been reading, for his own pleasure, Malthus' book on Population, and,
as he had long known from numerous observations, that every species
gives rise to many more descendants than ever attain to maturity, and
that, therefore, the greater number of the descendants of a species
perish without reproducing, the idea came to him that the decision as
to which member of a species was to perish and which was to attain to
maturity and reproduction might not be a matter of chance, but might
be determined by the constitution of the individuals themselves,
according as they were more or less fitted for survival. With this
idea the foundation of the theory of selection was laid.
In _artificial selection_ the breeder chooses out for pairing only
such individuals as possess the character desired by him in a somewhat
higher degree than the rest of the race. Some of the descendants
inherit this character, often in a still higher degree, and if this
method be pursued throughout several generations, the race is
transformed in respect of that particular character.
_Natural selection_ depends on the same three factors as _artificial
selection_: on _variability_, _inheritance_, and _selection for
breeding_, but this last is here carried out not by a breeder but by
what Darwin called the "struggle for existence." This last factor is
one of the special features of the Darwinian conception of nature.
That there are carnivorous animals which take heavy toll in every
generation of the progeny of the animals on which they prey, and that
there are herbivores which decimate the plants in every generation had
long been known, but it is only since Darwin's time that sufficient
attention has been paid to the facts that, in addition to this regular
destruction, there exists between the members of a species a keen
competition for space and food, which limits multiplication, and that
numerous individuals of each species perish because of unfavourable
climatic conditions. The "struggle for existence," which Darwin
regarded as taking the place of the human breeder in free nature, is
not a direct struggle between carnivores and their prey, but is the
assumed competition for survival between individuals _of the same_
species, of which, on an average, only those survive to reproduce
which have the greatest power of resistance, while the others, less
favourably constituted, perish early. This struggle is so keen, that,
within a limited area, where the conditions of life have long remained
unchanged, of every species, whatever be the degree of fertility, only
two, _on an average_, of the descendants of each pair survive; the
others succumb either to enemies, or to disadvantages of climate, or
to accident. A high degree of fertility is thus not an indication of
the special success of a species, but of the numerous dangers that
have attended its evolution. Of the six young brought forth by a pair
of elephants in the course of their lives only two survive in a given
area; similarly, of the millions of eggs which two thread-worms leave
behind them only two survive. It is thus possible to estimate the
dangers which threaten a species by its ratio of elimination, or,
since this cannot be done directly, by its fertility.
Although a great number of the descendants of each generation fall
victims to accident, among those that remain it is still the greater
or less fitness of the organism that determines the "selection for
breeding purposes," and it would be incomprehensible if, in this
competition, it were not ultimately, that is, on an average, the best
equipped which survive, in the sense of living long enough to
reproduce.
Thus the principle of natural selection is _the selection of the best
for reproduction_, whether the "best" refers to the whole
constitution, to one or more parts of the organism, or to one or more
stages of development. Every organ, every part, every character of an
animal, fertility and intelligence included, must be improved in this
manner, and be gradually brought up in the course of generations to
its highest attainable state of perfection. And not only may
improvement of parts be brought about in this way, but new parts and
organs may arise, since, through the slow and minute steps of
individual or "fluctuating" variations, a part may be added here or
dropped out there, and thus something new is produced.
The principle of selection solved the riddle as to how what was
purposive could conceivably be brought about without the intervention
of a directing power, the riddle which animate nature presents to our
intelligence at every turn, and in face of which the mind of a Kant
could find no way out, for he regarded a solution of it as not to be
hoped for. For, even if we were to assume an evolutionary force that
is continually transforming the most primitive and the simplest forms
of life into ever higher forms, and the homogeneity of primitive times
into the infinite variety of the present, we should still be unable to
infer from this alone how each of the numberless forms adapted to
particular conditions of life should have appeared _precisely at the
right moment in the history of the earth_ to which their adaptations
were appropriate, and precisely at the proper place in which all the
conditions of life to which they were adapted occurred: the
humming-birds at the same time as the flowers; the trichina at the
same time as the pig; the bark-coloured moth at the same time as the
oak, and the wasp-like moth at the same time as the wasp which
protects it. Without processes of selection we should be obliged to
assume a "pre-established harmony" after the famous Leibnitzian model,
by means of which the clock of the evolution of organisms is so
regulated as to strike in exact synchronism with that of the history
of the earth! All forms of life are strictly adapted to the conditions
of their life, and can persist under these conditions alone.
There must therefore be an intrinsic connection between the conditions
and the structural adaptations of the organism, and, _since the
conditions of life cannot be determined by the animal itself, the
adaptations must be called forth by the conditions_.
