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Darwin and Modern Science by A.C. Seward

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tells us, in the most emphatic manner, that she abhors perpetual self-
fertilisation...For may we not further infer as probable, in accordance
with the belief of the vast majority of the breeders of our domestic
productions, that marriage between near relations is likewise in some way
injurious, that some unknown great good is derived from the union of
individuals which have been kept distinct for many generations?" (Ibid.,
page 359.)

This view was supported by observations on plants of other families, e.g.
Papilionaceae; it could, however, in the absence of experimental proof, be
regarded only as a "working hypothesis."

All adaptations to cross-pollination might also be of use simply because
they made pollination possible when for any reason self-pollination had
become difficult or impossible. Cross-pollination would, therefore, be of
use, not as such, but merely as a means of pollination in general; it would
to some extent serve as a remedy for a method unsuitable in itself, such as
a modification standing in the way of self-pollination, and on the other
hand as a means of increasing the chance of pollination in the case of
flowers in which self-pollination was possible, but which might, in
accidental circumstances, be prevented. It was, therefore, very important
to obtain experimental proof of the conclusion to which Darwin was led by
the belief of the majority of breeders and by the evidence of the
widespread occurrence of cross-pollination and of the remarkable
adaptations thereto.

This was supplied by the researches which are described in the two other
works named above. The researches on which the conclusions rest had, in
part at least, been previously published in separate papers: this is the
case as regards the heterostyled plants. The discoveries which Darwin made
in the course of his investigations of these plants belong to the most
brilliant in biological science.

The case of Primula is now well known. C.K. Sprengel and others were
familiar with the remarkable fact that different individuals of the
European species of Primula bear differently constructed flowers; some
plants possess flowers in which the styles project beyond the stamens
attached to the corolla-tube (long-styled form), while in others the
stamens are inserted above the stigma which is borne on a short style
(short-styled form). It has been shown by Breitenbach that both forms of
flower may occur on the same plant, though this happens very rarely. An
analogous case is occasionally met with in hybrids, which bear flowers of
different colour on the same plant (e.g. Dianthus caryophyllus). Darwin
showed that the external differences are correlated with others in the
structure of the stigma and in the nature of the pollen. The long-styled
flowers have a spherical stigma provided with large stigmatic papillae; the
pollen grains are oblong and smaller than those of the short-styled
flowers. The number of the seeds produced is smaller and the ovules
larger, probably also fewer in number. The short-styled flowers have a
smooth compressed stigma and a corolla of somewhat different form; they
produce a greater number of seeds.

These different forms of flowers were regarded as merely a case of
variation, until Darwin showed "that these heterostyled plants are adapted
for reciprocal fertilisation; so that the two or three forms, though all
are hermaphrodites, are related to one another almost like the males and
females of ordinary unisexual animals." ("Forms of Flowers" (1st edition),
page 2.) We have here an example of hermaphrodite flowers which are
sexually different. There are essential differences in the manner in which
fertilisation occurs. This may be effected in four different ways; there
are two legitimate and two illegitimate types of fertilisation. The
fertilisation is legitimate if pollen from the long-styled flowers reaches
the stigma of the short-styled form or if pollen of the short-styled
flowers is brought to the stigma of the long-styled flower, that is the
organs of the same length of the two different kinds of flower react on one
another. Illegitimate fertilisation is represented by the two kinds of
self-fertilisation, also by cross-fertilisation, in which the pollen of the
long-styled form reaches the stigma of the same type of flower and,
similarly, by cross-pollination in the case of the short-styled flowers.

The applicability of the terms legitimate and illegitimate depends, on the
one hand, upon the fact that insects which visit the different forms of
flowers pollinate them in the manner suggested; the pollen of the short-
styled flowers adhere to that part of the insect's body which touches the
stigma of the long-styled flower and vice versa. On the other hand, it is
based also on the fact that experiment shows that artificial pollination
produces a very different result according as this is legitimate or
illegitimate; only the legitimate union ensures complete fertility, the
plants thus produced being stronger than those which are produced

If we take 100 as the number of flowers which produce seeds as the result
of legitimate fertilisation, we obtain the following numbers from
illegitimate fertilisation:

Primula officinalis (P. veris) (Cowslip) ... 69
Primula elatior (Oxlip) .................... 27
Primula acaulis (P. vulgaris) (Primrose) ... 60

Further, the plants produced by the illegitimate method of fertilisation
showed, e.g. in P. officinalis, a decrease in fertility in later
generations, sterile pollen and in the open a feebler growth. (Under very
favourable conditions (in a greenhouse) the fertility of the plants of the
fourth generation increases--a point, which in view of various theoretical
questions, deserves further investigation.) They behave in fact precisely
in the same way as hybrids between species of different genera. This
result is important, "for we thus learn that the difficulty in sexually
uniting two organic forms and the sterility of their offspring, afford no
sure criterion of so-called specific distinctness" ("Forms of Flowers",
page 242): the relative or absolute sterility of the illegitimate unions
and that of their illegitimate descendants depend exclusively on the nature
of the sexual elements and on their inability to combine in a particular
manner. This functional difference of sexual cells is characteristic of
the behaviour of hybrids as of the illegitimate unions of heterostyled
plants. The agreement becomes even closer if we regard the Primula plants
bearing different forms of flowers not as belonging to a systematic entity
or "species," but as including several elementary species. The
legitimately produced plants are thus true hybrids (When Darwin wrote in
reference to the different forms of heterostyled plants, "which all belong
to the same species as certainly as do the two sexes of the same species"
("Cross and Self fertilisation", page 466), he adopted the term species in
a comprehensive sense. The recent researches of Bateson and Gregory ("On
the inheritance of Heterostylism in Primula"; "Proc. Roy. Soc." Ser. B,
Vol. LXXVI. 1905, page 581) appear to me also to support the view that the
results of illegitimate crossing of heterostyled Primulas correspond with
those of hybridisation. The fact that legitimate pollen effects
fertilisation, even if illegitimate pollen reaches the stigma a short time
previously, also points to this conclusion. Self-pollination in the case
of the short-styled form, for example, is not excluded. In spite of this,
the numerical proportion of the two forms obtained in the open remains
approximately the same as when the pollination was exclusively legitimate,
presumably because legitimate pollen is prepotent.), with which their
behaviour in other respects, as Darwin showed, presents so close an
agreement. This view receives support also from the fact that descendants
of a flower fertilised illegitimately by pollen from another plant with the
same form of flower belong, with few exceptions, to the same type as that
of their parents. The two forms of flower, however, behave differently in
this respect. Among 162 seedlings of the long-styled illegitimately
pollinated plants of Primula officinalis, including five generations, there
were 156 long-styled and only six short-styled forms, while as the result
of legitimate fertilisation nearly half of the offspring were long-styled
and half short-styled. The short-styled illegitimately pollinated form
gave five long-styled and nine short-styled; the cause of this difference
requires further explanation. The significance of heterostyly, whether or
not we now regard it as an arrangement for the normal production of
hybrids, is comprehensively expressed by Darwin: "We may feel sure that
plants have been rendered heterostyled to ensure cross-fertilisation, for
we now know that a cross between the distinct individuals of the same
species is highly important for the vigour and fertility of the offspring."
("Forms of Flowers", page 258.) If we remember how important the
interpretation of heterostyly has become in all general problems as, for
example, those connected with the conditions of the formation of hybrids, a
fact which was formerly overlooked, we can appreciate how Darwin was able
to say in his autobiography: "I do not think anything in my scientific
life has given me so much satisfaction as making out the meaning of the
structure of these plants." ("Life and Letters", Vol. I. page 91.)

The remarkable conditions represented in plants with three kinds of
flowers, such as Lythrum and Oxalis, agree in essentials with those in
Primula. These cannot be considered in detail here; it need only be noted
that the investigation of these cases was still more laborious. In order
to establish the relative fertility of the different unions in Lythrum
salicaria 223 different fertilisations were made, each flower being
deprived of its male organs and then dusted with the appropriate pollen.

In the book containing the account of heterostyled plants other species are
dealt with which, in addition to flowers opening normally (chasmogamous),
also possess flowers which remain closed but are capable of producing
fruit. These cleistogamous flowers afford a striking example of habitual
self-pollination, and H. von Mohl drew special attention to them as such
shortly after the appearance of Darwin's Orchid book. If it were only a
question of producing seed in the simplest way, cleistogamous flowers would
be the most conveniently constructed. The corolla and frequently other
parts of the flower are reduced; the development of the seed may,
therefore, be accomplished with a smaller expenditure of building material
than in chasmogamous flowers; there is also no loss of pollen, and thus a
smaller amount suffices for fertilisation.

Almost all these plants, as Darwin pointed out, have also chasmogamous
flowers which render cross-fertilisation possible. His view that
cleistogamous flowers are derived from originally chasmogamous flowers has
been confirmed by more recent researches. Conditions of nutrition in the
broader sense are the factors which determine whether chasmogamous or
cleistogamous flowers are produced, assuming, of course, that the plants in
question have the power of developing both forms of flower. The former may
fail to appear for some time, but are eventually developed under favourable
conditions of nourishment. The belief of many authors that there are
plants with only cleistogamous flowers cannot therefore be accepted as
authoritative without thorough experimental proof, as we are concerned with
extra-european plants for which it is often difficult to provide
appropriate conditions in cultivation.

Darwin sees in cleistogamous flowers an adaptation to a good supply of
seeds with a small expenditure of material, while chasmogamous flowers of
the same species are usually cross-fertilised and "their offspring will
thus be invigorated, as we may infer from a wide-spread analogy." ("Forms
of Flowers" (1st edition), page 341.) Direct proof in support of this has
hitherto been supplied in a few cases only; we shall often find that the
example set by Darwin in solving such problems as these by laborious
experiment has unfortunately been little imitated.

Another chapter of this book treats of the distribution of the sexes in
polygamous, dioecious, and gyno-dioecious plants (the last term, now in
common use, we owe to Darwin). It contains a number of important facts and
discussions and has inspired the experimental researches of Correns and

The most important of Darwin's work on floral biology is, however, that on
cross and self-fertilisation, chiefly because it states the results of
experimental investigations extending over many years. Only such
experiments, as we have pointed out, could determine whether cross-
fertilisation is in itself beneficial, and self-fertilisation on the other
hand injurious; a conclusion which a merely comparative examination of
pollination-mechanisms renders in the highest degree probable. Later
floral biologists have unfortunately almost entirely confined themselves to
observations on floral mechanisms. But there is little more to be gained
by this kind of work than an assumption long ago made by C.K. Sprengel that
"very many flowers have the sexes separate and probably at least as many
hermaphrodite flowers are dichogamous; it would thus appear that Nature was
unwilling that any flower should be fertilised by its own pollen."

It was an accidental observation which inspired Darwin's experiments on the
effect of cross and self-fertilisation. Plants of Linaria vulgaris were
grown in two adjacent beds; in the one were plants produced by cross-
fertilisation, that is, from seeds obtained after fertilisation by pollen
of another plant of the same species; in the other grew plants produced by
self-fertilisation, that is from seed produced as the result of pollination
of the same flower. The first were obviously superior to the latter.

Darwin was surprised by this observation, as he had expected a prejudicial
influence of self-fertilisation to manifest itself after a series of
generations: "I always supposed until lately that no evil effects would be
visible until after several generations of self-fertilisation, but now I
see that one generation sometimes suffices and the existence of dimorphic
plants and all the wonderful contrivances of orchids are quite intelligible
to me." ("More Letters", Vol. II. page 373.)

The observations on Linaria and the investigations of the results of
legitimate and illegitimate fertilisation in heterostyled plants were
apparently the beginning of a long series of experiments. These were
concerned with plants of different families and led to results which are of
fundamental importance for a true explanation of sexual reproduction.

The experiments were so arranged that plants were shielded from insect-
visits by a net. Some flowers were then pollinated with their own pollen,
others with pollen from another plant of the same species. The seeds were
germinated on moist sand; two seedlings of the same age, one from a cross
and the other from a self-fertilised flower, were selected and planted on
opposite sides of the same pot. They grew therefore under identical
external conditions; it was thus possible to compare their peculiarities
such as height, weight, fruiting capacity, etc. In other cases the
seedlings were placed near to one another in the open and in this way their
capacity of resisting unfavourable external conditions was tested. The
experiments were in some cases continued to the tenth generation and the
flowers were crossed in different ways. We see, therefore, that this book
also represents an enormous amount of most careful and patient original

The general result obtained is that plants produced as the result of cross-
fertilisation are superior, in the majority of cases, to those produced as
the result of self-fertilisation, in height, resistance to external
injurious influences, and in seed-production.

