DARWIN’S THEORIES OF ARTIFICIAL AND OF NATURAL SELECTION
The Principle of Selection
Darwin’s theory of natural selection is preëminently a theory of
adaptation. It appears, in fact, better suited to explain this
phenomenon than that of the “origin of species.” Darwin prepared his
reader for the ideas contained in the theory of natural selection by a
brief consideration of the results of artificial selection; and since
the key to the situation is, I believe, to be found in just this
supposed resemblance, we cannot do better than examine the theories in
the order followed by Darwin himself.
One of the means by which the artificial races of animals and plants
have been formed by man is selection. The breeder picks out individuals
having a certain peculiarity, and allows them to breed together. He
hopes to find among the offspring, not only individuals like the parent
forms, but also some that have the special peculiarity even more
strongly developed. If such are found, they are isolated and allowed to
breed, and in the next generation it is hoped to find one or more new
individuals that show still more developed the special character that is
sought. This process, repeated through a number of generations, is
supposed to have led to the formation of many of our various forms of
domesticated animals and plants.
This heaping up as a result of the union of similar individuals cannot
for a moment be supposed to be the outcome of the addition of the two
variations to each other. Such an idea is counter to all the most
familiar facts of inheritance. For instance, when two similar forms
unite, we do not find that the young show all the characters of the
mother plus all those of the father, _i.e._ each peculiarity that is the
same in both, increased twofold. On the contrary, the young are in the
vast majority of cases not essentially different from either parent.
A more thorough examination of the facts shows that the problem is by no
means so simple as the preceding general statement might lead one to
suppose, for our experience shows that it is not always possible to
increase all variations by selection, and, furthermore, there is very
soon found a limit, even in favorable cases, to the extent to which the
process can be carried. The most important point appears to be the
nature of the variations themselves which may arise from different
causes, and which have different values in relation to the possibility
of their continuation.
We may begin, therefore, by following Darwin in his analysis of
variation, as given in the opening chapter of the “Origin of Species.”
He thinks that the great amount of variation shown by domesticated
animals and plants is due, in the first place, to the new conditions of
life to which they are exposed, and also to the lack of uniformity of
these conditions. Darwin thinks, also, that there is some probability
that this variability is due, in part, to an excess of food. “It seems
clear that organic beings must be exposed during several generations to
new conditions to cause any great amount of variation, and that when the
organization has once begun to vary, it generally continues varying for
many generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat,
still yield new varieties; our oldest domesticated animals are still
capable of rapid improvement or modification.” In this statement of
Darwin, full of significance, we must be careful to notice that he does
not mean to imply, when he states that an organism that has once begun
to vary continues to vary for many generations, that this continuous
variation is always in the same direction, but only that new
combinations, scattering in all directions, continue to appear.
The nature of the organism seemed to Darwin to be a more important
factor in the origin of new variations than the external conditions,
“for nearly similar variations sometimes arise under, as far as we can
judge, dissimilar conditions; and, on the other hand, dissimilar
variations arise under conditions which appear to be nearly uniform.”
The following statement is important in connection with the origin of
“definite” variations. “Each of the endless variations which we see in
the plumage of our fowls must have had some efficient cause; and if the
same causes were to act uniformly during a long series of generations on
many individuals, all probably would be modified in the same direction.”
Here we find an explicit statement in regard to the accumulation of
variation in a given direction as the result of an external agent, but
Darwin hastens to add: “Indefinite variability is a much more common
result of changed conditions than definite variability, and has probably
played a more important part in the formation of our domestic races. We
see indefinite variability in the endless slight peculiarities which
distinguish the individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from some more remote
ancestor. Even strongly marked differences occasionally appear in the
young of the same litter, and in seedlings from the same seed capsule.
At long intervals of time, out of millions of individuals reared in the
same country and fed on nearly the same food, deviations of structure so
strongly pronounced as to deserve to be called monstrosities arise; but
monstrosities cannot be separated by any distinct line from slighter
variations.”
Another cause of variation, Darwin believes, is in the inherited effect
of “habit and of the use and disuse of parts,” or what is generally
known as the Lamarckian factor of heredity. Darwin believes that changes
in the body of the parent, that are the result of the use or of the
disuse of a part, may be transmitted to the descendants, and cites a
number of cases which he credits to this process. As we shall deal more
fully with this topic in another chapter, we may treat it here quite
briefly. As an example of the inheritance of disuse, Darwin gives the
following case: “I find in the domestic duck that the bones of the wing
weigh less and the bones of the leg more in proportion to the whole
skeleton than do the same bones in the wild duck, and this change may be
safely attributed to the domestic duck flying much less and walking more
than its wild parents.” The great and inherited development of the
udders of cows and of goats in countries where they are habitually
milked, in comparison with these organs in other countries, is given as
another instance of the effect of use. “Not one of our domestic animals
can be named that in some country has not drooping ears, and the view
has been suggested that the drooping is due to the disuse of the muscles
of the ears from the animals being seldom much alarmed.”
It need scarcely be pointed out here, that, in the first case given,
those ducks would have been most likely to remain in confinement that
had less well-developed wings, and hence at the start artificial
selection may have served to bring about the result. The great
development of the udders of cows and of goats is obviously connected
with the greater milk-giving qualities of these animals, which may have
been selected for this purpose.
Another “law” of variation recognized by Darwin is what is called
correlated variation. For example, it has been found that cats which are
entirely white and have blue eyes are generally deaf, and this is stated
to be confined to the males. The teeth of hairless dogs are imperfect;
pigeons with feathered feet have skin between the outer toes, and those
with short beaks have small feet, and _vice versa_.
Another source of variation is that of reversion, or the reappearance in
the offspring of characters once possessed by the ancestors. Finally,
Darwin thinks that a source of variation is to be found in modifications
due to the influence of a previous union with another male, or, as it is
generally called, telegony. As an example Darwin cites the famous case
of Lord Morton’s mare. “A nearly purely bred Arabian chestnut mare bore
a hybrid to a quagga. She subsequently produced two colts by a black
Arabian horse. These colts were partially dun-colored and were striped
on the legs more plainly than the real hybrid or even than the
quagga.”[13] This case, however, is not above suspicion, since it is
well known that stripes often appear on young horses, and the careful
analysis made later by Ewart, as well as his other experiments on the
possibility of the transmission of influences of this sort, puts the
whole matter in a very dubious light.