The selection theory teaches us how this is conceivable, since it
enables us to understand that there is a continual production of what
is non-purposive as well as of what is purposive, but the purposive
alone survives, while the non-purposive perishes in the very act of
arising. This is the old wisdom taught long ago by Empedocles.
II. THE LAMARCKIAN PRINCIPLE
Lamarck, as is well known, formulated a definite theory of evolution
at the beginning of the nineteenth century, exactly fifty years before
the Darwin-Wallace principle of selection was given to the world. This
brilliant investigator also endeavoured to support his theory by
demonstrating forces which might have brought about the
transformations of the organic world in the course of the ages. In
addition to other factors, he laid special emphasis on the increased
or diminished use of the parts of the body, assuming that the
strengthening or weakening which takes place from this cause during
the individual life, could be handed on to the offspring, and thus
intensified and raised to the rank of a specific character. Darwin
also regarded this _Lamarckian principle_, as it is now generally
called, as a factor in evolution, but he was not fully convinced of
the transmissibility of acquired characters.
As I have here to deal only with the theory of selection, I need not
discuss the Lamarckian hypothesis, but I must express my opinion that
there is room for much doubt as to the cooeperation of this principle
in evolution. Not only is it difficult to imagine how the transmission
of functional modifications could take place, but, up to the present
time, notwithstanding the endeavours of many excellent investigators,
not a single actual proof of such inheritance has been brought
forward. Semon's experiments on plants are, according to the botanist
Pfeffer, not to be relied on, and even the recent, beautiful
experiments made by Dr. Kammerer on salamanders, cannot, as I hope to
show elsewhere, be regarded as proof, if only because they do not deal
at all with functional modifications, that is, with modifications
brought about by use, and it is to these _alone_ that the Lamarckian
principle refers.
III. OBJECTIONS TO THE THEORY OF SELECTION
(_a_) _Saltatory evolution_
The Darwinian doctrine of evolution depends essentially on _the
cumulative augmentation_ of minute variations in the direction of
utility. But can such minute variations, which are undoubtedly
continually appearing among the individuals of the same species,
possess any selection-value; can they determine which individuals are
to survive, and which are to succumb; can they be increased by natural
selection till they attain to the highest development of a purposive
variation?
To many this seems so improbable that they have urged a theory of
evolution by leaps from species to species. Koelliker, in 1872,
compared the evolution of species with the processes which we can
observe in the individual life in cases of alternation of generations.
But a polyp only gives rise to a medusa because it has itself arisen
from one, and there can be no question of a medusa ever having arisen
suddenly and _de novo_ from a polyp-bud, if only because both forms
are adapted in their structure as a whole, and in every detail to the
conditions of their life. A sudden origin, in a natural way, of
numerous adaptations is inconceivable. Even the degeneration of a
medusoid from a free-swimming animal to a mere brood-sac (gonophore)
is not sudden and saltatory, but occurs by imperceptible modifications
throughout hundreds of years, as we can learn from the numerous stages
of the process of degeneration persisting at the same time in
different species.
If, then, the degeneration to a simple brood-sac takes place only by
very slow transitions, each stage of which may last for centuries, how
could the much more complex _ascending_ evolution possibly have taken
place by sudden leaps? I regard this argument as capable of further
extension, for wherever in nature we come upon degeneration, it is
taking place by minute steps and with a slowness that makes it not
directly perceptible, and I believe that this in itself justifies us
in concluding that _the same must be true of ascending_ evolution. But
in the latter case the goal can seldom be distinctly recognised while
in cases of degeneration the starting-point of the process can often
be inferred, because several nearly related species may represent
different stages.
In recent years Bateson in particular has championed the idea of
saltatory, or so-called discontinuous evolution, and has collected a
number of cases in which more or less marked variations have suddenly
appeared. These are taken for the most part from among domesticated
animals which have been bred and crossed for a long time, and it is
hardly to be wondered at that their much mixed and much influenced
germ-plasm should, under certain conditions, give rise to remarkable
phenomena, often indeed producing forms which are strongly suggestive
of monstrosities, and which would undoubtedly not survive in free
nature, unprotected by man. I should regard such cases as due to an
intensified germinal selection--though this is to anticipate a
little--and from this point of view it cannot be denied that they have
a special interest. But they seem to me to have no significance as far
as the transformation of species is concerned, if only because of the
extreme rarity of their occurrence.
There are, however, many variations which have appeared in a sudden
and saltatory manner, and some of these Darwin pointed out and
discussed in detail: the copper beech, the weeping trees, the oak with
"fern-like leaves," certain garden-flowers, etc. But none of them have
persisted in free nature, or evolved into permanent types.