Ipomoea purpurea may be quoted as an example. If we express the result of
cross-fertilisation by 100, we obtain the following numbers for the
fertilised plants.

Generation. Height. Number of seeds.

1 100 : 76 100 : 64
2 100 : 79 -
3 100 : 68 100 : 94
4 100 : 86 100 : 94
5 100 : 75 100 : 89
6 100 : 72 -
7 100 : 81 -
8 100 : 85 -
9 100 : 79 100 : 26 (Number of capsules)
10 100 : 54 -

Taking the average, the ratio as regards growth is 100:77. The
considerable superiority of the crossed plants is apparent in the first
generation and is not increased in the following generations; but there is
some fluctuation about the average ratio. The numbers representing the
fertility of crossed and self-fertilised plants are more difficult to
compare with accuracy; the superiority of the crossed plants is chiefly
explained by the fact that they produce a much larger number of capsules,
not because there are on the average more seeds in each capsule. The ratio
of the capsules was, e.g. in the third generation, 100:38, that of the
seeds in the capsules 100:94. It is also especially noteworthy that in the
self-fertilised plants the anthers were smaller and contained a smaller
amount of pollen, and in the eighth generation the reduced fertility showed
itself in a form which is often found in hybrids, that is the first flowers
were sterile. (Complete sterility was not found in any of the plants
investigated by Darwin. Others appear to be more sensitive; Cluer found
Zea Mais "almost sterile" after three generations of self-fertilisation.
(Cf. Fruwirth, "Die Zuchtung der Landwirtschaftlichen Kulturpflanzen",
Berlin, 1904, II. page 6.)

The superiority of crossed individuals is not exhibited in the same way in
all plants. For example in Eschscholzia californica the crossed seedlings
do not exceed the self-fertilised in height and vigour, but the crossing
considerably increases the plant's capacity for flower-production, and the
seedlings from such a mother-plant are more fertile.

The conception implied by the term crossing requires a closer analysis. As
in the majority of plants, a large number of flowers are in bloom at the
same time on one and the same plant, it follows that insects visiting the
flowers often carry pollen from one flower to another of the same stock.
Has this method, which is spoken of as Geitonogamy, the same influence as
crossing with pollen from another plant? The results of Darwin's
experiments with different plants (Ipomoea purpurea, Digitalis purpurea,
Mimulus luteus, Pelargonium, Origanum) were not in complete agreement; but
on the whole they pointed to the conclusion that Geitonogamy shows no
superiority over self-fertilisation (Autogamy). (Similarly crossing in the
case of flowers of Pelargonium zonale, which belong to plants raised from
cuttings from the same parent, shows no superiority over self-
fertilisation.) Darwin, however, considered it possible that this may
sometimes be the case. "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." ("Cross and Self
fertilisation" (1st edition), page 444.)

As regards the importance of this question from the point of view of the
significance of cross-fertilisation in general, it may be noted that later
observers have definitely discovered a difference between the results of
autogamy and geitonogamy. Gilley and Fruwirth found that in Brassica
Napus, the length and weight of the fruits as also the total weight of the
seeds in a single fruit were less in the case of autogamy than in
geitonogamy. With Sinapis alba a better crop of seeds was obtained after
geitonogamy, and in the Sugar Beet the average weight of a fruit in the
case of a self-fertilised plant was 0.009 gr., from geitonogamy 0.012 gr.,
and on cross-fertilisation 0.013 gr.

On the whole, however, the results of geitonogamy show that the favourable
effects of cross-fertilisation do not depend simply on the fact that the
pollen of one flower is conveyed to the stigma of another. But the plants
which are crossed must in some way be different. If plants of Ipomoea
purpurea (and Mimulus luteus) which have been self-fertilised for seven
generations and grown under the same conditions of cultivation are crossed
together, the plants so crossed would not be superior to the self-
fertilised; on the other hand crossing with a fresh stock at once proves
very advantageous. The favourable effect of crossing is only apparent,
therefore, if the parent plants are grown under different conditions or if
they belong to different varieties. "It is really wonderful what an effect
pollen from a distinct seedling plant, which has been exposed to different
conditions of life, has on the offspring in comparison with pollen from the
same flower or from a distinct individual, but which has been long
subjected to the same conditions. The subject bears on the very principle
of life, which seems almost to require changes in the conditions." ("More
Letters", Vol. II. page 406.)

The fertility--measured by the number or weight of the seeds produced by an
equal number of plants--noticed under different conditions of fertilisation
may be quoted in illustration.

On crossing On crossing On self-
with a fresh plants of the fertilisation
stock same stock
Mimuleus luteus
(First and ninth generation) 100 4 3

Eschscholzia californica
(second generation) 100 45 40

Dianthus caryophyllus
(third and fourth generation) 100 45 33

Petunia violacea 100 54 46

Crossing under very similar conditions shows, therefore, that the
difference between the sexual cells is smaller and thus the result of
crossing is only slightly superior to that given by self-fertilisation.
Is, then, the favourable result of crossing with a foreign stock to be
attributed to the fact that this belongs to another systematic entity or to
the fact that the plants, though belonging to the same entity were exposed
to different conditions? This is a point on which further researches must
be taken into account, especially since the analysis of the systematic
entities has been much more thorough than formerly. (In the case of garden
plants, as Darwin to a large extent claimed, it is not easy to say whether
two individuals really belong to the same variety, as they are usually of
hybrid origin. In some instances (Petunia, Iberis) the fresh stock
employed by Darwin possessed flowers differing in colour from those of the
plant crossed with it.) We know that most of Linneaus's species are
compound species, frequently consisting of a very large number of smaller
or elementary species formerly included under the comprehensive term
varieties. Hybridisation has in most cases affected our garden and
cultivated plants so that they do not represent pure species but a mixture
of species.

But this consideration has no essential bearing on Darwin's point of view,
according to which the nature of the sexual cells is influenced by external
conditions. Even individuals growing close to one another are only
apparently exposed to identical conditions. Their sexual cells may
therefore be differently influenced and thus give favourable results on
crossing, as "the benefits which so generally follow from a cross between
two plants apparently depend on the two differing somewhat in constitution
or character." As a matter of fact we are familiar with a large number of
cases in which the condition of the reproductive organs is influenced by
external conditions. Darwin has himself demonstrated this for self-sterile
plants, that is plants in which self-fertilisation produces no result.
This self-sterility is affected by climatic conditions: thus in Brazil
Eschscholzia californica is absolutely sterile to the pollen of its own
flowers; the descendants of Brazilian plants in Darwin's cultures were
partially self-fertile in one generation and in a second generation still
more so. If one has any doubt in this case whether it is a question of the
condition of the style and stigma, which possibly prevents the entrance of
the pollen-tube or even its development, rather than that of the actual
sexual cells, in other cases there is no doubt that an influence is exerted
on the latter.

Janczewski (Janczewski, "Sur les antheres steriles des Groseilliers",
"Bull. de l'acad. des sciences de Cracovie", June, 1908.) has recently
shown that species of Ribes cultivated under unnatural conditions
frequently produce a mixed (i.e. partly useless) or completely sterile
pollen, precisely as happens with hybrids. There are, therefore,
substantial reasons for the conclusion that conditions of life exert an
influence on the sexual cells. "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." ("Cross and Self
fertilisation" (1st edition), page 444.)

We thus obtain an insight into the significance of sexuality. If an
occasional and slight alteration in the conditions under which plants and
animals live is beneficial (Reasons for this are given by Darwin in
"Variation under Domestication" (2nd edition), Vol. II. page 127.),
crossing between organisms which have been exposed to different conditions
becomes still more advantageous. The entire constitution is in this way
influenced from the beginning, at a time when the whole organisation is in
a highly plastic state. The total life-energy, so to speak, is increased,
a gain which is not produced by asexual reproduction or by the union of
sexual cells of plants which have lived under the same or only slightly
different conditions. All the wonderful arrangements for cross-
fertilisation now appear to be useful adaptations. Darwin was, however,
far from giving undue prominence to this point of view, though this has
been to some extent done by others. He particularly emphasised the
following consideration:--"But we should always keep in mind that two
somewhat opposed ends have to be gained; the first and more important one
being the production of seeds by any means, and the second, cross-
fertilisation." ("Cross and Self fertilisation" (1st edition), page 371.)
Just as in some orchids and cleistogamic flowers self-pollination regularly
occurs, so it may also occur in other cases. Darwin showed that Pisum
sativum and Lathyrus odoratus belong to plants in which self-pollination is
regularly effected, and that this accounts for the constancy of certain
sorts of these plants, while a variety of form is produced by crossing.
Indeed among his culture plants were some which derived no benefit from
crossing. Thus in the sixth self-fertilised generation of his Ipomoea
cultures the "Hero" made its appearance, a form slightly exceeding its
crossed companion in height; this was in the highest degree self-fertile
and handed on its characteristics to both children and grandchildren.
Similar forms were found in Mimulus luteus and Nicotiana (In Pisum sativum
also the crossing of two individuals of the same variety produced no
advantage; Darwin attributed this to the fact that the plants had for
several generations been self-fertilised and in each generation cultivated
under almost the same conditions. Tschermak ("Ueber kunstliche Kreuzung an
Pisum sativum") afterwards recorded the same result; but he found on
crossing different varieties that usually there was no superiority as
regards height over the products of self-fertilisation, while Darwin found
a greater height represented by the ratios 100:75 and 100:60.), types
which, after self-fertilisation, have an enhanced power of seed-production
and of attaining a greater height than the plants of the corresponding
generation which are crossed together and self-fertilised and grown under
the same conditions. "Some observations made on other plants lead me to
suspect that self-fertilisation is in some respects beneficial; although
the benefit thus derived is as a rule very small compared with that from a
cross with a distinct plant." ("Cross and Self fertilisation", page 350.)
We are as ignorant of the reason why plants behave differently when crossed
and self-fertilised as we are in regard to the nature of the
differentiation of the sexual cells, which determines whether a union of
the sexual cells will prove favourable or unfavourable.

It is impossible to discuss the different results of cross-fertilisation;
one point must, however, be emphasised, because Darwin attached
considerable importance to it. It is inevitable that pollen of different
kinds must reach the stigma. It was known that pollen of the same
"species" is dominant over the pollen of another species, that, in other
words, it is prepotent. Even if the pollen of the same species reaches the
stigma rather later than that of another species, the latter does not
effect fertilisation.

Darwin showed that the fertilising power of the pollen of another variety
or of another individual is greater than that of the plant's own pollen.
("Cross and Self fertilisation", page 391.) This has been demonstrated in
the case of Mimulus luteus (for the fixed white-flowering variety) and
Iberis umbellata with pollen of another variety, and observations on
cultivated plants, such as cabbage, horseradish, etc. gave similar results.
It is, however, especially remarkable that pollen of another individual of
the same variety may be prepotent over the plant's own pollen. This
results from the superiority of plants crossed in this manner over self-
fertilised plants. "Scarcely any result from my experiments has surprised
me so much as this of the prepotency of pollen from a distinct individual
over each plant's own pollen, as proved by the greater constitutional
vigour of the crossed seedlings." (Ibid. page 397.) Similarly, in self-
fertile plants the flowers of which have not been deprived of the male
organs, pollen brought to the stigma by the wind or by insects from another
plant effects fertilisation, even if the plant's own pollen has reached the
stigma somewhat earlier.

Have the results of his experimental investigations modified the point of
view from which Darwin entered on his researches, or not? In the first
place the question is, whether or not the opinion expressed in the Orchid
book that there is "Something injurious" connected with self-fertilisation,
has been confirmed. We can, at all events, affirm that Darwin adhered in
essentials to his original position; but self-fertilisation afterwards
assumed a greater importance than it formerly possessed. Darwin emphasised
the fact that "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 self-fertilisation." (Ibid. page 437.) But he had no
doubt that in favourable circumstances self-fertilised plants were able to
persist for several generations without crossing. An occasional crossing
appears to be useful but not indispensable in all cases; its sporadic
occurrence in plants in which self-pollination habitually occurs is not
excluded. Self-fertilisation is for the most part relatively and not
absolutely injurious and always better than no fertilisation. "Nature
abhors perpetual self-fertilisation" (It is incorrect to say, as a writer
has lately said, that the aphorism expressed by Darwin in 1859 and 1862,
"Nature abhors perpetual self-fertilisation," is not repeated in his later
works. The sentence is repeated in "Cross and Self fertilisation" (page
8), with the addition, "If the word perpetual had been omitted, the
aphorism would have been false. As it stands, I believe that it is true,
though perhaps rather too strongly expressed.") is, however, a pregnant
expression of the fact that cross-fertilisation is exceedingly widespread
and has been shown in the majority of cases to be beneficial, and that in
those plants in which we find self-pollination regularly occurring cross-
pollination may occasionally take place.