These citations show that Darwin recognized quite a number of sources of
variation, and, although he freely admits that “our ignorance of the
laws of variation is profound,” yet some at least of these sources of
variation are very questionable. Be this as it may, it is important to
emphasize that Darwin recognized two main sources of variation,—one of
which is the indefinite, or fluctuating, variability that appears
constantly in domesticated animals and plants, and the other, definite
variability, or a change in a definite direction, that can often be
traced to the direct action of the environment on the parent or on its
reproductive cells. It is the former, _i.e._ the fluctuating
variability, that, according to Darwin, has been used by the breeder to
produce most of our domestic races. In regard to the other source of
variation, the definite kind, we must analyze the facts more closely.
Footnote 13:
“Animals and Plants under Domestication,” Chap. IX.
A definite change in the surroundings might bring about a definite
change in the next generation, because the new condition acts either on
the developing organism, or on the egg itself from which the individual
develops. The distinction may be one of importance, for, if the new
condition only effects the developing organism directly, then, when the
influence is removed, there should be a return to the former condition;
but if the egg itself is affected, so that it is fundamentally changed,
then the effect might persist even if the animal were returned to its
former environment. More important still is Darwin’s recognition of the
cumulative effect in a given direction of external influences, for a new
variation, that was slight at first, might, through prolonged action,
continue to become more developed without any other processes affecting
the organism.
From the Darwinian point of view, however, the all-important source for
the origin of new forms is the fluctuating variation, which is made use
of both in the process of artificial and of natural selection. We may
now proceed to inquire how this is supposed to take place.
It has been stated that, by means of artificial selection, Darwin
believes the breeder has produced the greater number of domesticated
animals and plants. The most important question is what sort of
variations he has made use of in order to produce his result. Has he
made use of the fluctuating variations, or of the definite ones? It is
difficult, if not impossible, to answer this question in most cases,
because the breeder does not always distinguish between the two. There
can be little question, however, that he may sometimes have made use of
the definite kinds, whether these are the outcome of external or of
internal influences. The question has been seriously raised only in
recent years, and we are still uncertain how far we can accumulate and
fix a variation that is of the fluctuating kind. In a few cases it has
been found that the upper limit is soon reached, as shown by De Vries’s
experiments with clover, and it is always possible that a definite
variation of the right sort may arise at any stage of the process. If
this should occur, then a new standard is introduced from which, as from
a new base, variations fluctuating in the desired direction may be
selected.
This question, before all others, ought to be settled before we begin to
speculate further as to what selection is able to accomplish.
Darwin’s theory is often stated in such a general way that it would be
applicable to either sort of variation; but if definite variation can go
on accumulating without selection, then possibly we could account for
evolution without supposing any other process to intervene. Under these
circumstances all that could be claimed for selection would be the
destruction of those variations incapable of living, or of competing
with other forms. Hence the process of selection would have an entirely
negative value.
The way in which domesticated animals and plants have originated is
explained by Darwin in the following significant passage:—
“Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect
may be attributed to the direct and definite action of the external
conditions of life, and some to habit; but he would be a bold man who
would account by such agencies for the differences between a dray- and
race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
One of the most remarkable features in our domesticated races is that we
see in them adaptation, not indeed to the animal’s or plant’s own good,
but to man’s use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step; many botanists, for instance, believe
that the fuller’s-teasel, with its hooks, which cannot be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus; and
this amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed for another
purpose; when we compare the many breeds of dogs, each good for man in
different ways; when we compare the game-cock, so pertinacious in
battle, with other breeds so little quarrelsome, with ‘everlasting
layers’ which never desire to sit, and with the bantam so small and
elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different
seasons and for different purposes, or so beautiful in his eyes, we
must, I think, look further than to mere variability. We cannot suppose
that all the breeds were suddenly produced as perfect and as useful as
we now see them; indeed, in many cases, we know that this has not been
their history. The key is man’s power of accumulative selection: nature
gives successive variations; man adds them up in certain directions
useful to him. In this sense he may be said to have made for himself
useful breeds.”
Darwin also gives the following striking examples, which make probable
the view that domestic forms have really been made by man selecting
those variations that are useful to him:—
“In regard to plants, there is another means of observing the
accumulated effects of selection—namely, by comparing the diversity of
flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers of
the same varieties; and the diversity of fruit of the same species in
the orchard, in comparison with the leaves and flowers of the same set
of varieties. See how different the leaves of the cabbage are, and how
extremely alike the flowers; how unlike the flowers of the heartsease
are, and how alike the leaves; how much the fruit of the different kinds
of gooseberries differ in size, color, shape, and hairiness, and yet the
flowers present very slight differences. It is not that the varieties
which differ largely in some one point do not differ at all in other
points; this is hardly ever,—I speak after careful observation,—perhaps
never, the case. The law of correlated variation, the importance of
which should never be overlooked, will insure some differences; but, as
a general rule, it cannot be doubted that the continued selection of
slight variations, either in the leaves, the flowers, or the fruit, will
produce races differing from each other chiefly in these characters.”
Exception may perhaps be taken to the concluding sentence, for,
interesting as the facts here recorded certainly are, it does not
necessarily follow that all domestic products have arisen “by the
continued selection of slight variations,” however probable the
conclusion may appear. Darwin also believes that a process of
“unconscious selection” has given even more important “results than
methodical selection.” By unconscious selection is meant the outcome of
“every one trying to possess and breed from best individual animals.”
“Thus a man who intends keeping pointers naturally tries to get as good
dogs as he can, and afterwards breeds from his own best dogs, but he has
no wish, or expectation of permanently altering the breed. Nevertheless
we may infer that this process, continued during centuries, would
improve and modify any breed.... There is reason to believe that the
King Charles spaniel has been unconsciously modified to a large extent
since the time of that monarch.”
The enormous length of time required to produce new species by the
selection of fluctuating variations is everywhere admitted by Darwin;
nowhere perhaps more strikingly than in the following statement: “If it
has taken centuries or thousands of years to improve or modify most of
our plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope, nor
any other region inhabited by quite uncivilized man has afforded us a
single plant worth culture. It is not that these countries, so rich in
species, do not by a strange chance possess the aboriginal stocks of any
useful plants, but that the native plants have not been improved by
continued selection up to a standard of perfection comparable with that
acquired by the plants in countries anciently civilized.”