On the other hand, wherever enduring types have arisen, we find traces
of a gradual origin by successive stages, even if, at first sight,
their origin may appear to have been sudden. This is the case with
_seasonal Dimorphism_, the first known cases of which exhibited marked
differences between the two generations, the winter and the summer
brood. Take for instance the much discussed and studied form
_Vanessa_ (_Araschnia_) _levana-prorsa_. Here the differences between
the two forms are so great and so apparently disconnected, that one
might almost believe it to be a sudden mutation, were it not that old
transition-stages can be called forth by particular temperatures, and
we know other butterflies, as for instance our Garden Whites, in which
the differences between the two generations are not nearly so marked;
indeed, they are so little apparent that they are scarcely likely to
be noticed except by experts. Thus here again there are small initial
steps, some of which, indeed, must be regarded as adaptations, such as
the green-sprinkled or lightly tinted under-surface which gives them a
deceptive resemblance to parsley or to Cardamine leaves.
Even if saltatory variations do occur, we cannot assume that these
_have ever led to forms which are capable of survival under the
conditions of wild life_. Experience has shown that in plants which
have suddenly varied the power of persistence is diminished.
Korschinsky attributes to them weaknesses of organisation in general;
"they bloom late, ripen few of their seeds, and show great
sensitiveness to cold." These are not the characters which make for
success in the struggle for existence.
We must briefly refer here to the views--much discussed in the last
decade--of H. de Vries, who believes that the roots of transformation
must be sought for in _saltatory variations arising from internal
causes_, and distinguishes such _mutations_, as he has called them,
from ordinary individual variations, in that they breed true, that is,
with strict in-breeding they are handed on pure to the next
generation. I have elsewhere endeavoured to point out the weaknesses
of this theory,[33] and I am the less inclined to return to it here
that it now appears[34] that the far-reaching conclusions drawn by de
Vries from his observations on the Evening Primrose, _Oenothera
lamarckiana_, rest upon a very insecure foundation. The plant from
which de Vries saw numerous "species"--his "mutations"--arise was not,
as he assumed, a _wild species_ that had been introduced to Europe
from America, but was probably a hybrid form which was first
discovered in the Jardin des Plantes in Paris, and which does not
appear to exist anywhere in America as a wild species.
This gives a severe shock to the "Mutation theory," for the other
_actually wild_ species with which de Vries experimented showed no
"mutations" but yielded only negative results.
Thus we come to the conclusion that Darwin[35] was right in regarding
transformations as taking place by minute steps, which, if useful, are
augmented in the course of innumerable generations, because their
possessors more frequently survive in the struggle for existence.
(_b_) _Selection-value of the initial steps_
Is it possible that the insignificant deviations which we know as
"individual variations" can form the beginning of a process of
selection? Can they decide which is to perish and which to survive? To
use a phrase of Romanes, can they have _selection-value_?
Darwin himself answered this question, and brought together many
excellent examples to show that differences, apparently insignificant
because very small, might be of decisive importance for the life of
the possessor. But it is by no means enough to bring forward cases of
this kind, for the question is not merely whether finished adaptations
have selection-value, but whether the first beginnings of these, and
whether the small, I might almost say minimal increments, which have
led up from these beginnings to the perfect adaptation, have also had
selection-value. To this question even one who, like myself, has been
for many years a convinced adherent of the theory of selection, can
only reply: _We must assume so, but we cannot prove it in any case_.
It is not upon demonstrative evidence that we rely when we champion
the doctrine of selection as a scientific truth; we base our argument
on quite other grounds. Undoubtedly there are many apparently
insignificant features, which can nevertheless be shown to be
adaptations--for instance, the thickness of the basin-shaped shell of
the limpets that live among the breakers on the shore. There can be no
doubt that the thickness of these shells, combined with their flat
forms, protects the animals from the force of the waves breaking upon
them,--but how have they become so thick? What proportion of thickness
was sufficient to decide that of two variants of a limpet one should
survive, the other be eliminated? We can say nothing more than that we
infer from the present state of the shell, that it must have varied in
regard to differences in shell-thickness, and that these differences
must have had selection-value,--no proof therefore, but an assumption
which we must show to be convincing.
For a long time the marvellously complex _radiate_ and _lattice-work_
skeletons of Radiolarians were regarded as a mere outflow of "Nature's
infinite wealth of form," as an instance of a purely morphological
character with no biological significance. But recent investigations
have shown that these, too, have an adaptive significance (Haecker).