An attempt has been made to express in brief the main results of Darwin's
work on the biology of flowers. We have seen that his object was to
elucidate important general questions, particularly the question of the
significance of sexual reproduction.

It remains to consider what influence his work has had on botanical
science. That this influence has been very considerable, is shown by a
glance at the literature on the biology of flowers published since Darwin
wrote. Before the book on orchids was published there was nothing but the
old and almost forgotten works of Kolreuter and Sprengel with the exception
of a few scattered references. Darwin's investigations gave the first
stimulus to the development of an extensive literature on floral biology.
In Knuth's "Handbuch der Blutenbiologie" ("Handbook of Flower Pollination",
Oxford, 1906) as many as 3792 papers on this subject are enumerated as
having been published before January 1, 1904. These describe not only the
different mechanisms of flowers, but deal also with a series of remarkable
adaptations in the pollinating insects. As a fertilising rain quickly
calls into existence the most varied assortment of plants on a barren
steppe, so activity now reigns in a field which men formerly left deserted.
This development of the biology of flowers is of importance not only on
theoretical grounds but also from a practical point of view. The rational
breeding of plants is possible only if the flower-biology of the plants in
question (i.e. the question of the possibility of self-pollination, self-
sterility, etc.) is accurately known. And it is also essential for plant-
breeders that they should 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." ("Cross and Self fertilisation" (1st edition), page

But the influence of Darwin on floral biology was not confined to the
development of this branch of Botany. Darwin's activity in this domain has
brought about (as Asa Gray correctly pointed out) the revival of teleology
in Botany and Zoology. Attempts were now made to determine, not only in
the case of flowers but also in vegetative organs, in what relation the
form and function of organs stand to one another and to what extent their
morphological characters exhibit adaptation to environment. A branch of
Botany, which has since been called Ecology (not a very happy term) has
been stimulated to vigorous growth by floral biology.

While the influence of the work on the biology of flowers was
extraordinarily great, it could not fail to elicit opinions at variance
with Darwin's conclusions. The opposition was based partly on reasons
valueless as counterarguments, partly on problems which have still to be
solved; to some extent also on that tendency against teleological
conceptions which has recently become current. This opposing trend of
thought is due to the fact that many biologists are content with
teleological explanations, unsupported by proof; it is also closely
connected with the fact that many authors estimate the importance of
natural selection less highly than Darwin did. We may describe the
objections which are based on the widespread occurrence of self-
fertilisation and geitonogamy as of little importance. Darwin did not deny
the occurrence of self-fertilisation, even for a long series of
generations; his law states only that "Nature abhors PERPETUAL self-
fertilisation." (It is impossible (as has been attempted) to express
Darwin's point of view in a single sentence, such as H. Muller's statement
of the "Knight-Darwin law." The conditions of life in organisms are so
various and complex that laws, such as are formulated in physics and
chemistry, can hardly be conceived.) An exception to this rule would
therefore occur only in the case of plants in which the possibility of
cross-pollination is excluded. Some of the plants with cleistogamous
flowers might afford examples of such cases. We have already seen,
however, that such a case has not as yet been shown to occur. Burck
believed that he had found an instance in certain tropical plants
(Anonaceae, Myrmecodia) of the complete exclusion of cross-fertilisation.
The flowers of these plants, in which, however,--in contrast to the
cleistogamous flowers--the corolla is well developed, remain closed and
fruit is produced.

Loew (E. Loew, "Bemerkungen zu Burck...", "Biolog. Centralbl." XXVI.
(1906).) has shown that cases occur in which cross-fertilisation may be
effected even in these "cleistopetalous" flowers: humming birds visit the
permanently closed flowers of certain species of Nidularium and transport
the pollen. The fact that the formation of hybrids may occur as the result
of this shows that pollination may be accomplished.

The existence of plants for which self-pollination is of greater importance
than it is for others is by no means contradictory to Darwin's view. Self-
fertilisation is, for example, of greater importance for annuals than for
perennials as without it seeds might fail to be produced. Even in the case
of annual plants with small inconspicuous flowers in which self-
fertilisation usually occurs, such as Senecio vulgaris, Capsella bursa-
pastoris and Stellaria media, A. Bateson (Anna Bateson, "The effects of
cross-fertilisation on inconspicuous flowers", "Annals of Botany", Vol. I.
1888, page 255.) found that cross-fertilisation gave a beneficial result,
although only in a slight degree. If the favourable effects of sexual
reproduction, according to Darwin's view, are correlated with change of
environment, it is quite possible that this is of less importance in plants
which die after ripening their seeds ("hapaxanthic") and which in any case
constantly change their situation. Objections which are based on the proof
of the prevalence of self-fertilisation are not, therefore, pertinent. At
first sight another point of view, which has been more recently urged,
appears to have more weight.

W. Burck (Burck, "Darwin's Kreuzeungsgesetz...", "Biol. Centralbl". XXVIII.
1908, page 177.) has expressed the opinion that the beneficial results of
cross-fertilisation demonstrated by Darwin concern only hybrid plants.
These alone become weaker by self-pollination; while pure species derive no
advantage from crossing and no disadvantage from self-fertilisation. It is
certain that some of the plants used by Darwin were of hybrid origin. (It
is questionable if this was always the case.) This is evident from his
statements, which are models of clearness and precision; he says that his
Ipomoea plants "were probably the offspring of a cross." ("Cross and Self
fertilisation" (1st edition), page 55.) The fixed forms of this plant,
such as Hero, which was produced by self-fertilisation, and a form of
Mimulus with white flowers spotted with red probably resulted from
splitting of the hybrids. It is true that the phenomena observed in self-
pollination, e.g. in Ipomoea, agree with those which are often noticed in
hybrids; Darwin himself drew attention to this.

Let us next call to mind some of the peculiarities connected with
hybridisation. We know that hybrids are often characterized by their large
size, rapidity of growth, earlier production of flowers, wealth of flower-
production and a longer life; hybrids, if crossed with one of the two
parent forms, are usually more fertile than when they are crossed together
or with another hybrid. But the characters which hybrids exhibit on self-
fertilisation are rather variable. The following instance may be quoted
from Gartner: "There are many hybrids which retain the self-fertility of
the first generation during the second and later generations, but very
often in a less degree; a considerable number, however, become sterile."
But the hybrid varieties may be more fertile in the second generation than
in the first, and in some hybrids the fertility with their own pollen
increases in the second, third, and following generations. (K.F. Gartner,
"Versuche uber die Bastarderzeugung", Stuttgart, 1849, page 149.) As yet
it is impossible to lay down rules of general application for the self-
fertility of hybrids. That the beneficial influence of crossing with a
fresh stock rests on the same ground--a union of sexual cells possessing
somewhat different characters--as the fact that many hybrids are
distinguished by greater luxuriance, wealth of flowers, etc. corresponds
entirely with Darwin's conclusions. It seems to me to follow clearly from
his investigations that there is no essential difference between cross-
fertilisation and hybridisation. The heterostyled plants are normally
dependent on a process corresponding to hybridisation. The view that
specifically distinct species could at best produce sterile hybrids was
always opposed by Darwin. But if the good results of crossing were
EXCLUSIVELY dependent on the fact that we are concerned with hybrids, there
must then be a demonstration of two distinct things. First, that crossing
with a fresh stock belonging to the same systematic entity or to the same
hybrid, but cultivated for a considerable time under different conditions,
shows no superiority over self-fertilisation, and that in pure species
crossing gives no better results than self-pollination. If this were the
case, we should be better able to understand why in one plant crossing is
advantageous while in others, such as Darwin's Hero and the forms of
Mimulus and Nicotiana no advantage is gained; these would then be pure
species. But such a proof has not been supplied; the inference drawn from
cleistogamous and cleistopetalous plants is not supported by evidence, and
the experiments on geitonogamy and on the advantage of cross-fertilisation
in species which are usually self-fertilised are opposed to this view.
There are still but few researches on this point; Darwin found that in
Ononis minutissima, which produces cleistogamous as well as self-fertile
chasmogamous flowers, the crossed and self-fertilised capsules produced
seed in the proportion of 100:65 and that the average bore the proportion
100:86. Facts previously mentioned are also applicable to this case.
Further, it is certain that the self-sterility exhibited by many plants has
nothing to do with hybridisation. Between self-sterility and reduced
fertility as the result of self-fertilisation there is probably no
fundamental difference.

It is certain that so difficult a problem as that of the significance of
sexual reproduction requires much more investigation. Darwin was anything
but dogmatic and always ready to alter an opinion when it was not based on
definite proof: he wrote, "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." He has also
shown us the way along which to follow up this problem; it is that of
carefully planned and exact experimental research. It may be that
eventually many things will be viewed in a different light, but Darwin's
investigations will always form the foundation of Floral Biology on which
the future may continue to build.



In developing his conception of organic evolution Charles Darwin was of
necessity brought into contact with some of the problems of mental
evolution. In "The Origin of Species" he devoted a chapter to "the
diversities of instinct and of the other mental faculties in animals of the
same class." ("Origin of Species" (6th edition), page 205.) When he
passed to the detailed consideration of "The Descent of Man", it was part
of his object to show "that there is no fundamental difference between man
and the higher mammals in their mental faculties." ("Descent of Man" (2nd
edition 1888), Vol. I. page 99; Popular edition page 99.) "If no organic
being excepting man," he said, "had possessed any mental power, or if his
powers had been of a wholly different nature from those of the lower
animals, then we should never have been able to convince ourselves that our
high faculties had been gradually developed." (Ibid. page 99.) In his
discussion of "The Expression of the Emotions" it was important for his
purpose "fully to recognise that actions readily become associated with
other actions and with various states of the mind." ("The Expression of
the Emotions" (2nd edition), page 32.) His hypothesis of sexual selection
is largely dependent upon the exercise of choice on the part of the female
and her preference for "not only the more attractive but at the same time
the more vigorous and victorious males." ("Descent of Man", Vol. II. page
435.) Mental processes and physiological processes were for Darwin closely
correlated; and he accepted the conclusion "that the nervous system not
only regulates most of the existing functions of the body, but has
indirectly influenced the progressive development of various bodily
structures and of certain mental qualities." (Ibid. pages 437, 438.)

Throughout his treatment, mental evolution was for Darwin incidental to and
contributory to organic evolution. For specialised research in comparative
and genetic psychology, as an independent field of investigation, he had
neither the time nor the requisite training. None the less his writings
and the spirit of his work have exercised a profound influence on this
department of evolutionary thought. And, for those who follow Darwin's
lead, mental evolution is still in a measure subservient to organic
evolution. Mental processes are the accompaniments or concomitants of the
functional activity of specially differentiated parts of the organism.
They are in some way dependent on physiological and physical conditions.
But though they are not physical in their nature, and though it is
difficult or impossible to conceive that they are physical in their origin,
they are, for Darwin and his followers, factors in the evolutionary process
in its physical or organic aspect. By the physiologist within his special
and well-defined universe of discourse they may be properly regarded as
epiphenomena; but by the naturalist in his more catholic survey of nature
they cannot be so regarded, and were not so regarded by Darwin.
Intelligence has contributed to evolution of which it is in a sense a

The facts of observation or of inference which Darwin accepted are these:
Conscious experience accompanies some of the modes of animal behaviour; it
is concomitant with certain physiological processes; these processes are
the outcome of development in the individual and evolution in the race; the
accompanying mental processes undergo a like development. Into the subtle
philosophical questions which arise out of the naive acceptance of such a
creed it was not Darwin's province to enter; "I have nothing to do," he
said ("Origin of Species" (6th edition), page 205.), "with the origin of
the mental powers, any more than I have with that of life itself." He
dealt with the natural history of organisms, including not only their
structure but their modes of behaviour; with the natural history of the
states of consciousness which accompany some of their actions; and with the
relation of behaviour to experience. We will endeavour to follow Darwin in
his modesty and candour in making no pretence to give ultimate
explanations. But we must note one of the implications of this self-
denying ordinance of science. Development and evolution imply continuity.
For Darwin and his followers the continuity is organic through physical
heredity. Apart from speculative hypothesis, legitimate enough in its
proper place but here out of court, we know nothing of continuity of mental
evolution as such: consciousness appears afresh in each succeeding
generation. Hence it is that for those who follow Darwin's lead, mental
evolution is and must ever be, within his universe of discourse,
subservient to organic evolution. Only in so far as conscious experience,
or its neural correlate, effects some changes in organic structure can it
influence the course of heredity; and conversely only in so far as changes
in organic structure are transmitted through heredity, is mental evolution
rendered possible. Such is the logical outcome of Darwin's teaching.