In reply to this, it may be said that if the selection of fluctuating
variations leads to an accumulation in the given direction, it is not
apparent why it should take thousands of years to produce a new race, or
require such a high degree of skill as Darwin supposes the breeder to
possess.
The conditions favorable to artificial selection are, according to
Darwin: 1. The possession of a large number of individuals, for in this
way the chance of the desired variation appearing is increased. 2.
Prevention of intercrossing, such as results when the land is enclosed,
so that new forms may be kept apart. 3. Changed conditions, as
introducing variability. 4. The intercrossing of aboriginally distinct
species. 5. The intercrossing of new breeds, “but the importance of
intercrossing has been much exaggerated.” 6. In plants propagation of
bud variations by means of cuttings. The chapter concludes with the
statement, “Over all these causes of Change, the accumulative action of
Selection, whether applied methodically and quickly, or unconsciously
and slowly, but more efficiently, seems to have been the predominant
Power.”
Variability, Darwin says, is governed by many unknown laws, and the
final result is “infinitely complex.” If this is so, we may at least
hesitate before we accept the statement that selection of fluctuating
variations has been the only principle that has brought about these
results. This is a most important point, for, as we shall see, the
central question in the theory of natural selection has come to be
whether by the accumulation of fluctuating variations a new species
could ever be produced. If it be admitted that the evidence from
artificial selection is far from convincing, in showing that selection
of fluctuating variations could have been the main source, even in the
formation of new races, we need not be prejudiced in favor of such a
process, when we come to examine the formation of species in nature.
There are still other questions raised in this same chapter that demand
serious consideration. Darwin writes as follows:—
“When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often have
a somewhat monstrous character; by which I mean, that, although
differing from each other, and from other species of the same genus, in
several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially
when compared with the species under nature to which they are nearest
allied. With these exceptions (and with that of the perfect fertility of
varieties when crossed,—a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner as
do the closely allied species of the same genus in a state of nature,
but the differences in most cases are less in degree. This must be
admitted as true, for the domestic races of many animals and plants have
been ranked by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere varieties. If
any well-marked distinction existed between a domestic race and a
species, this source of doubt would not so perpetually recur.”
The point here raised in regard to the systematic value of the new forms
is the question that first demands our attention. We must exclude all
those cases in which several original species have been blended to make
a new form, because the results are too complicated to make use of at
present. The domesticated races of dogs appear to have had such a
multiple origin, the origin of horses is in doubt; but the domesticated
pigeons, ducks, rabbits, and fowls are supposed, by Darwin, to have come
each from one original wild species. The great variety of the domestic
pigeons gives perhaps the most striking illustration of changes that
have taken place under domestication; and Darwin lays great stress on
the evidence from this source.
It seems probable in this case, (1) that all the different races of
pigeons have come from one original species; (2) that the structural
differences are in some respects as great as those recognized by
systematists as specifically distinct; (3) that the different races
breed true to their kind; (4) that the result has been reached mainly by
selecting and isolating variations that have appeared under
domestication, and that probably some, at least, of these variations
were fluctuating ones.
Does not this grant all that Darwin contends for? In one sense, yes; in
another, no! The results appear to show that by artificial selection of
some kind a group of new forms may be produced that in many respects
resemble a natural family, or a genus; but if this is to be interpreted
to mean that the result is the same as that by which natural groups have
arisen, then I think that there are good reasons for dissenting from
such a conclusion. Moreover, we must not grant too readily that the
different races of pigeons have arisen by the selection of _fluctuating
variations_ alone, for this is not established with any great degree of
probability by the evidence.
In regard to the first point we find that one of the most striking
differences between species in nature is their infertility, and the
infertility of their offspring when intercrossed. This is a very general
rule, so far as we know. In regard to the different races of
domesticated forms, the most significant fact is that, no matter how
different they may be, they are perfectly fertile _inter se_. In this
respect, as well as in others, there are important differences between
domesticated races and wild species. The further difference, that has
been pointed out by a number of writers, should also not pass unnoticed,
namely, that the domestic forms differ from each other in the extreme
development of some one character, and not in a large number of less
conspicuous characters, as is the case in wild species.
These considerations show that, interesting and suggestive as are the
facts of artificial selection, they fail to demonstrate the main point
for which they are used by Darwin. With the most rigorous attention to
the process of artificial selection, new species comparable in all
respects to wild ones have not been formed, even in those cases in which
the variation has been carried farthest (where the history of the forms
is most completely known).
There is another point on which emphasis should be laid. If by selecting
the most extreme forms in each generation and breeding from them the
standard can be raised, it might appear that we could go on indefinitely
in the same direction, and produce, for instance, pigeons with legs five
metres long, and with necks of corresponding length. But experience has
shown that this cannot be done. As Darwin frequently remarks, the
breeder is entirely helpless until the desired variation appears. It
seems possible, by selecting the more extreme of the fluctuating
variations in each generation, that a higher plane of variation is
established, and even that more extreme forms are likely to arise for a
few generations; but, even if this is the case, a limit is soon reached
beyond which it is impossible to go.
The facts of observation show, that when a new variety appears its
descendants are more likely, on the average, to produce proportionately
more individuals that show the same variation, and some even that may go
still farther in the same direction. If these latter are chosen to be
the parents of the next generation, then once more the offspring may
show the same advance; but little by little the advance slows down,
until before very long it may cease altogether. Unless, then, a new kind
of variation appears, or a new standard of variation develops of a
different kind, the result of selection of fluctuating variations has
reached its limit. Our experience seems, therefore, to teach us that
selection of fluctuating variations leads us to only a certain point,
and then stops in this direction. We get no evidence from the facts in
favor of the view that the process, if carried on for a long time, could
ever produce such great changes, or the kind of changes, as those seen
in wild animals and plants.
Variation and Competition in Nature
Darwin rests his theory on the small individual variations which occur
in nature, as the following quotation shows:—
“It may be doubted whether sudden and considerable deviations of
structure such as we occasionally see in our domestic productions, more
especially with plants, are ever permanently propagated in a state of
nature. Almost every part of every organic being is so beautifully
related to its complex conditions of life that it seems as improbable
that any part should have been suddenly produced perfect, as that a
complex machine should have been invented by man in a perfect state.