The same thing has been shown by Schuett in regard to the lowly
unicellular plants, the Peridineae, which abound alike on the surface
of the ocean and in its depths. It has been shown that the long
skeletal processes which grow out from these organisms have
significance not merely as a supporting skeleton, but also as an
extension of the superficial area, which increases the contact with
the water-particles, and prevents the floating organisms from sinking.
It has been established that the processes are considerably shorter in
the colder layers of the ocean, and that they may be twelve times as
long[36] in the warmer layers, thus corresponding to the greater or
smaller amount of friction which takes place in the denser and less
dense layers of the water.
The Peridineae of the warmer ocean layers have thus become long-rayed,
those of the colder layers short-rayed, not through the direct effect
of friction on the protoplasm, but through processes of selection,
which favoured the longer rays in warm water, since they kept the
organism afloat, while those with short rays sank and were eliminated.
If we put the question as to selection-value in this case, and ask how
great the variations in the length of processes must be in order to
possess selection-value; what can we answer except that these
variations must have been minimal, and yet sufficient to prevent too
rapid sinking and consequent elimination? Yet this very case would
give the ideal opportunity for a mathematical calculation of the
minimal selection-value, although of course it is not feasible from
lack of data to carry out the actual calculation.
But even in organisms of more than microscopic size there must
frequently be minute, even microscopic differences which set going the
process of selection, and regulate its progress to the highest
possible perfection.
Many tropical trees possess thick, leathery leaves, as a protection
against the force of the tropical raindrops. The _direct_ influence of
the rain cannot be the cause of this power of resistance, for the
leaves, while they were still thin, would simply have been torn to
pieces. Their toughness must therefore be referred to selection, which
would favour the trees with slightly thicker leaves, though we cannot
calculate with any exactness how great the first stages of increase in
thickness must have been. Our hypothesis receives further support from
the fact that, in many such trees, the leaves are drawn out into a
beak-like prolongation (Stahl and Haberlandt) which facilitates the
rapid falling off of the rain water, and also from the fact that the
leaves, while they are still young, hang limply down in bunches which
offer the least possible resistance to the rain. Thus there are here
three adaptations which can only be interpreted as due to selection.
The initial stages of these adaptations must undoubtedly have had
selection-value.
But even in regard to this case we are reasoning in a circle, not
giving "proofs," and no one who does not wish to believe in the
selection-value of the initial stages can be forced to do so. Among
the many pieces of presumptive evidence a particularly weighty one
seems to me to be _the smallness of the steps of progress_ which we
can observe in certain cases, as for instance in leaf-imitation among
butterflies, and in mimicry generally. The resemblance to a leaf, for
instance of a particular Kallima, seems to us so close as to be
deceptive, and yet we find in another individual, or it may be in many
others, a spot added which increases the resemblance, and which could
not have become fixed unless the increased deceptiveness so produced
had frequently led to the overlooking of its much persecuted
possessor. But if we take the selection-value of the initial stages
for granted, we are confronted with the further question which I
myself formulated many years ago: How does it happen _that the
necessary beginnings of a useful variation are always present_? How
could insects which live upon or among green leaves become all green,
while those that live on bark become brown? How have the desert
animals become yellow and the Arctic animals white? Why were the
necessary variations always present? How could the green locust lay
brown eggs, or the privet caterpillar develop white and lilac-coloured
lines on its green skin?
It is of no use answering to this that the question is wrongly
formulated[37] and that it is the converse that is true; that the
process of selection takes place in accordance with the variations
that present themselves. This proposition is undeniably true, but so
also is another, which apparently negatives it: the variation required
has in the majority of cases actually presented itself. Selection
cannot solve this contradiction; it does not call forth the useful
variation, but simply works upon it. The ultimate reason why one and
the same insect should occur in green and in brown, as often happens
in caterpillars and locusts, lies in the fact that variations towards
brown presented themselves, and so also did variations towards green:
_the kernel of the riddle lies in the varying_, and for the present we
can only say, that small variations in different directions present
themselves in every species. Otherwise so many different kinds of
variations could not have arisen. I have endeavoured to explain this
remarkable fact by means of the intimate processes that must take
place within the germ-plasm, and I shall return to the problem when
dealing with "germinal selection."
We have, however, to make still greater demands on variation, for it
is not enough that the necessary variation should occur in isolated
individuals, because in that case there would be small prospect of its
being preserved, notwithstanding its utility. Darwin at first
believed, that even single variations might lead to transformation of
the species, but later he became convinced that this was impossible,
at least without the cooeperation of other factors, such as isolation
and sexual selection.