Those who abide by the cardinal results of this teaching are bound to
regard all behaviour as the expression of the functional activities of the
living tissues of the organism, and all conscious experience as correlated
with such activities. For the purposes of scientific treatment, mental
processes are one mode of expression of the same changes of which the
physiological processes accompanying behaviour are another mode of
expression. This is simply accepted as a fact which others may seek to
explain. The behaviour itself is the adaptive application of the energies
of the organism; it is called forth by some form of presentation or
stimulation brought to bear on the organism by the environment. This
presentation is always an individual or personal matter. But in order that
the organism may be fitted to respond to the presentation of the
environment it must have undergone in some way a suitable preparation.
According to the theory of evolution this preparation is primarily racial
and is transmitted through heredity. Darwin's main thesis was that the
method of preparation is predominantly by natural selection. Subordinate
to racial preparation, and always dependent thereon, is individual or
personal preparation through some kind of acquisition; of which the
guidance of behaviour through individually won experience is a typical
example. We here introduce the mental factor because the facts seem to
justify the inference. Thus there are some modes of behaviour which are
wholly and solely dependent upon inherited racial preparation; there are
other modes of behaviour which are also dependent, in part at least, on
individual preparation. In the former case the behaviour is adaptive on
the first occurrence of the appropriate presentation; in the latter case
accommodation to circumstances is only reached after a greater or less
amount of acquired organic modification of structure, often accompanied (as
we assume) in the higher animals by acquired experience. Logically and
biologically the two classes of behaviour are clearly distinguishable: but
the analysis of complex cases of behaviour where the two factors cooperate,
is difficult and requires careful and critical study of life-history.

The foundations of the mental life are laid in the conscious experience
that accompanies those modes of behaviour, dependent entirely on racial
preparation, which may broadly be described as instinctive. In the eighth
chapter of "The Origin of Species" Darwin says ("Origin of Species" (6th
edition), page 205.), "I will not attempt any definition of
instinct...Every one understands what is meant, when it is said that
instinct impels the cuckoo to migrate and to lay her eggs in other birds'
nests. An action, which we ourselves require experience to enable us to
perform, when performed by an animal, more especially by a very young one,
without experience, and when performed by many individuals in the same way,
without their knowing for what purpose it is performed, is usually said to
be instinctive." And in the summary at the close of the chapter he says
("Origin of Species" (6th edition), page 233.), "I have endeavoured briefly
to show that the mental qualities of our domestic animals vary, and that
the variations are inherited. Still more briefly I have attempted to show
that instincts vary slightly in a state of nature. No one will dispute
that instincts are of the highest importance to each animal. Therefore
there is no real difficulty, under changing conditions of life, in natural
selection accumulating to any extent slight modifications of instinct which
are in any way useful. In many cases habit or use and disuse have probably
come into play."

Into the details of Darwin's treatment there is neither space nor need to
enter. There are some ambiguous passages; but it may be said that for him,
as for his followers to-day, instinctive behaviour is wholly the result of
racial preparation transmitted through organic heredity. For the
performance of the instinctive act no individual preparation under the
guidance of personal experience is necessary. It is true that Darwin
quotes with approval Huber's saying that "a little dose of judgment or
reason often comes into play, even with animals low in the scale of
nature." (Ibid. page 205.) But we may fairly interpret his meaning to be
that in behaviour, which is commonly called instinctive, some element of
intelligent guidance is often combined. If this be conceded the strictly
instinctive performance (or part of the performance) is the outcome of
heredity and due to the direct transmission of parental or ancestral
aptitudes. Hence the instinctive response as such depends entirely on how
the nervous mechanism has been built up through heredity; while intelligent
behaviour, or the intelligent factor in behaviour, depends also on how the
nervous mechanism has been modified and moulded by use during its
development and concurrently with the growth of individual experience in
the customary situations of daily life. Of course it is essential to the
Darwinian thesis that what Sir E. Ray Lankester has termed "educability,"
not less than instinct, is hereditary. But it is also essential to the
understanding of this thesis that the differentiae of the hereditary
factors should be clearly grasped.

For Darwin there were two modes of racial preparation, (1) natural
selection, and (2) the establishment of individually acquired habit. He
showed that instincts are subject to hereditary variation; he saw that
instincts are also subject to modification through acquisition in the
course of individual life. He believed that not only the variations but
also, to some extent, the modifications are inherited. He therefore held
that some instincts (the greater number) are due to natural selection but
that others (less numerous) are due, or partly due, to the inheritance of
acquired habits. The latter involve Lamarckian inheritance, which of late
years has been the centre of so much controversy. It is noteworthy however
that Darwin laid especial emphasis on the fact that many of the most
typical and also the most complex instincts--those of neuter insects--do
not admit of such an interpretation. "I am surprised," he says ("Origin of
Species" (6th edition), page 233.), "that no one has hitherto advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck." None the less Darwin admitted
this doctrine as supplementary to that which was more distinctively his
own--for example in the case of the instincts of domesticated animals.
Still, even in such cases, "it may be doubted," he says (Ibid. pages 210,
211.), "whether any one would have thought of training a dog to point, had
not some one dog naturally shown a tendency in this line...so that habit
and some degree of selection have probably concurred in civilising by
inheritance our dogs." But in the interpretation of the instincts of
domesticated animals, a more recently suggested hypothesis, that of organic
selection (Independently suggested, on somewhat different lines, by Profs.
J. Mark Baldwin, Henry F. Osborn and the writer.), may be helpful.
According to this hypothesis any intelligent modification of behaviour
which is subject to selection is probably coincident in direction with an
inherited tendency to behave in this fashion. Hence in such behaviour
there are two factors: (1) an incipient variation in the line of such
behaviour, and (2) an acquired modification by which the behaviour is
carried further along the same line. Under natural selection those
organisms in which the two factors cooperate are likely to survive. Under
artificial selection they are deliberately chosen out from among the rest.

Organic selection has been termed a compromise between the more strictly
Darwinian and the Lamarckian principles of interpretation. But it is not
in any sense a compromise. The principle of interpretation of that which
is instinctive and hereditary is wholly Darwinian. It is true that some of
the facts of observation relied upon by Lamarckians are introduced. For
Lamarckians however the modifications which are admittedly factors in
survival, are regarded as the parents of inherited variations; for
believers in organic selection they are only the foster parents or nurses.
It is because organic selection is the direct outcome of and a natural
extension of Darwin's cardinal thesis that some reference to it here is
justifiable. The matter may be put with the utmost brevity as follows.
(1) Variations (V) occur, some of which are in the direction of increased
adaptation (+), others in the direction of decreased adaptation (-). (2)
Acquired modifications (M) also occur. Some of these are in the direction
of increased accommodation to circumstances (+), while others are in the
direction of diminished accommodation (-). Four major combinations are

(a) + V with + M,
(b) + V with - M,
(c) - V with + M,
(d) - V with - M.

Of these (d) must inevitably be eliminated while (a) are selected. The
predominant survival of (a) entails the survival of the adaptive variations
which are inherited. The contributory acquisitions (+M) are not inherited;
but they are none the less factors in determining the survival of the
coincident variations. It is surely abundantly clear that this is
Darwinism and has no tincture of Lamarck's essential principle, the
inheritance of acquired characters.

Whether Darwin himself would have accepted this interpretation of some at
least of the evidence put forward by Lamarckians is unfortunately a matter
of conjecture. The fact remains that in his interpretation of instinct and
in allied questions he accepted the inheritance of individually acquired
modifications of behaviour and structure.

Darwin was chiefly concerned with instinct from the biological rather than
from the psychological point of view. Indeed it must be confessed that,
from the latter standpoint, his conception of instinct as a "mental
faculty" which "impels" an animal to the performance of certain actions,
scarcely affords a satisfactory basis for genetic treatment. To carry out
the spirit of Darwin's teaching it is necessary to link more closely
biological and psychological evolution. The first step towards this is to
interpret the phenomena of instinctive behaviour in terms of stimulation
and response. It may be well to take a particular case. Swimming on the
part of a duckling is, from the biological point of view, a typical example
of instinctive behaviour. Gently lower a recently hatched bird into water:
coordinated movements of the limbs follow in rhythmical sequence. The
behaviour is new to the individual though it is no doubt closely related to
that of walking, which is no less instinctive. There is a group of stimuli
afforded by the "presentation" which results from partial immersion: upon
this there follows as a complex response an application of the functional
activities in swimming; the sequence of adaptive application on the
appropriate presentation is determined by racial preparation. We know, it
is true, but little of the physiological details of what takes place in the
central nervous system; but in broad outline the nature of the organic
mechanism and the manner of its functioning may at least be provisionally
conjectured in the present state of physiological knowledge. Similarly in
the case of the pecking of newly-hatched chicks; there is a visual
presentation, there is probably a cooperating group of stimuli from the
alimentary tract in need of food, there is an adaptive application of the
activities in a definite mode of behaviour. Like data are afforded in a
great number of cases of instinctive procedure, sometimes occurring very
early in life, not infrequently deferred until the organism is more fully
developed, but all of them dependent upon racial preparation. No doubt
there is some range of variation in the behaviour, just such variation as
the theory of natural selection demands. But there can be no question that
the higher animals inherit a bodily organisation and a nervous system, the
functional working of which gives rise to those inherited modes of
behaviour which are termed instinctive.

It is to be noted that the term "instinctive" is here employed in the
adjectival form as a descriptive heading under which may be grouped many
and various modes of behaviour due to racial preparation. We speak of
these as inherited; but in strictness what is transmitted through heredity
is the complex of anatomical and physiological conditions under which, in
appropriate circumstances, the organism so behaves. So far the term
"instinctive" has a restricted biological connotation in terms of
behaviour. But the connecting link between biological evolution and
psychological evolution is to be sought,--as Darwin fully realised,--in the
phenomena of instinct, broadly considered. The term "instinctive" has also
a psychological connotation. What is that connotation?

Let us take the case of the swimming duckling or the pecking chick, and fix
our attention on the first instinctive performance. Grant that just as
there is, strictly speaking, no inherited behaviour, but only the
conditions which render such behaviour under appropriate circumstances
possible; so too there is no inherited experience, but only the conditions
which render such experience possible; then the cerebral conditions in both
cases are the same. The biological behaviour-complex, including the total
stimulation and the total response with the intervening or resultant
processes in the sensorium, is accompanied by an experience-complex
including the initial stimulation-consciousness and resulting response-
consciousness. In the experience-complex are comprised data which in
psychological analysis are grouped under the headings of cognition,
affective tone and conation. But the complex is probably experienced as an
unanalysed whole. If then we use the term "instinctive" so as to comprise
all congenital modes of behaviour which contribute to experience, we are in
a position to grasp the view that the net result in consciousness
constitutes what we may term the primary tissue of experience. To the
development of this experience each instinctive act contributes. The
nature and manner of organisation of this primary tissue of experience are
dependent on inherited biological aptitudes; but they are from the outset
onwards subject to secondary development dependent on acquired aptitudes.
Biological values are supplemented by psychological values in terms of
satisfaction or the reverse.

In our study of instinct we have to select some particular phase of animal
behaviour and isolate it so far as is possible from the life of which it is
a part. But the animal is a going concern, restlessly active in many ways.
Many instinctive performances, as Darwin pointed out ("Origin of Species"
(6th edition), page 206.), are serial in their nature. But the whole of
active life is a serial and coordinated business. The particular
instinctive performance is only an episode in a life-history, and every
mode of behaviour is more or less closely correlated with other modes.
This coordination of behaviour is accompanied by a correlation of the modes
of primary experience. We may classify the instinctive modes of behaviour
and their accompanying modes of instinctive experience under as many heads
as may be convenient for our purposes of interpretation, and label them
instincts of self-preservation, of pugnacity, of acquisition, the
reproductive instincts, the parental instincts, and so forth. An instinct,
in this sense of the term (for example the parental instinct), may be
described as a specialised part of the primary tissue of experience
differentiated in relation to some definite biological end. Under such an
instinct will fall a large number of particular and often well-defined
modes of behaviour, each with its own peculiar mode of experience.