Under domestication monstrosities sometimes occur which resemble normal
structures in widely different animals. Thus pigs have occasionally been
born with a sort of proboscis, and if any wild species of the same genus
had naturally possessed a proboscis, it might have been argued that this
had appeared as a monstrosity; but I have as yet failed to find, after
diligent search, cases of monstrosities resembling normal structures in
nearly allied forms, and these alone bear on the question. If monstrous
forms of this kind ever do appear in a state of nature and are capable
of reproduction (which is not always the case), as they occur rarely and
singly, their preservation would depend on unusually favorable
circumstances. They would, also, during the first and succeeding
generations cross with the ordinary form, and thus their abnormal
character would almost inevitably be lost.”
It is clear that Darwin does not think that the sudden and large
variations that sometimes occur furnish the basis for natural selection,
and the final statement in the last citation (which was added in later
editions of the “Origin of Species”), to the effect that if such
monstrous variations appeared as single or occasional variations they
would be lost by intercrossing implies that, in general, single
variations would likewise be lost unless they appeared in a sufficient
number of individuals to maintain themselves against the swamping
effects of intercrossing.
It is necessary to quote again, in order to show that, in some cases at
least, Darwin believed selection plays little or no part in the origin
and maintenance of certain peculiarities that are of no use to the
species. “There is one point connected with individual differences,
which is extremely perplexing: I refer to those genera which have been
called protean or ‘polymorphic,’ in which the species present an
inordinate amount of variation. With respect to many of these forms,
hardly two naturalists agree, whether to rank them as species or as
varieties. We may instance Rubus, Rosa, and Hieracium amongst plants,
several genera of insects and of Brachiopod shells. In most polymorphic
genera some of the species have fixed and definite characters. Genera
which are polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on by selection to act on and accumulate, in the same manner as man
accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the
most experienced naturalist would be surprised at the number of cases of
variability, even in important parts of structure, which he could
collect on good authority, as I have collected, during a course of
years.”
After pointing out that naturalists have no definite standard to
determine whether a group of individuals is a variety or a species,
Darwin makes the highly important admissions contained in the following
paragraph: “Hence, I look at individual differences, though of small
interest to the systematist, as of the highest importance for us, as
being the first steps toward such slight varieties as are barely thought
worth recording in works on natural history. And I look at varieties
which are in any degree more distinct and permanent, as steps toward
more strongly marked and permanent varieties; and at the latter, as
leading to subspecies, and then to species. The passage from one stage
of difference to another may, in many cases, be the simple result of the
nature of the organism and of the different physical conditions to which
it has long been exposed; but with respect to the more important and
adaptive characters, the passage from one stage of difference to another
may be safely attributed to the cumulative action of natural selection,
hereafter to be explained, and to the effects of the increased use or
disuse of parts. A well-marked variety may therefore be called an
incipient species; but whether this belief is justifiable must be judged
by the weight of the various facts and considerations to be given
throughout this work.”
In this paragraph attention should be called especially, first, to the
statement in respect to the origin of varieties, which are said to arise
through individual differences. It is not clear whether these
differences are supposed to have appeared first in one, or in a few
individuals, or in large numbers at the same time. Again, especial note
should be made of the striking admission, that the passage from one
stage to another may, in many cases, be the simple result of the nature
of the organism and of the physical conditions surrounding it; but with
respect to the more important and adaptive differences, natural
selection “may safely” be supposed to have intervened. Is it to be
wondered at that Darwin’s critics have sometimes accused him of playing
fast and loose with the origin of varieties? And since this question is
fundamental for the theory of natural selection, it is much to be
regretted that Darwin leaves the matter in such a hazy condition. It may
be said that, at the time when he wrote, he made the best of the
evidence in regard to the origin of varieties. Be this as it may, a
theory standing on no better foundations than this is not likely to be
found satisfactory at the present time.
We come now to the most important chapters, the third and the fourth, of
the “Origin of Species,” dealing with “the struggle for existence,”
“natural selection,” or the “survival of the fittest.” Behind these
fatal phrases, which have become almost household words, lurk many
dangers for the unwary.
“It has been seen in the last chapter that amongst organic beings in a
state of nature there is some individual variability: indeed I am not
aware that this has ever been disputed. It is immaterial for us whether
a multitude of doubtful forms be called species or subspecies or
varieties; what rank, for instance, the two or three hundred doubtful
forms of British plants are entitled to hold, if the existence of any
well-marked varieties be admitted. But the mere existence of individual
variability and of some few well-marked varieties, though necessary as
the foundation for the work, helps us but little in understanding how
species arise in nature. How have all those exquisite adaptions of one
part of the organization to another part, and to the conditions of life,
and of one organic being to another being, been perfected? We see these
beautiful coadaptions most plainly in the woodpecker and the mistletoe;
and only a little less plainly in the humblest parasite which clings to
the hairs of a quadruped or feathers of a bird; in the structure of the
beetle which dives through the water; in the plumed seed which is wafted
by the gentlest breeze; in short, we see beautiful adaptions everywhere
and in every part of the organic world.
“Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term Natural
Selection, in order to mark its relation to man’s power of selection.
But the expression often used by Mr. Herbert Spencer of the Survival of
the Fittest is more accurate, and is sometimes equally convenient. We
have seen that man by selection can certainly produce great results, and
can adapt organic beings to his own uses, through the accumulation of
slight but useful variations, given to him by the hand of Nature. But
Natural Selection, as we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man’s feeble
efforts, as the works of Nature are to those of Art.”
Darwin gives the following explicit statement of the way in which he
intends the term “struggle for existence” to be understood: “I should
premise that I use this term in a large and metaphorical sense,
including dependence of one being on another, and including (which is
more important) not only the life of the individual, but success in
leaving progeny. Two canine animals, in time of dearth, may be truly
said to struggle with each other which shall get food and live. But a
plant on the edge of a desert is said to struggle for life against the
drought, though more properly it should be said to be dependent on the
moisture. A plant which actually produces a thousand seeds of which only
one on an average comes to maturity may be more truly said to struggle
with the plants of the same and other kinds which already clothe the
ground. The mistletoe is dependent on the apple, and a few other trees,
but can only in a far-fetched sense be said to struggle with these
trees, for if too many of these parasites grow on the same tree, it
languishes and dies. But several seedling mistletoes, growing close
together on the same branch, may more truly be said to struggle with
each other. As the mistletoe is disseminated by birds, its existence
depends on them, and it may metaphorically be said to struggle with
other fruit-bearing plants, in tempting the birds to devour and thus
disseminate its seeds. In these several senses, which pass into each
other, I use for convenience’ sake the general term ‘Struggle for
Existence.’”
