditions for several generations.

Colours of flowers attractive to insects.
not the sole guide to bees.



Convolvulus major.
-- tricolor.

Corolla, removal of.
perforation by bees.


Corydalis cava.
-- halleri.
-- intermedia.
-- lutea.
-- ochroleuca.
-- solida.

Corylus avellana.



Crossed plants, greater constitutional vigour of.

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