It is no doubt exceedingly difficult as a matter of observation and of
inference securely based thereon to distinguish what is primary from what
is in part due to secondary acquisition--a fact which Darwin fully
appreciated. Animals are educable in different degrees; but where they are
educable they begin to profit by experience from the first. Only,
therefore, on the occasion of the first instinctive act of a given type can
the experience gained be weighed as WHOLLY primary; all subsequent
performance is liable to be in some degree, sometimes more, sometimes less,
modified by the acquired disposition which the initial behaviour engenders.
But the early stages of acquisition are always along the lines
predetermined by instinctive differentiation. It is the task of
comparative psychology to distinguish the primary tissue of experience from
its secondary and acquired modifications. We cannot follow up the matter
in further detail. It must here suffice to suggest that this conception of
instinct as a primary form of experience lends itself better to natural
history treatment than Darwin's conception of an impelling force, and that
it is in line with the main trend of Darwin's thought.

In a characteristic work,--characteristic in wealth of detail, in closeness
and fidelity of observation, in breadth of outlook, in candour and
modesty,--Darwin dealt with "The Expression of the Emotions in Man and
Animals". Sir Charles Bell in his "Anatomy of Expression" had contended
that many of man's facial muscles had been specially created for the sole
purpose of being instrumental in the expression of his emotions. Darwin
claimed that a natural explanation, consistent with the doctrine of
evolution, could in many cases be given and would in other cases be
afforded by an extension of the principles he advocated. "No doubt," he
said ("Expression of the Emotions", page 13. The passage is here somewhat
condensed.), "as long as man and all other animals are viewed as
independent creations, an effectual stop is put to our natural desire to
investigate as far as possible the causes of Expression. By this doctrine,
anything and everything can be equally well explained...With mankind, some
expressions...can hardly be understood, except on the belief that man once
existed in a much lower and animal-like condition. The community of
certain expressions in distinct though allied species...is rendered
somewhat more intelligible, if we believe in their descent from a common
progenitor. He who admits on general grounds that the structure and habits
of all animals have been gradually evolved, will look at the whole subject
of Expression in a new and interesting light."

Darwin relied on three principles of explanation. "The first of these
principles is, that movements which are serviceable in gratifying some
desire, or in relieving some sensation, if often repeated, become so
habitual that they are performed, whether or not of any service, whenever
the same desire or sensation is felt, even in a very weak degree." (Ibid.
page 368.) The modes of expression which fall under this head have become
instinctive through the hereditary transmission of acquired habit. "As far
as we can judge, only a few expressive movements are learnt by each
individual; that is, were consciously and voluntarily performed during the
early years of life for some definite object, or in imitation of others,
and then became habitual. The far greater number of the movements of
expression, and all the more important ones, are innate or inherited; and
such cannot be said to depend on the will of the individual. Nevertheless,
all those included under our first principle were at first voluntarily
performed for a definite object,--namely, to escape some danger, to relieve
some distress, or to gratify some desire." (Ibid. pages 373, 374.)

"Our second principle is that of antithesis. The habit of voluntarily
performing opposite movements under opposite impulses has become firmly
established in us by the practice of our whole lives. Hence, if certain
actions have been regularly performed, in accordance with our first
principle, under a certain frame of mind, there will be a strong and
involuntary tendency to the performance of directly opposite actions,
whether or not these are of any use, under the excitement of an opposite
frame of mind." ("Expression of the Emotions", page 368.) This principle
of antithesis has not been widely accepted. Nor is Darwin's own position
easy to grasp.

"Our third principle," he says (Ibid. page 369.), "is the direct action of
the excited nervous system on the body, independently of the will, and
independently, in large part, of habit. Experience shows that nerve-force
is generated and set free whenever the cerebro-spinal system is excited.
The direction which this nerve-force follows is necessarily determined by
the lines of connection between the nerve-cells, with each other and with
various parts of the body."

Lack of space prevents our following up the details of Darwin's treatment
of expression. Whether we accept or do not accept his three principles of
explanation we must regard his work as a masterpiece of descriptive
analysis, packed full of observations possessing lasting value. For a
further development of the subject it is essential that the instinctive
factors in expression should be more fully distinguished from those which
are individually acquired--a difficult task--and that the instinctive
factors should be rediscussed in the light of modern doctrines of heredity,
with a view to determining whether Lamarckian inheritance, on which Darwin
so largely relied, is necessary for an interpretation of the facts.

The whole subject as Darwin realised is very complex. Even the term
"expression" has a certain amount of ambiguity. When the emotion is in
full flood the animal fights, flees, or faints. Is this full-tide effect
to be regarded as expression; or are we to restrict the term to the
premonitory or residual effects--the bared canine when the fighting mood is
being roused, the ruffled fur when reminiscent representations of the
object inducing anger cross the mind? Broadly considered both should be
included. The activity of premonitory expression as a means of
communication was recognised by Darwin; he might, perhaps, have emphasised
it more strongly in dealing with the lower animals. Man so largely relies
on a special means of communication, that of language, that he sometimes
fails to realise that for animals with their keen powers of perception, and
dependent as they are on such means of communication, the more strictly
biological means of expression are full of subtle suggestiveness. Many
modes of expression, otherwise useless, are signs of behaviour that may be
anticipated,--signs which stimulate the appropriate attitude of response.
This would not, however, serve to account for the utility of the organic
accompaniments--heart-affection, respiratory changes, vaso-motor effects
and so forth, together with heightened muscular tone,--on all of which
Darwin lays stress ("Expression of the Emotions", pages 65 ff.) under his
third principle. The biological value of all this is, however, of great
importance, though Darwin was hardly in a position to take it fully into

Having regard to the instinctive and hereditary factors of emotional
expression we may ask whether Darwin's third principle does not alone
suffice as an explanation. Whether we admit or reject Lamarckian
inheritance it would appear that all hereditary expression must be due to
pre-established connections within the central nervous system and to a
transmitted provision for coordinated response under the appropriate
stimulation. If this be so, Darwin's first and second principles are
subordinate and ancillary to the third, an expression, so far as it is
instinctive or hereditary, being "the direct result of the constitution of
the nervous system."

Darwin accepted the emotions themselves as hereditary or acquired states of
mind and devoted his attention to their expression. But these emotions
themselves are genetic products and as such dependent on organic
conditions. It remained, therefore, for psychologists who accepted
evolution and sought to build on biological foundations to trace the
genesis of these modes of animal and human experience. The subject has
been independently developed by Professors Lange and James (Cf. William
James, "Principles of Psychology", Vol. II. Chap. XXV, London, 1890.); and
some modification of their view is regarded by many evolutionists as
affording the best explanation of the facts. We must fix our attention on
the lower emotions, such as anger or fear, and on their first occurrence in
the life of the individual organism. It is a matter of observation that if
a group of young birds which have been hatched in an incubator are
frightened by an appropriate presentation, auditory or visual, they
instinctively respond in special ways. If we speak of this response as the
expression, we find that there are many factors. There are certain visible
modes of behaviour, crouching at once, scattering and then crouching,
remaining motionless, the braced muscles sustaining an attitude of arrest,
and so forth. There are also certain visceral or organic effects, such as
affections of the heart and respiration. These can be readily observed by
taking the young bird in the hand. Other effects cannot be readily
observed; vaso-motor changes, affections of the alimentary canal, the skin
and so forth. Now the essence of the James-Lange view, as applied to these
congenital effects, is that though we are justified in speaking of them as
effects of the stimulation, we are not justified, without further evidence,
in speaking of them as effects of the emotional state. May it not rather
be that the emotion as a primary mode of experience is the concomitant of
the net result of the organic situation--the initial presentation, the
instinctive mode of behaviour, the visceral disturbances? According to
this interpretation the primary tissue of experience of the emotional
order, felt as an unanalysed complex, is generated by the stimulation of
the sensorium by afferent or incoming physiological impulses from the
special senses, from the organs concerned in the responsive behaviour, from
the viscera and vaso-motor system.

Some psychologists, however, contend that the emotional experience is
generated in the sensorium prior to, and not subsequent to, the behaviour-
response and the visceral disturbances. It is a direct and not an indirect
outcome of the presentation to the special senses. Be this as it may,
there is a growing tendency to bring into the closest possible relation, or
even to identify, instinct and emotion in their primary genesis. The
central core of all such interpretations is that instinctive behaviour and
experience, its emotional accompaniments, and its expression, are but
different aspects of the outcome of the same organic occurrences. Such
emotions are, therefore, only a distinguishable aspect of the primary
tissue of experience and exhibit a like differentiation. Here again a
biological foundation is laid for a psychological doctrine of the mental
development of the individual.

The intimate relation between emotion as a psychological mode of experience
and expression as a group of organic conditions has an important bearing on
biological interpretation. The emotion, as the psychological accompaniment
of orderly disturbances in the central nervous system profoundly influences
behaviour and often renders it more vigorous and more effective. The
utility of the emotions in the struggle for existence can, therefore,
scarcely be over-estimated. Just as keenness of perception has survival-
value; just as it is obviously subject to variation; just as it must be
enhanced under natural selection, whether individually acquired increments
are inherited or not; and just as its value lies not only in this or that
special perceptive act but in its importance for life as a whole; so the
vigorous effectiveness of activity has survival-value; it is subject to
variation; it must be enhanced under natural selection; and its importance
lies not only in particular modes of behaviour but in its value for life as
a whole. If emotion and its expression as a congenital endowment are but
different aspects of the same biological occurrence; and if this is a
powerful supplement to vigour effectiveness and persistency of behaviour,
it must on Darwin's principles be subject to natural selection.

If we include under the expression of the emotions not only the premonitory
symptoms of the initial phases of the organic and mental state, not only
the signs or conditions of half-tide emotion, but the full-tide
manifestation of an emotion which dominates the situation, we are naturally
led on to the consideration of many of the phenomena which are discussed
under the head of sexual selection. The subject is difficult and complex,
and it was treated by Darwin with all the strength he could summon to the
task. It can only be dealt with here from a special point of view--that
which may serve to illustrate the influence of certain mental factors on
the course of evolution. From this point of view too much stress can
scarcely be laid on the dominance of emotion during the period of courtship
and pairing in the more highly organised animals. It is a period of
maximum vigour, maximum activity, and, correlated with special modes of
behaviour and special organic and visceral accompaniments, a period also of
maximum emotional excitement. The combats of males, their dances and
aerial evolutions, their elaborate behaviour and display, or the flood of
song in birds, are emotional expressions which are at any rate coincident
in time with sexual periodicity. From the combat of the males there
follows on Darwin's principles the elimination of those which are deficient
in bodily vigour, deficient in special structures, offensive or protective,
which contribute to success, deficient in the emotional supplement of which
persistent and whole-hearted fighting is the expression, and deficient in
alertness and skill which are the outcome of the psychological development
of the powers of perception. Few biologists question that we have here a
mode of selection of much importance, though its influence on psychological
evolution often fails to receive its due emphasis. Mr Wallace
("Darwinism", pages 282, 283, London, 1889.) regards it as "a form of
natural selection"; "to it," he says, "we must impute the development of
the exceptional strength, size, and activity of the male, together with the
possession of special offensive and defensive weapons, and of all other
characters which arise from the development of these or are correlated with
them." So far there is little disagreement among the followers of Darwin--
for Mr Wallace, with fine magnanimity, has always preferred to be ranked as
such, notwithstanding his right, on which a smaller man would have
constantly insisted, to the claim of independent originator of the doctrine
of natural selection. So far with regard to sexual selection Darwin and Mr
Wallace are agreed; so far and no farther. For Darwin, says Mr Wallace
(Ibid. page 283.), "has extended the principle into a totally different
field of action, which has none of that character of constancy and of
inevitable result that attaches to natural selection, including male
rivalry; for by far the larger portion of the phenomena, which he
endeavours to explain by the direct action of sexual selection, can only be
so explained on the hypothesis that the immediate agency is female choice
or preference. It is to this that he imputes the origin of all secondary
sexual characters other than weapons of offence and defence...In this
extension of sexual selection to include the action of female choice or
preference, and in the attempt to give to that choice such wide-reaching
effects, I am unable to follow him more than a very little way."