A number of writers have objected to the general and often vague way in
which Darwin makes use of this phrase; but it does not seem to me that
this is a serious objection, provided we are on our guard as to what the
outcome will be in each case. In each instance we must consider the
question on its own merits, and if it is found convenient to have a
sufficiently general and non-committal term, such as the “struggle for
existence,” to include all cases, I see no serious objection to the use
of such an expression, although it is true the outcome has been that it
has become a catchword, that is used too often by those who have no
knowledge of its contents.
Were it not that each animal and plant gives birth, on an average, to
more than two offspring, the species would soon become exterminated by
accidents, etc. We find in some of the lower animals, and in some of the
higher plants, that thousands and even millions of eggs are produced by
a single individual in the course of its life. A single nematode may lay
sixty million eggs, and a tapeworm one thousand million. A starfish may
produce about thirty-nine million eggs, a salmon may contain fifteen
thousand, and a large shad as many as one hundred thousand. The queen of
a termite nest is said to lay eighty thousand eggs a day.
In the higher vertebrates the number of young is considerably less, but
since the young stages are passed within the body of the parent,
proportionately more of them reach maturity, so that even in man the
population may be doubled in twenty-five years, and in the elephant,
slowest breeder of all animals, Darwin has calculated that, if it begins
breeding when about thirty years old and goes on until ninety years,
bringing forth six young in the interval, after 750 years there will be
nearly nineteen million elephants alive which have descended from the
first pair.
Obviously, then, if all the descendants of all the individuals of a
species were to remain alive, the world would be over-crowded in a very
short time, and the want of room would in itself lead to the destruction
of countless individuals, if for no other reason than lack of food. We
can easily carry out on a small scale an experiment that shows how the
overstocking, resulting from favorable conditions, comes about, and how
it checks itself. If we make a meat broth suitable for the life of a
particular bacterium, and sow in the broth a very few individuals, we
find in the course of several days the fluid swarming with the
descendants of the original individuals. Thus it has been shown that, if
we start with a few hundred bacteria, there will be five thousand after
twenty-four hours, and twenty thousand, forty-eight hours later; and
after four days they are beyond calculation.
Cohn found that a single bacterium produces two individuals in one hour,
and four in two hours, and if they continue to multiply at this rate
there will be produced at the end of three days 4,772 billions of
descendants. If these are reduced to weight, they would weigh
seventy-five hundred tons. Thus when the conditions are favorable,
bacteria are able to increase at such an enormous rate that they could
cover the surface of the earth in a very few days. The reason that they
do not go on increasing at this rate is that they soon exhaust the food
supply, and the rate of increase slows down, and will finally cease
altogether. If the bacteria were dependent on a continuous supply of
food, they would perish after the supply had been exhausted, so that the
rapid rate of multiplication would serve only to bring the career of the
organism to an untimely end. If the weaker individuals were to die
first, the products of their disintegration might serve to nourish the
stronger individuals; hunger coming on again, the next weakest might
die; and the same process continuing, we might imagine that the bacteria
were finally reduced to a single one which would then die in turn for
lack of food. Like a starving shipload of men, reduced by hunger to
cannibalism, the life of some and finally of the last individual might
be prolonged in the hope of rescue, but if this did not arrive, the last
and perhaps the strongest individual would perish. But this is not what
we find occurring in these lower organisms, for, as a rule, they
gradually cease to increase when the food supply becomes lessened, and
their activities slow down. Finally, when the food is gone, they pass
into a resting stage, in which condition they can remain dormant for a
long time, even for years. If they should again find themselves in
favorable surroundings, they become active, and begin once more their
round of multiplication. We cannot follow the individuals in such a
culture of bacteria, but there is nothing to be seen that suggests a
struggle for existence, if this idea conveys the impression of the
destruction of certain individuals by competition with others. In fact,
the results are in some respects exactly the reverse. Millions of
individuals are present at the time when the food supply becomes
exhausted, and they all pass into a protected resting stage.
The enormous rate of increase in this case finds its counterpart in
higher animals when the food supply, or the absence of enemies, allows a
species to multiply at its maximum rate of increase. The introduction of
rabbits into Australia was followed by an enormous increase in a few
years, and the introduction of the English sparrow into the United
States has had a similar result. But in no country can such a process
continue beyond a certain point, because, in the first place, the
scarcity of food will begin to keep the birth-rate down, and in the
second place, the increase in numbers may lead to an increase in the
number of its enemies, or even induce other forms to feed on it.
Crowding will also give an opportunity for the spread of disease, which
again may check the increase. Sooner or later a sort of ever shifting
balance will be reached for each species, and after this, if the
conditions remain the same, the number of individuals will keep
approximately constant.
Darwin admits that the “causes which check the natural tendency of each
species to increase are most obscure.” “We know not exactly what the
checks are even in a single instance.” This admission may well put us on
our guard against a too ready acceptation of a theory in which the whole
issue turns on just this very point, namely, the nature of the checks to
increase. Darwin gives the following general cases to show what some of
the checks to increase are. He states that eggs and very young animals
and seeds suffer more than the adults; that “the amount of food for each
species of course gives the extreme limit to which each can increase;
but very frequently it is not the obtaining food, but the serving as
prey to other animals which determines the average numbers of a species.
Thus, there seems to be little doubt that the stock of partridges,
grouse, and hares on any large estate depends largely on the destruction
of the vermin.” “On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.” “Climate
plays an important part in determining the average number of a species,
and periodical seasons of extreme cold or drought seem to be the most
effective of all checks.” “The action of climate seems at first sight to
be quite independent of the struggle for existence; but in so far as
climate acts in reducing food, it brings on the most severe struggle
between the individuals, whether of the same, or of distinct species
which subsist on the same kind of food.”