Into the details of Mr Wallace's criticisms it is impossible to enter here.
We cannot discuss either the mode of origin of the variations in structure
which have rendered secondary sexual characters possible or the modes of
selection other than sexual which have rendered them, within narrow limits,
specifically constant. Mendelism and mutation theories may have something
to say on the subject when these theories have been more fully correlated
with the basal principles of selection. It is noteworthy that Mr Wallace
says ("Darwinism", pages 283, 284.): "Besides the acquisition of weapons
by the male for the purpose of fighting with other males, there are some
other sexual characters which may have been produced by natural selection.
Such are the various sounds and odours which are peculiar to the male, and
which serve as a call to the female or as an indication of his presence.
These are evidently a valuable addition to the means of recognition of the
two sexes, and are a further indication that the pairing season has
arrived; and the production, intensification, and differentiation of these
sounds and odours are clearly within the power of natural selection. The
same remark will apply to the peculiar calls of birds, and even to the
singing of the males." Why the same remark should not apply to their
colours and adornments is not obvious. What is obvious is that "means of
recognition" and "indication that the pairing season has arrived" are
dependent on the perceptive powers of the female who recognises and for
whom the indication has meaning. The hypothesis of female preference,
stripped of the aesthetic surplusage which is psychologically both
unnecessary and unproven, is really only different in degree from that
which Mr Wallace admits in principle when he says that it is probable that
the female is pleased or excited by the display.

Let us for our present purpose leave on one side and regard as sub judice
the question whether the specific details of secondary sexual characters
are the outcome of female choice. For us the question is whether certain
psychological accompaniments of the pairing situation have influenced the
course of evolution and whether these psychological accompaniments are
themselves the outcome of evolution. As a matter of observation, specially
differentiated modes of behaviour, often very elaborate, frequently
requiring highly developed skill, and apparently highly charged with
emotional tone, are the precursors of pairing. They are generally confined
to the males, whose fierce combats during the period of sexual activity are
part of the emotional manifestation. It is inconceivable that they have no
biological meaning; and it is difficult to conceive that they have any
other biological end than to evoke in the generally more passive female the
pairing impulse. They are based on instinctive foundations ingrained in
the nervous constitution through natural (or may we not say sexual?)
selection in virtue of their profound utility. They are called into play
by a specialised presentation such as the sight or the scent of the female
at, or a little in advance of, a critical period of the physiological
rhythm. There is no necessity that the male should have any knowledge of
the end to which his strenuous activity leads up. In presence of the
female there is an elaborate application of all the energies of behaviour,
just because ages of racial preparation have made him biologically and
emotionally what he is--a functionally sexual male that must dance or sing
or go through hereditary movements of display, when the appropriate
stimulation comes. Of course after the first successful courtship his
future behaviour will be in some degree modified by his previous
experience. No doubt during his first courtship he is gaining the primary
data of a peculiarly rich experience, instinctive and emotional. But the
biological foundations of the behaviour of courtship are laid in the
hereditary coordinations. It would seem that in some cases, not indeed in
all, but perhaps especially in those cases in which secondary sexual
behaviour is most highly evolved,--correlative with the ardour of the male
is a certain amount of reluctance in the female. The pairing act on her
part only takes place after prolonged stimulation, for affording which the
behaviour of male courtship is the requisite presentation. The most
vigorous, defiant and mettlesome male is preferred just because he alone
affords a contributory stimulation adequate to evoke the pairing impulse
with its attendant emotional tone.

It is true that this places female preference or choice on a much lower
psychological plane than Darwin in some passages seems to contemplate
where, for example, he says that the female appreciates the display of the
male and places to her credit a taste for the beautiful. But Darwin
himself distinctly states ("Descent of Man" (2nd edition), Vol. II. pages
136, 137; (Popular edition), pages 642, 643.) that "it is not probable that
she consciously deliberates; but she is most excited or attracted by the
most beautiful, or melodious, or gallant males." The view here put
forward, which has been developed by Prof. Groos ("The Play of Animals",
page 244, London, 1898.), therefore seems to have Darwin's own sanction.
The phenomena are not only biological; there are psychological elements as
well. One can hardly suppose that the female is unconscious of the male's
presence; the final yielding must surely be accompanied by heightened
emotional tone. Whether we call it choice or not is merely a matter of
definition of terms. The behaviour is in part determined by supplementary
psychological values. Prof. Groos regards the coyness of females as "a
most efficient means of preventing the too early and too frequent yielding
to the sexual impulse." (Ibid. page 283.) Be that as it may, it is, in
any case, if we grant the facts, a means through which male sexual
behaviour with all its biological and psychological implications, is raised
to a level otherwise perhaps unattainable by natural means, while in the
female it affords opportunities for the development in the individual and
evolution in the race of what we may follow Darwin in calling appreciation,
if we empty this word of the aesthetic implications which have gathered
round it in the mental life of man.

Regarded from this standpoint sexual selection, broadly considered, has
probably been of great importance. The psychological accompaniments of the
pairing situation have profoundly influenced the course of biological
evolution and are themselves the outcome of that evolution.

Darwin makes only passing reference to those modes of behaviour in animals
which go by the name of play. "Nothing," he says ("Descent of Man", Vol.
II. page 60; (Popular edition), page 566.), "is more common than for
animals to take pleasure in practising whatever instinct they follow at
other times for some real good." This is one of the very numerous cases in
which a hint of the master has served to stimulate research in his
disciples. It was left to Prof. Groos to develop this subject on
evolutionary lines and to elaborate in a masterly manner Darwin's
suggestion. "The utility of play," he says ("The Play of Animals", page
76.), "is incalculable. This utility consists in the practice and exercise
it affords for some of the more important duties of life,"--that is to say,
for the performance of activities which will in adult life be essential to
survival. He urges (Ibid. page 75.) that "the play of young animals has
its origin in the fact that certain very important instincts appear at a
time when the animal does not seriously need them." It is, however,
questionable whether any instincts appear at a time when they are not
needed. And it is questionable whether the instinctive and emotional
attitude of the play-fight, to take one example, can be identified with
those which accompany fighting in earnest, though no doubt they are closely
related and have some common factors. It is probable that play, as
preparatory behaviour, differs in biological detail (as it almost certainly
does in emotional attributes) from the earnest of after-life and that it
has been evolved through differentiation and integration of the primary
tissue of experience, as a preparation through which certain essential
modes of skill may be acquired--those animals in which the preparatory
play-propensity was not inherited in due force and requisite amount being
subsequently eliminated in the struggle for existence. In any case there
is little question that Prof. Groos is right in basing the play-propensity
on instinctive foundations. ("The Play of Animals" page 24.) None the
less, as he contends, the essential biological value of play is that it is
a means of training the educable nerve-tissue, of developing that part of
the brain which is modified by experience and which thus acquires new
characters, of elaborating the secondary tissue of experience on the
predetermined lines of instinctive differentiation and thus furthering the
psychological activities which are included under the comprehensive term

In "The Descent of Man" Darwin dealt at some length with intelligence and
the higher mental faculties. ("Descent of Man" (1st edition), Chapters II,
III, V; (2nd edition), Chapters III, IV, V.) His object, he says, is to
show that there is no fundamental difference between man and the higher
mammals in their mental faculties; that these faculties are variable and
the variations tend to be inherited; and that under natural selection
beneficial variations of all kinds will have been preserved and injurious
ones eliminated.

Darwin was too good an observer and too honest a man to minimise the
"enormous difference" between the level of mental attainment of civilised
man and that reached by any animal. His contention was that the
difference, great as it is, is one of degree and not of kind. He realised
that, in the development of the mental faculties of man, new factors in
evolution have supervened--factors which play but a subordinate and
subsidiary part in animal intelligence. Intercommunication by means of
language, approbation and blame, and all that arises out of reflective
thought, are but foreshadowed in the mental life of animals. Still he
contends that these may be explained on the doctrine of evolution. He
urges (Ibid. Vol. I. pages 70, 71; (Popular edition), pages 70, 71.)" that
man is variable in body and mind; and that the variations are induced,
either directly or indirectly, by the same general causes, and obey the
same general laws, as with the lower animals." He correlates mental
development with the evolution of the brain. (Ibid. page 81.) "As the
various mental faculties gradually developed themselves, the brain would
almost certainly become larger. No one, I presume, doubts that the large
proportion which the size of man's brain bears to his body, compared to the
same proportion in the gorilla or orang, is closely connected with his
higher mental powers." "With respect to the lower animals," he says
("Descent of Man" (Popular edition), page 82.), "M.E. Lartet ("Comptes
Rendus des Sciences", June 1, 1868.), by comparing the crania of tertiary
and recent mammals belonging to the same groups, has come to the remarkable
conclusion that the brain is generally larger and the convolutions are more
complex in the more recent form."

Sir E. Ray Lankester has sought to express in the simplest terms the
implications of the increase in size of the cerebrum. "In what," he asks,
"does the advantage of a larger cerebral mass consist?" "Man," he replies
"is born with fewer ready-made tricks of the nerve-centres--these
performances of an inherited nervous mechanism so often called by the ill-
defined term 'instincts'--than are the monkeys or any other animal.
Correlated with the absence of inherited ready-made mechanism, man has a
greater capacity of developing in the course of his individual growth
similar nervous mechanisms (similar to but not identical with those of
'instinct') than any other animal...The power of being educated--
'educability' as we may term it--is what man possesses in excess as
compared with the apes. I think we are justified in forming the hypothesis
that it is this 'educability' which is the correlative of the increased
size of the cerebrum." There has been natural selection of the more
educable animals, for "the character which we describe as 'educability' can
be transmitted, it is a congenital character. But the RESULTS of education
can NOT be transmitted. In each generation they have to be acquired
afresh, and with increased 'educability' they are more readily acquired and
a larger variety of them...The fact is that there is no community between
the mechanisms of instinct and the mechanisms of intelligence, and that the
latter are later in the history of the evolution of the brain than the
former and can only develop in proportion as the former become feeble and
defective." ("Nature", Vol. LXI. pages 624, 625 (1900).)

In this statement we have a good example of the further development of
views which Darwin foreshadowed but did not thoroughly work out. It states
the biological case clearly and tersely. Plasticity of behaviour in
special accommodation to special circumstances is of survival value; it
depends upon acquired characters; it is correlated with increase in size
and complexity of the cerebrum; under natural selection therefore the
larger and more complex cerebrum as the organ of plastic behaviour has been
the outcome of natural selection. We have thus the biological foundations
for a further development of genetic psychology.

There are diversities of opinion, as Darwin showed, with regard to the
range of instinct in man and the higher animals as contrasted with lower
types. Darwin himself said ("Descent of Man", Vol. I. page 100.) that
"Man, perhaps, has somewhat fewer instincts than those possessed by the
animals which come next to him in the series." On the other hand, Prof.
Wm. James says ("Principles of Psychology," Vol. II. page 289.) that man is
probably the animal with most instincts. The true position is that man and
the higher animals have fewer complete and self-sufficing instincts than
those which stand lower in the scale of mental evolution, but that they
have an equally large or perhaps larger mass of instinctive raw material
which may furnish the stuff to be elaborated by intelligent processes.
There is, perhaps, a greater abundance of the primary tissue of experience
to be refashioned and integrated by secondary modification; there is
probably the same differentiation in relation to the determining biological
ends, but there is at the outset less differentiation of the particular and
specific modes of behaviour. The specialised instinctive performances and
their concomitant experience-complexes are at the outset more indefinite.
Only through acquired connections, correlated with experience, do they
become definitely organised.

The full working-out of the delicate and subtle relationship of instinct
and educability--that is, of the hereditary and the acquired factors in the
mental life--is the task which lies before genetic and comparative
psychology. They interact throughout the whole of life, and their
interactions are very complex. No one can read the chapters of "The
Descent of Man" which Darwin devotes to a consideration of the mental
characters of man and animals without noticing, on the one hand, how
sedulous he is in his search for hereditary foundations, and, on the other
hand, how fully he realises the importance of acquired habits of mind. The
fact that educability itself has innate tendencies--is in fact a partially
differentiated educability--renders the unravelling of the factors of
mental progress all the more difficult.

In his comparison of the mental powers of men and animals it was essential
that Darwin should lay stress on points of similarity rather than on points
of difference. Seeking to establish a doctrine of evolution, with its
basal concept of continuity of process and community of character, he was
bound to render clear and to emphasise the contention that the difference
in mind between man and the higher animals, great as it is, is one of
degree and not of kind. To this end Darwin not only recorded a large
number of valuable observations of his own, and collected a considerable
body of information from reliable sources, he presented the whole subject
in a new light and showed that a natural history of mind might be written
and that this method of study offered a wide and rich field for
investigation. Of course those who regarded the study of mind only as a
branch of metaphysics smiled at the philosophical ineptitude of the mere
man of science. But the investigation, on natural history lines, has been
prosecuted with a large measure of success. Much indeed still remains to
be done; for special training is required, and the workers are still few.
Promise for the future is however afforded by the fact that investigation
is prosecuted on experimental lines and that something like organised
methods of research are taking form. There is now but little reliance on
casual observations recorded by those who have not undergone the necessary
discipline in these methods. There is also some change of emphasis in
formulating conclusions. Now that the general evolutionary thesis is fully
and freely accepted by those who carry on such researches, more stress is
laid on the differentiation of the stages of evolutionary advance than on
the fact of their underlying community of nature. The conceptual
intelligence which is especially characteristic of the higher mental
procedure of man is more firmly distinguished from the perceptual
intelligence which he shares with the lower animals--distinguished now as a
higher product of evolution, no longer as differing in origin or different
in kind. Some progress has been made, on the one hand in rendering an
account of intelligent profiting by experience under the guidance of
pleasure and pain in the perceptual field, on lines predetermined by
instinctive differentiation for biological ends, and on the other hand in
elucidating the method of conceptual thought employed, for example, by the
investigator himself in interpreting the perceptual experience of the lower

Thus there is a growing tendency to realise more fully that there are two
orders of educability--first an educability of the perceptual intelligence
based on the biological foundation of instinct, and secondly an educability
of the conceptual intelligence which refashions and rearranges the data
afforded by previous inheritance and acquisition. It is in relation to
this second and higher order of educability that the cerebrum of man shows
so large an increase of mass and a yet larger increase of effective surface
through its rich convolutions. It is through educability of this order
that the human child is brought intellectually and affectively into touch
with the ideal constructions by means of which man has endeavoured, with
more or less success, to reach an interpretation of nature, and to guide
the course of the further evolution of his race--ideal constructions which
form part of man's environment.

It formed no part of Darwin's purpose to consider, save in broad outline,
the methods, or to discuss in any fulness of detail the results of the
process by which a differentiation of the mental faculties of man from
those of the lower animals has been brought about--a differentiation the
existence of which he again and again acknowledges. His purpose was rather
to show that, notwithstanding this differentiation, there is basal
community in kind. This must be remembered in considering his treatment of
the biological foundations on which man's systems of ethics are built. He
definitely stated that he approached the subject "exclusively from the side
of natural history." ("Descent of Man", Vol. I. page 149.) His general
conclusion is that the moral sense is fundamentally identical with the
social instincts, which have been developed for the good of the community;
and he suggests that the concept which thus enables us to interpret the
biological ground-plan of morals also enables us to frame a rational ideal
of the moral end. "As the social instincts," he says (Ibid. page 185.),
"both of man and the lower animals have no doubt been developed by nearly
the same steps, it would be advisable, if found practicable, to use the
same definition in both cases, and to take as the standard of morality, the
general good or welfare of the community, rather than the general
happiness." But the kind of community for the good of which the social
instincts of animals and primitive men were biologically developed may be
different from that which is the product of civilisation, as Darwin no
doubt realised. Darwin's contention was that conscience is a social
instinct and has been evolved because it is useful to the tribe in the
struggle for existence against other tribes. On the other hand, J.S. Mill
urged that the moral feelings are not innate but acquired, and Bain held
the same view, believing that the moral sense is acquired by each
individual during his life-time. Darwin, who notes (Ibid. page 150
(footnote).) their opinion with his usual candour, adds that "on the
general theory of evolution this is at least extremely improbable. It is
impossible to enter into the question here: much turns on the exact
connotation of the terms "conscience" and "moral sense," and on the meaning
we attach to the statement that the moral sense is fundamentally identical
with the social instincts.

Presumably the majority of those who approach the subjects discussed in the
third, fourth and fifth chapters of "The Descent of Man" in the full
conviction that mental phenomena, not less than organic phenomena, have a
natural genesis, would, without hesitation, admit that the intellectual and
moral systems of civilised man are ideal constructions, the products of
conceptual thought, and that as such they are, in their developed form,
acquired. The moral sentiments are the emotional analogues of highly
developed concepts. This does not however imply that they are outside the
range of natural history treatment. Even though it may be desirable to
differentiate the moral conduct of men from the social behaviour of animals
(to which some such term as "pre-moral" or "quasi-moral" may be applied),
still the fact remains that, as Darwin showed, there is abundant evidence
of the occurrence of such social behaviour--social behaviour which, even
granted that it is in large part intelligently acquired, and is itself so
far a product of educability, is of survival value. It makes for that
integration without which no social group could hold together and escape
elimination. Furthermore, even if we grant that such behaviour is
intelligently acquired, that is to say arises through the modification of
hereditary instincts and emotions, the fact remains that only through these
instinctive and emotional data is afforded the primary tissue of the
experience which is susceptible of such modification.

Darwin sought to show, and succeeded in showing, that for the intellectual
and moral life there are instinctive foundations which a biological
treatment alone can disclose. It is true that he did not in all cases
analytically distinguish the foundations from the superstructure. Even to-
day we are scarcely in a position to do so adequately. But his treatment
was of great value in giving an impetus to further research. This value
indeed can scarcely be overestimated. And when the natural history of the
mental operations shall have been written, the cardinal fact will stand
forth, that the instinctive and emotional foundations are the outcome of
biological evolution and have been ingrained in the race through natural
selection. We shall more clearly realise that educability itself is a
product of natural selection, though the specific results acquired through
cerebral modifications are not transmitted through heredity. It will,
perhaps, also be realised that the instinctive foundations of social
behaviour are, for us, somewhat out of date and have undergone but little
change throughout the progress of civilisation, because natural selection
has long since ceased to be the dominant factor in human progress. The
history of human progress has been mainly the history of man's higher
educability, the products of which he has projected on to his environment.
This educability remains on the average what it was a dozen generations
ago; but the thought-woven tapestry of his surroundings is refashioned and
improved by each succeeding generation. Few men have in greater measure
enriched the thought-environment with which it is the aim of education to
bring educable human beings into vital contact, than has Charles Darwin.
His special field of work was the wide province of biology; but he did much
to help us realise that mental factors have contributed to organic
evolution and that in man, the highest product of Evolution, they have
reached a position of unquestioned supremacy.


Professor of Philosophy in the University of Copenhagen.


It is difficult to draw a sharp line between philosophy and natural
science. The naturalist who introduces a new principle, or demonstrates a
fact which throws a new light on existence, not only renders an important
service to philosophy but is himself a philosopher in the broader sense of
the word. The aim of philosophy in the stricter sense is to attain points
of view from which the fundamental phenomena and the principles of the
special sciences can be seen in their relative importance and connection.
But philosophy in this stricter sense has always been influenced by
philosophy in the broader sense. Greek philosophy came under the influence
of logic and mathematics, modern philosophy under the influence of natural
science. The name of Charles Darwin stands with those of Galileo, Newton,
and Robert Mayer--names which denote new problems and great alterations in
our conception of the universe.

First of all we must lay stress on Darwin's own personality. His deep love
of truth, his indefatigable inquiry, his wide horizon, and his steady self-
criticism make him a scientific model, even if his results and theories
should eventually come to possess mainly an historical interest. In the
intellectual domain the primary object is to reach high summits from which
wide surveys are possible, to reach them toiling honestly upwards by way of
experience, and then not to turn dizzy when a summit is gained. Darwinians
have sometimes turned dizzy, but Darwin never. He saw from the first the
great importance of his hypothesis, not only because of its solution of the
old problem as to the value of the concept of species, not only because of
the grand picture of natural evolution which it unrolls, but also because
of the life and inspiration its method would impart to the study of
comparative anatomy, of instinct and of heredity, and finally because of
the influence it would exert on the whole conception of existence. He
wrote in his note-book in the year 1837: "My theory would give zest to
recent and fossil comparative anatomy; it would lead to the study of
instinct, heredity, and mind-heredity, whole (of) metaphysics." ("Life and
Letters of Charles Darwin", Vol. I. page 8.)

We can distinguish four main points in which Darwin's investigations
possess philosophical importance.

The evolution hypothesis is much older than Darwin; it is, indeed, one of
the oldest guessings of human thought. In the eighteenth century it was
put forward by Diderot and Lamettrie and suggested by Kant (1786). As we
shall see later, it was held also by several philosophers in the first half
of the nineteenth century. In his preface to "The Origin of Species",
Darwin mentions the naturalists who were his forerunners. But he has set
forth the hypothesis of evolution in so energetic and thorough a manner
that it perforce attracts the attention of all thoughtful men in a much
higher degree than it did before the publication of the "Origin".

And further, the importance of his teaching rests on the fact that he, much
more than his predecessors, even than Lamarck, sought a foundation for his
hypothesis in definite facts. Modern science began by demanding--with
Kepler and Newton--evidence of verae causae; this demand Darwin
industriously set himself to satisfy--hence the wealth of material which he
collected by his observations and his experiments. He not only revived an
old hypothesis, but he saw the necessity of verifying it by facts. Whether
the special cause on which he founded the explanation of the origin of
species--Natural Selection--is sufficient, is now a subject of discussion.
He himself had some doubt in regard to this question, and the criticisms
which are directed against his hypothesis hit Darwinism rather than Darwin.
In his indefatigable search for empirical evidence he is a model even for
his antagonists: he has compelled them to approach the problems of life
along other lines than those which were formerly followed.

Whether the special cause to which Darwin appealed is sufficient or not, at
least to it is probably due the greater part of the influence which he has
exerted on the general trend of thought. "Struggle for existence" and
"natural selection" are principles which have been applied, more or less,
in every department of thought. Recent research, it is true, has
discovered greater empirical discontinuity--leaps, "mutations"--whereas
Darwin believed in the importance of small variations slowly accumulated.
It has also been shown by the experimental method, which in recent
biological work has succeeded Darwin's more historical method, that types
once constituted possess great permanence, the fluctuations being
restricted within clearly defined boundaries. The problem has become more
precise, both as to variation and as to heredity. The inner conditions of
life have in both respects shown a greater independence than Darwin had
supposed in his theory, though he always admitted that the cause of
variation was to him a great enigma, "a most perplexing problem," and that
the struggle for life could only occur where variation existed. But, at
any rate, it was of the greatest importance that Darwin gave a living
impression of the struggle for life which is everywhere going on, and to
which even the highest forms of existence must be amenable. The
philosophical importance of these ideas does not stand or fall with the
answer to the question, whether natural selection is a sufficient
explanation of the origin of species or not: it has an independent,
positive value for everyone who will observe life and reality with an
unbiassed mind.

In accentuating the struggle for life Darwin stands as a characteristically
English thinker: he continues a train of ideas which Hobbes and Malthus
had already begun. Moreover in his critical views as to the conception of
species he had English forerunners; in the middle ages Occam and Duns
Scotus, in the eighteenth century Berkeley and Hume. In his moral
philosophy, as we shall see later, he is an adherent of the school which is
represented by Hutcheson, Hume and Adam Smith. Because he is no
philosopher in the stricter sense of the term, it is of great interest to
see that his attitude of mind is that of the great thinkers of his nation.

In considering Darwin's influence on philosophy we will begin with an
examination of the attitude of philosophy to the conception of evolution at
the time when "The Origin of Species" appeared. We will then examine the
effects which the theory of evolution, and especially the idea of the
struggle for life, has had, and naturally must have, on the discussion of
philosophical problems.


When "The Origin of Species" appeared fifty years ago Romantic speculation,
Schelling's and Hegel's philosophy, still reigned on the continent, while
in England Positivism, the philosophy of Comte and Stuart Mill, represented
the most important trend of thought. German speculation had much to say on
evolution, it even pretended to be a philosophy of evolution. But then the
word "evolution" was to be taken in an ideal, not in a real, sense. To
speculative thought the forms and types of nature formed a system of ideas,
within which any form could lead us by continuous transitions to any other.
It was a classificatory system which was regarded as a divine world of
thought or images, within which metamorphoses could go on--a condition
comparable with that in the mind of the poet when one image follows another
with imperceptible changes. Goethe's ideas of evolution, as expressed in
his "Metamorphosen der Pflanzen und der Thiere", belong to this category;
it is, therefore, incorrect to call him a forerunner of Darwin. Schelling
and Hegel held the same idea; Hegel expressly rejected the conception of a
real evolution in time as coarse and materialistic. "Nature," he says, "is
to be considered as a SYSTEM OF STAGES, the one necessarily arising from
the other, and being the nearest truth of that from which it proceeds; but
not in such a way that the one is NATURALLY generated by the other; on the
contrary (their connection lies) in the inner idea which is the ground of
nature. The METAMORPHOSIS can be ascribed only to the notion as such,
because it alone is evolution...It has been a clumsy idea in the older as
well as in the newer philosophy of nature, to regard the transformation and
the transition from one natural form and sphere to a higher as an outward
and actual production." ("Encyclopaedie der philosophischen
Wissenschaften" (4th edition), Berlin, 1845, paragraph 249.)