We need not follow Darwin through his account of how complex are the
relations of all animals and plants to one another in the struggle for
existence, for, if true, it only goes to show more plainly how
impossible it is to establish any safe scientific hypothesis, where the
conditions are so complex and so impossible to estimate. To show that
the young Scotch fir in an enclosed pasture is kept down by the browsing
of the cattle, and in other parts of the world, Paraguay for instance,
the number of cattle is determined by insects, and that the increase of
these flies is _probably_ habitually checked by other insects, leads to
a bewilderingly complex set of conditions. We cannot do better than to
quote Darwin’s conclusion: “Hence, if certain insectivorous birds were
to decrease in Paraguay, the parasitic insects would probably increase;
and this would lessen the number of the navel-frequenting flies—then
cattle and horses would become feral, and this would certainly greatly
alter (as indeed I have observed in parts of South America) the
vegetation: this again would largely affect the insects; and this, as we
have just seen in Staffordshire, the insectivorous birds, and so onwards
in ever increasing circles of complexity. Not that under nature the
relations will ever be as simple as this. Battle within battle must be
continually recurring with varying success; and yet in the long run the
forces are so nicely balanced, that the face of nature remains for long
periods of time uniform, though assuredly the merest trifle would give
the victory to one organic being over another. Nevertheless, so profound
is our ignorance, and so high our presumption, that we marvel when we
hear of the extinction of an organic being; and as we do not see the
cause, we invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!”
The effect of the struggle for existence in determining _the
distribution of species_ is well illustrated in the following cases:—
“As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under the
most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. In Australia the
imported hive-bee is rapidly exterminating the small, stingless native
bee. One species of charlock has been known to supplant another species;
and so in other cases. We can dimly see why the competition should be
most severe between allied forms, which fill nearly the same place in
the economy of nature; but probably in no one case could we precisely
say why one species has been victorious over another in the great battle
of life.”
All this goes to show, if it really shows anything at all, that the
distribution of a species is determined, in part, by its relation to
other animals and plants—a truism that is recognized by every
naturalist. The statement has no necessary bearing on the origin of new
species through competition, as the incautious reader might infer. Not
that I mean in any way to imply that Darwin intended to produce this
effect on the reader; but Darwin is not always careful to discriminate
as to the full bearing of the interesting illustrations with which his
book so richly abounds.
At the end of his treatment of the subject, Darwin emphasizes once more
how little we know about the subject of the struggle for existence.
“It is good thus to try in imagination to give to any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult, to acquire. All that we can do, is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each at some period of its life, during some season of the year,
during each generation or at intervals, has to struggle for life and to
suffer great destruction. When we reflect on this struggle, we may
console ourselves with the full belief, that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.”
The kindliness of heart that prompted the concluding sentence may arouse
our admiration for the humanity of the writer, but need not, therefore,
dull our criticism of his theory. For whether no fear is felt, and
whether death is prompt or slow, has no bearing on the question at
issue—except as it prepares the gentle reader to accept the dreadful
calamity of nature, pictured in this battle for existence, and make more
contented with their lot “the vigorous, the healthy, and the happy.”
The Theory of Natural Selection
We have already anticipated, to some extent, Darwin’s conclusion in
regard to the outcome of the competition of animals and plants. This
result is supposed to lead to the survival of the fittest. The
competition is carried out by nature, who is personified as selecting
those forms for further experiments that have won in the struggle for
existence.
“Can the principle of selection, which we have seen is so potent in the
hands of man, apply under Nature? I think we shall see that it can act
most efficiently. Let the endless number of slight variations and
individual differences occurring in our domestic productions, and, in a
lesser degree, in those under Nature, be borne in mind; as well as the
strength of the hereditary tendency. Can it, then, be thought
improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle for life, should occur in the course of many
successive generations? If such do occur can we doubt (remembering how
many more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others, would
have the best chance of surviving and of procreating their kind? On the
other hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed.”
The process of natural selection is defined as follows, “The
preservation of favorable individual differences and variations and the
destruction of those that are injurious I have called Natural Selection
or the Survival of the Fittest.” And immediately there follows the
significant statement, that, “Variations neither useful nor injurious
would not be affected by natural selection, and would be left either a
fluctuating element, as perhaps we see in certain polymorphic species,
or would ultimately become fixed, owing to the nature of the organism
and the nature of the conditions.” It will be seen from this quotation,
as well as from others already given, that Darwin leaves many structures
outside of the pale of natural selection, and uses his theory to explain
only those cases that are of sufficient use to be decisive in the life
and death struggle of the individuals with each other and with the
surrounding conditions.
Darwin states that we can best understand “the probable course of
natural selection by taking the case of a country undergoing some slight
physical change, for instance, of climate. The proportional numbers of
its inhabitants will almost immediately undergo a change, and some
species will probably become extinct. We may conclude, from what we have
seen of the intimate and complex manner in which the inhabitants of each
country are bound together, that any change in the numerical proportions
of the inhabitants, independency of the change of climate itself, would
seriously affect the others.... In such cases, slight modifications,
which in any way favored the individuals of any species, by better
adapting them to their altered conditions, would tend to be preserved;
and natural selection would have free scope for the work of
improvement.”
The first half of the first of these two quotations seems so plausible,
that without further thought we may be tempted to give a ready assent to
the second, yet the whole issue is contained in this statement. In the
abstract, it undoubtedly appears true that any slightly useful
modification might tend to be preserved. Whether it will, in reality, be
preserved must depend on many things that should be taken into account.
This question will come up later for further consideration; but it
should be pointed out here, that, even assuming that one or more
individuals happen to possess a favorable variation, it by no means
follows that natural selection would have free scope for the work of
improvement, because the question of the inheritance of this variation,
and of its accumulation and building up through successive generations,
must be determined before we can be expected to give assent to this
argument, that appears so attractive when stated in an abstract and
vague way.
Darwin again makes the statement that under the term _variation_ it must
never be forgotten that mere individual differences are meant. “As a man
can produce a great result with his domestic animals and plants by
adding up in any given direction individual differences, so could
natural selection, but far more easily from having incomparably longer
time for action.” Too much emphasis cannot be laid on the fact that
Darwin believed that selection takes place amongst the small individual
differences that we find in animals and plants. Some of his followers,
as we shall see, are apt to put into the background this fundamental
conception of Darwin’s view. His constant comparison between the results
of artificial and natural selection leaves no room for doubt as to his
meaning. Darwin himself seems, at times, not unconscious of the weakness
of this comparison. He says: “How fleeting are the wishes and efforts of
man! how short his time! and consequently how poor will be his results,
compared with those accumulated by Nature during whole geological
periods. Can we wonder then that Nature’s productions should be far
‘truer’ in character than man’s productions; that they should be
infinitely better adapted to the most complex conditions of life, and
should plainly bear the stamp of far higher workmanship?” We should not
lose sight of the fact that even after the most rigorous selective
process has been brought to bear on organisms, namely, by isolation
under domestication, we do not apparently find ourselves gradually
approaching nearer and nearer to the formation of new species, but we
find, on the contrary, that we have produced something quite different.
In the light of this truth, the relation between the two selective
theories may appear quite different from the interpretation that Darwin
gives of it. We may well doubt whether nature does select so much better
than does man, and whether she has ever _made_ new species in this way.
We come now to a point that touches the theory of natural selection in a
very vital spot.
“It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the
course of natural selection. For instance, a vast number of eggs or
seeds are annually devoured, and these could be modified through natural
selection only if they varied in some manner which protected them from
their enemies. Yet many of these eggs or seeds would perhaps, if not
destroyed, have yielded individuals better adapted to their conditions
of life than any of those which happened to survive. So again a vast
number of mature animals and plants, whether or not they be the best
adapted to their conditions, must be annually destroyed by accidental
causes, which would not be in the least degree mitigated by certain
changes of structure or constitution which would in other ways be
beneficial to the species. But let the destruction of the adults be ever
so heavy, if the number which can exist in any district be not wholly
kept down by such causes,—or again let the destruction of eggs or seeds
be so great that only a hundredth or a thousandth part are
developed,—yet of those which do survive, the best adapted individuals,
supposing that there is any variability in a favorable direction, will
tend to propagate their kind in larger numbers than the less well
adapted. If the numbers be wholly kept down by the causes just
indicated, as will often have been the case, natural selection will be
powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we are
far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.”
Some of the admissions made in this paragraph have an important bearing
on the theory of natural selection. Far from supposing that fortuitous
destruction would have no influence on the course of natural selection,
it can be shown that it would have a most disastrous effect. In many
cases the destruction comes in the form of a catastrophe to the
individuals, so that small differences in structure, whether
advantageous or not, are utterly unavailing. Our experience shows us
that a destruction of this sort is going on around us all the time, and
accounts in large part for the way in which the majority of animals and
plants are destroyed. Unless, for example, a seed happen to fall on a
place suitable for its growth, it will perish without respect to a
slight advantage it may have over other seeds of its kind. Of the
thousands of eggs laid by one starfish, chance alone will decide whether
one or another embryo is destroyed by larger animals, or if they escape
this danger, the majority of them may be carried out to sea, where it
will not be of the least avail if one individual has a slight advantage
over the others. Darwin admits this, but adds that, if only a thousandth
part is developed, yet of those that do survive the best adapted
individuals will tend to propagate their kind in larger numbers than the
less well adapted. The argument is not, however, so simple as it appears
to be on the surface. I pass over, for the present, the apparent
inconsequence in this statement that the best adapted individuals will
tend to propagate their kind in larger numbers. It is not by any means
certain that this is the case. Darwin’s meaning is, however, fairly
clear, and can be interpreted to mean this: after the fortuitous
destruction has finished, there will be a further competition of the
survivors amongst themselves and with the surrounding conditions. In
this higher competition, which is less severe, small individual
differences suffice to determine the survival of certain individuals.
These are, therefore, selected.
In this argument it is assumed that a second competition takes place
after the first destruction of individuals has occurred, and this
presupposes that more individuals reach maturity than there is room for
in the economy of nature. But we do not know to what extent this takes
place. If only as many mature as can survive, then the second
competition does not take place. If, on the other hand, fewer mature
than there is room for, then again competition does not take place. And
if at all times selection is not rigorously carried out, everything may
be lost that has been so laboriously gained. We see then that the result
that Darwin imagines would take place, can be carried out only when more
individuals reach maturity than there is room for (if it is a case of
competition with one another), or that escape their enemies (if it is a
question of competition with other forms).
It is instructive to consider some of the examples that Darwin has given
to illustrate how the process of natural selection is carried out. The
first example is the imaginary case of a species of wolf, the
individuals of which secure their prey sometimes by craft, sometimes by
strength, and sometimes by fleetness. If the prey captured by the first
two methods should fail, then all the wolves would be obliged to capture
their food by fleetness, and consequently the fleetest alone would
survive. “I can see no more reason to doubt that this would be the
result than that man should improve the fleetness of his greyhounds.”
But even if the fleetness of the race could be kept up in this way, it
does not follow that a new species of wolf would be formed in
consequence, as Darwin implies. His own comment on this illustration is,
perhaps, the best criticism that can be made.
“It should be observed that, in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked
variation having been preserved. In former editions of this work I
sometimes spoke as if this latter alternative had frequently occurred. I
saw the great importance of individual differences, and this led me
fully to discuss the results of unconscious selection by man, which
depends on the preservation of all the more or less valuable
individuals, and on the destruction of the worst. I saw, also, that the
preservation in a state of nature of any occasional deviation of
structure, such as a monstrosity, would be a rare event; and that, if at
first preserved, it would generally be lost by subsequent intercrossing
with ordinary individuals. Nevertheless, until reading an able and
valuable article in the _North British Review_ (1867), I did not
appreciate how rarely single variations, whether slight or strongly
marked, could be perpetuated. The author takes the case of a pair of
animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average
survive to procreate their kind. This is rather an extreme estimate for
most of the higher animals, but by no means so for many of the lower
organisms. He then shows that if a single individual were born, which
varied in some manner, giving it twice as good a chance of life as that
of the other individuals, yet the chances would be strongly against its
survival. Supposing it to survive and to breed, and that half its young
inherited the favourable variation; still, as the reviewer goes on to
show, the young would have only a slightly better chance of surviving
and breeding; and this chance would go on decreasing in the succeeding
generations. The justice of these remarks cannot, I think, be disputed.
If, for instance, a bird of some kind could procure its food more easily
by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be a
very poor chance of this one individual perpetuating its kind to the
exclusion of the common form; but there can hardly be a doubt, judging
by what we see taking place under domestication, that this result would
follow from the preservation during many generations of a large number
of individuals with more or less strongly curved beaks, and from the
destruction of a still larger number with the straightest beaks.”
There then follows what, I believe, is one of the most significant
admissions in the “Origin of Species”:—
“It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organization being
similarly acted on—of which fact numerous instances could be given with
our domestic productions. In such cases, if the varying individual did
not actually transmit to its offspring its newly acquired character, it
would undoubtedly transmit to them, as long as the existing conditions
remained the same, a still stronger tendency to vary in the same manner.
There can also be little doubt that the tendency to vary in the same
manner has often been so strong that all the individuals of the same
species have been similarly modified without the aid of any form of
selection. Or only a third, fifth, or tenth part of the individuals may
have been thus affected, of which fact several instances could be given.