The only one of the philosophers of Romanticism who believed in a real,
historical evolution, a real production of new species, was Oken.
("Lehrbuch der Naturphilosophie", Jena, 1809.) Danish philosophers, such
as Treschow (1812) and Sibbern (1846), have also broached the idea of an
historical evolution of all living beings from the lowest to the highest.
Schopenhauer's philosophy has a more realistic character than that of
Schelling's and Hegel's, his diametrical opposites, though he also belongs
to the romantic school of thought. His philosophical and psychological
views were greatly influenced by French naturalists and philosophers,
especially by Cabanis and Lamarck. He praises the "ever memorable
Lamarck," because he laid so much stress on the "will to live." But he
repudiates as a "wonderful error" the idea that the organs of animals
should have reached their present perfection through a development in time,
during the course of innumerable generations. It was, he said, a
consequence of the low standard of contemporary French philosophy, that
Lamarck came to the idea of the construction of living beings in time
through succession! ("Ueber den Willen in der Natur" (2nd edition),
Frankfurt a. M., 1854, pages 41-43.)

The positivistic stream of thought was not more in favour of a real
evolution than was the Romantic school. Its aim was to adhere to positive
facts: it looked with suspicion on far-reaching speculation. Comte laid
great stress on the discontinuity found between the different kingdoms of
nature, as well as within each single kingdom. As he regarded as
unscientific every attempt to reduce the number of physical forces, so he
rejected entirely the hypothesis of Lamarck concerning the evolution of
species; the idea of species would in his eyes absolutely lose its
importance if a transition from species to species under the influence of
conditions of life were admitted. His disciples (Littre, Robin) continued
to direct against Darwin the polemics which their master had employed
against Lamarck. Stuart Mill, who, in the theory of knowledge, represented
the empirical or positivistic movement in philosophy--like his English
forerunners from Locke to Hume--founded his theory of knowledge and morals
on the experience of the single individual. He sympathised with the theory
of the original likeness of all individuals and derived their differences,
on which he practically and theoretically laid much stress, from the
influence both of experience and education, and, generally, of physical and
social causes. He admitted an individual evolution, and, in the human
species, an evolution based on social progress; but no physiological
evolution of species. He was afraid that the hypothesis of heredity would
carry us back to the old theory of "innate" ideas.

Darwin was more empirical than Comte and Mill; experience disclosed to him
a deeper continuity than they could find; closer than before the nature and
fate of the single individual were shown to be interwoven in the great web
binding the life of the species with nature as a whole. And the continuity
which so many idealistic philosophers could find only in the world of
thought, he showed to be present in the world of reality.


Darwin's energetic renewal of the old idea of evolution had its chief
importance in strengthening the conviction of this real continuity in the
world, of continuity in the series of form and events. It was a great
support for all those who were prepared to base their conception of life on
scientific grounds. Together with the recently discovered law of the
conservation of energy, it helped to produce the great realistic movement
which characterises the last third of the nineteenth century. After the
decline of the Romantic movement people wished to have firmer ground under
their feet and reality now asserted itself in a more emphatic manner than
in the period of Romanticism. It was easy for Hegel to proclaim that "the
real" was "the rational," and that "the rational" was "the real": reality
itself existed for him only in the interpretation of ideal reason, and if
there was anything which could not be merged in the higher unity of
thought, then it was only an example of the "impotence of nature to hold to
the idea." But now concepts are to be founded on nature and not on any
system of categories too confidently deduced a priori. The new devotion to
nature had its recompense in itself, because the new points of view made us
see that nature could indeed "hold to ideas," though perhaps not to those
which we had cogitated beforehand.

A most important question for philosophers to answer was whether the new
views were compatible with an idealistic conception of life and existence.
Some proclaimed that we have now no need of any philosophy beyond the
principles of the conservation of matter and energy and the principle of
natural evolution: existence should and could be definitely and completely
explained by the laws of material nature. But abler thinkers saw that the
thing was not so simple. They were prepared to give the new views their
just place and to examine what alterations the old views must undergo in
order to be brought into harmony with the new data.

The realistic character of Darwin's theory was shown not only in the idea
of natural continuity, but also, and not least, in the idea of the cause
whereby organic life advances step by step. This idea--the idea of the
struggle for life--implied that nothing could persist, if it had no power
to maintain itself under the given conditions. Inner value alone does not
decide. Idealism was here put to its hardest trial. In continuous
evolution it could perhaps still find an analogy to the inner evolution of
ideas in the mind; but in the demand for power in order to struggle with
outward conditions Realism seemed to announce itself in its most brutal
form. Every form of Idealism had to ask itself seriously how it was going
to "struggle for life" with this new Realism.

We will now give a short account of the position which leading thinkers in
different countries have taken up in regard to this question.

I. Herbert Spencer was the philosopher whose mind was best prepared by his
own previous thinking to admit the theory of Darwin to a place in his
conception of the world. His criticism of the arguments which had been put
forward against the hypothesis of Lamarck, showed that Spencer, as a young
man, was an adherent to the evolution idea. In his "Social Statics" (1850)
he applied this idea to human life and moral civilisation. In 1852 he
wrote an essay on "The Development Hypothesis", in which he definitely
stated his belief that the differentiation of species, like the
differentiation within a single organism, was the result of development.
In the first edition of his "Psychology" (1855) he took a step which put
him in opposition to the older English school (from Locke to Mill): he
acknowledged "innate ideas" so far as to admit the tendency of acquired
habits to be inherited in the course of generations, so that the nature and
functions of the individual are only to be understood through its
connection with the life of the species. In 1857, in his essay on
"Progress", he propounded the law of differentiation as a general law of
evolution, verified by examples from all regions of experience, the
evolution of species being only one of these examples. On the effect which
the appearance of "The Origin of Species" had on his mind he writes in his
"Autobiography": "Up to that time...I held that the sole cause of organic
evolution is the inheritance of functionally-produced modifications. The
"Origin of Species" made it clear to me that I was wrong, and that the
larger part of the facts cannot be due to any such cause...To have the
theory of organic evolution justified was of course to get further support
for that theory of evolution at large with which...all my conceptions were
bound up." (Spencer, "Autobiography", Vol. II. page 50, London, 1904.)
Instead of the metaphorical expression "natural selection," Spencer
introduced the term "survival of the fittest," which found favour with
Darwin as well as with Wallace.

In working out his ideas of evolution, Spencer found that differentiation
was not the only form of evolution. In its simplest form evolution is
mainly a concentration, previously scattered elements being integrated and
losing independent movement. Differentiation is only forthcoming when
minor wholes arise within a greater whole. And the highest form of
evolution is reached when there is a harmony between concentration and
differentiation, a harmony which Spencer calls equilibration and which he
defines as a moving equilibrium. At the same time this definition enables
him to illustrate the expression "survival of the fittest." "Every living
organism exhibits such a moving equilibrium--a balanced set of functions
constituting its life; and the overthrow of this balanced set of functions
or moving equilibrium is what we call death. Some individuals in a species
are so constituted that their moving equilibria are less easily overthrown
than those of other individuals; and these are the fittest which survive,
or, in Mr Darwin's language, they are the select which nature preserves."
(Ibid. page 100.) Not only in the domain of organic life, but in all
domains, the summit of evolution is, according to Spencer, characterised by
such a harmony--by a moving equilibrium.

Spencer's analysis of the concept of evolution, based on a great variety of
examples, has made this concept clearer and more definite than before. It
contains the three elements; integration, differentiation and
equilibration. It is true that a concept which is to be valid for all
domains of experience must have an abstract character, and between the
several domains there is, strictly speaking, only a relation of analogy.
So there is only analogy between psychical and physical evolution. But
this is no serious objection, because general concepts do not express more
than analogies between the phenomena which they represent. Spencer takes
his leading terms from the material world in defining evolution (in the
simplest form) as integration of matter and dissipation of movement; but as
he--not always quite consistently (Cf. my letter to him, 1876, now printed
in Duncan's "Life and Letters of Herbert Spencer", page 178, London,
1908.)--assumed a correspondence of mind and matter, he could very well
give these terms an indirect importance for psychical evolution. Spencer
has always, in my opinion with full right, repudiated the ascription of
materialism. He is no more a materialist than Spinoza. In his "Principles
of Psychology" (paragraph 63) he expressed himself very clearly: "Though
it seems easier to translate so-called matter into so-called spirit, than
to translate so-called spirit into so-called matter--which latter is indeed
wholly impossible--yet no translation can carry us beyond our symbols."
These words lead us naturally to a group of thinkers whose starting-point
was psychical evolution. But we have still one aspect of Spencer's
philosophy to mention.

Spencer founded his "laws of evolution" on an inductive basis, but he was
convinced that they could be deduced from the law of the conservation of
energy. Such a deduction is, perhaps, possible for the more elementary
forms of evolution, integration and differentiation; but it is not possible
for the highest form, the equilibration, which is a harmony of integration
and differentiation. Spencer can no more deduce the necessity for the
eventual appearance of "moving equilibria" of harmonious totalities than
Hegel could guarantee the "higher unities" in which all contradictions
should be reconciled. In Spencer's hands the theory of evolution acquired
a more decidedly optimistic character than in Darwin's; but I shall deal
later with the relation of Darwin's hypothesis to the opposition of
optimism and pessimism.

II. While the starting-point of Spencer was biological or cosmological,
psychical evolution being conceived as in analogy with physical, a group of
eminent thinkers--in Germany Wundt, in France Fouillee, in Italy Ardigo--
took, each in his own manner, their starting-point in psychical evolution
as an original fact and as a type of all evolution, the hypothesis of
Darwin coming in as a corroboration and as a special example. They
maintain the continuity of evolution; they find this character most
prominent in psychical evolution, and this is for them a motive to demand a
corresponding continuity in the material, especially in the organic domain.

To Wundt and Fouillee the concept of will is prominent. They see the type
of all evolution in the transformation of the life of will from blind
impulse to conscious choice; the theories of Lamarck and Darwin are used to
support the view that there is in nature a tendency to evolution in steady
reciprocity with external conditions. The struggle for life is here only a
secondary fact. Its apparent prominence is explained by the circumstance
that the influence of external conditions is easily made out, while inner
conditions can be verified only through their effects. For Ardigo the
evolution of thought was the starting-point and the type: in the evolution
of a scientific hypothesis we see a progress from the indefinite
(indistinto) to the definite (distinto), and this is a characteristic of
all evolution, as Ardigo has pointed out in a series of works. The
opposition between indistinto and distinto corresponds to Spencer's
opposition between homogeneity and heterogeneity. The hypothesis of the
origin of differences of species from more simple forms is a special
example of the general law of evolution.

In the views of Wundt and Fouillee we find the fundamental idea of
idealism: psychical phenomena as expressions of the innermost nature of
existence. They differ from the older Idealism in the great stress which
they lay on evolution as a real, historical process which is going on
through steady conflict with external conditions. The Romantic dread of
reality is broken. It is beyond doubt that Darwin's emphasis on the
struggle for life as a necessary condition of evolution has been a very
important factor in carrying philosophy back to reality from the heaven of
pure ideas. The philosophy of Ardigo, on the other side, appears more as a
continuation and deepening of positivism, though the Italian thinker
arrived at his point of view independently of French-English positivism.
The idea of continuous evolution is here maintained in opposition to
Comte's and Mill's philosophy of discontinuity. From Wundt and Fouillee
Ardigo differs in conceiving psychical evolution not as an immediate
revelation of the innermost nature of existence, but only as a single,
though the most accessible example, of evolution.

III. To the French philosophers Boutroux and Bergson, evolution proper is
continuous and qualitative, while outer experience and physical science
give us fragments only, sporadic processes and mechanical combinations. To
Bergson, in his recent work "L'Evolution Creatrice", evolution consists in
an elan de vie which to our fragmentary observation and analytic reflexion
appears as broken into a manifold of elements and processes. The concept
of matter in its scientific form is the result of this breaking asunder,
essential for all scientific reflexion. In these conceptions the strongest
opposition between inner and outer conditions of evolution is expressed:
in the domain of internal conditions spontaneous development of qualitative
forms--in the domain of external conditions discontinuity and mechanical

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