Thus Graba estimates that about one-fifth of the guillemots in the Faroe
Islands consist of a variety so well marked, that it was formerly ranked
as a distinct species under the name of _Uria lacrymans_. In cases of
this kind, if the variation were of a beneficial nature, the original
form would soon be supplanted by the modified form, through the survival
of the fittest.”
Do not the admissions in this paragraph almost amount to a withdrawal of
much that has preceded in regard to the survival of fluctuating,
individual differences? In the last edition, from which we have just
quoted, Darwin, in response to the criticisms which his book met,
inserted here and there statements that are in many ways in
contradiction to the statements in the first edition, and yet the
earlier statements have been allowed to stand for the most part.
The next example is also worthy of careful examination, since it appears
to prove too much:—
“It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosæ, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose
that the juice or nectar was excreted from the inside of the flowers of
a certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one
flower to another. The flowers of two distinct individuals of the same
species would thus get crossed; the act of crossing, as can be fully
proved, gives rise to vigorous seedlings, which consequently would have
the best chance of flourishing and surviving. The plants which produced
flowers with the largest glands or nectaries, excreting most nectar,
would oftenest be visited by insects, and would oftenest be crossed; and
so in the long run would gain the upper hand and form a local variety.”
The reader will notice that the sweet juice or nectar secreted by
certain plants is supposed to have first appeared independently of the
action of natural selection. Why then account for its presence in
flowers as the outcome of an entirely different process? If the nectar
is eagerly sought for by insects, without the plant benefiting in any
way by their visitations, why give a different explanation of its origin
in flowers where it is of benefit to the plant?
Darwin carries his illustration further: “When our plant, by the above
process long continued, had been rendered highly attractive to insects,
they would unintentionally, on their part, regularly carry pollen from
flower to flower; and that they do this effectually, I could easily show
by many striking facts. I will give only one, as likewise illustrating
one step in the separation of the sexes of plants.... As soon as the
plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
‘physiological division of labour’; hence we may believe that it would
be advantageous to a plant to produce stamens alone in one flower or on
one whole plant, and pistils alone in another flower or on another
plant. In plants under culture and placed under new conditions of life,
sometimes the male organs and sometimes the female organs become more or
less impotent; now if we suppose this to occur in ever so slight a
degree under nature, then, as pollen is already carried regularly from
flower to flower, and as a more complete separation of the sexes of our
plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased would be
continually favoured or selected, until at last a complete separation of
the sexes might be effected. It would take up too much space to show the
various steps, through dimorphism and other means, by which the
separation of the sexes in plants of various kinds is apparently now in
progress; but I may add that some of the species of holly in North
America are, according to Asa Gray, in an exactly intermediate
condition, or, as he expresses it, are more or less diœciously
polygamous.”
From this it will be seen that Darwin supposes that the separation of
the sexes in some of the higher plants has been brought about by natural
selection. Despite the supposed advantage of the so-called “division of
labor,” one may, I venture to suggest, be sceptical as to whether the
separation of the sexes can be explained in this way. The whole case is
largely supposititious, since in most of the higher hermaphroditic
plants and in nearly all hermaphroditic animals the sexual products
ripen at different times in the same individual. Hence there is no basis
for the assumption that unless the sexes are separated there will be
self-fertilization. Shall we assume that this difference in time of
ripening of the two kinds of sex-cells is also the outcome of natural
selection, and that there has existed an earlier stage in all animals
and plants, that now have different times for the ripening of their
sexual elements, a time when these products ripened simultaneously? I
doubt if even a Darwinian would give such loose rein to his fancy.
But this is not yet the whole story that Darwin has made out in this
connection, for he continues:—
“Let us now turn to the nectar-feeding insects; we may suppose the
plant, of which we have been slowly increasing the nectar by continued
selection, to be a common plant; and that certain insects depended in
main part on its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit of cutting
holes and sucking the nectar at the bases of certain flowers, which with
a very little more trouble, they can enter by the mouth. Bearing such
facts in mind, it may be believed that under certain circumstances
individual differences in the curvature or length of the proboscis,
etc., too slight to be appreciated by us, might profit a bee or other
insect, so that certain individuals would be able to obtain their food
more quickly than others; and thus the communities to which they
belonged would flourish and throw off many swarms inheriting the same
peculiarities.”
Aside from the general criticism that will suggest itself here also, it
should be pointed out that even if “certain individuals” of the bees had
slightly longer proboscides, this would, in the case of the hive-bees at
least, be of no avail, since they do not reproduce, and hence leave no
descendants with longer mouth-parts. Of course, it may be replied that
those colonies in which the queens produce more of the long-proboscis
kind of worker would have an advantage over other colonies not having so
many individuals of this sort. It would then be a competition of one
colony with another, as Darwin supposes to take place in colonial forms.
But whether slight differences of this sort would lead to the
elimination of the least well-endowed colonies is entirely a matter of
speculation. Since there are flowers with corolla-tubes of all lengths,
we can readily suppose that if one kind of flower excluded individuals
of certain colonies, they would search elsewhere for their nectar rather
than perish. While different races might arise in this way, the process
would not be the survival of the fittest, but a process of adaptation to
a new environment.
We come now to a topic on which Darwin lays much stress: the divergence
of character. He tries to show how the “lesser differences between the
varieties become augmented into the greater differences between
species.”
“Mere chance, as we may call it, might cause one variety to differ in
some character from its parents, and the offspring of this variety again
to differ from its parent in the very same character and in a greater
degree; but this alone would never account for so habitual and large a
degree of difference as that between the species of the same genus. As
has always been my practice, I have sought light on this head from our
domestic productions.”
Then, after pointing out that under domestication two different races,
the race-horse and the dray-horse, for instance, might arise by
selecting different sorts of variations, Darwin inquires:—
“But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), from the simple circumstance that the more
diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to
seize on many and widely diversified places in the polity of nature, and
so be enabled to increase in numbers.”
Here we touch on one of the fundamental principles of the doctrine of
evolution. It is intimated that the new form of animal or plant first
appears (without regard to any kind of selection), and then finds that
place in nature where it can remain in existence and propagate its kind.
Darwin refers here, of course, only to the less extensive variations,
the individual or fluctuating kind; but as we shall discuss at greater
length in another place, this same process, if extended to other kinds
of variation, may give us an explanation of evolution without
competition, or selection, or destruction of the individuals of the same
kind taking place at all.
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