THE THEORY OF NATURAL SELECTION (_Continued_)
Objections to the Theory of Natural Selection
Although in the preceding chapter a number of criticisms have been made
of the special parts of the theory of natural selection, there still
remain to be considered some further objections that have been made
since the first publication of the theory. It is a fortunate
circumstance from every point of view that Darwin himself was able in
the later editions of the “Origin of Species” to reply to those
criticisms that he thought of sufficient importance. He says:—
“Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the
theory.”
The first difficulty is this: “Why, if species have descended from other
species by fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion, instead of the
species being, as we see them, well defined?”
The answer that Darwin gives is, that by competition the new form will
crowd out its own less-improved parent form, and other less-favored
forms. But is this a sufficient or satisfactory answer? If we recall
what Darwin has said on the advantage that those forms will have, in
which a great number of new variations appear to fit them to the great
diversity of natural conditions, and if we recall the gradations that
exist in external conditions, I think we shall find that Darwin’s reply
fails to give a satisfactory answer to the question.
It is well known, and Darwin himself has commented on it, that the same
species often remains constant under very diverse external conditions,
both inorganic and organic. Hence I think the explanation fails, in so
far as it is based on the accumulation by selection of small individual
variations that are supposed to give the individuals some slight
advantage under each set of external conditions. Darwin admits that
“this difficulty for a long time quite confounded me. But I think it can
be in large part explained.” The first explanation that is offered is
that areas now continuous may not have been so in the past. This may be
true in places, but the great continents have had continuous areas for a
long time, and Darwin frankly acknowledges that he “will pass over this
way of explaining the difficulty.” The second attempt is based on the
supposed narrowness of the area, where two species, descended from a
common parent, overlap. In this region the change is often very abrupt,
and Darwin adds:—
“To those who look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But
when we bear in mind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not for other competing
species; that nearly all either prey on or serve as prey for others; in
short, that each organic being is either directly or indirectly related
in the most important manner to other organic beings,—we see that the
range of the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in a large part on the
presence of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects, not blending one into another by insensible
gradations, the range of any one species, depending as it does on the
range of others, will tend to be sharply defined.”
Here we have a _petitio principii_. The sharp definition of species,
that we started out to account for, is explained by the sharp definition
of other species!
A third part of the explanation is that, owing to the relative fewness
of individuals at the confines of the range during the fluctuations of
their enemies, or of their prey, or in the nature of the seasons, they
would be extremely liable to utter extermination. If this were really
the case, then new species themselves which, on the theory, are at first
few in numbers ought to be exterminated. On the whole, then, it does not
appear that Darwin has been very successful in his attempt to meet this
objection to the theory.
Darwin tries to meet the objection, that organs of extreme perfection
and complication cannot be accounted for by natural selection, as
follows:—
“To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I freely
confess, absurd in the highest degree.”
The following sketch that Darwin gives to show how he imagined the
vertebrate eye to have been formed is very instructive, as illustrating
how he supposed that natural selection acts:—
“If we must compare the eye to an optical instrument, we ought in
imagination to take a thick layer of transparent tissue, with spaces
filled with fluid, and with a nerve sensitive to light beneath, and then
suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and
thicknesses, placed at different distances from each other, and with the
surfaces of each layer slowly changing in form. Further we must suppose
that there is a power, represented by natural selection or the survival
of the fittest, always intently watching each slight alteration in the
transparent layers; and carefully preserving each which, under varied
circumstances, in any way or in any degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better one is
produced, and then the old ones to be all destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions of
years; and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might thus be
formed as superior to one of glass, as the works of the Creator are to
those of man.”
We may conclude in Darwin’s own words:—
“To arrive, however, at a just conclusion regarding the formation of the
eye, with all its marvellous yet not absolutely perfect characters, it
is indispensable that the reason should conquer the imagination; but I
have felt the difficulty far too keenly to be surprised at others
hesitating to extend the principle of natural selection to so startling
a length.”
The electric organs, present in several fish, offer a case of special
difficulty to the selection theory. When well developed, as in the
Torpedo and in Gymnotus, it is conceivable that it may serve as an organ
of defence, but in other forms the shock is so weak that it is not to be
supposed that it can have any such function. Romanes, who in many ways
was one of the stanchest followers of Darwin, admits that, so far as he
can see, the evolution of the electric organs cannot be explained by the
selection theory. Darwin offers no explanation, but bases his defence on
the grounds that we do not know of what use this organ can be to the
animal.
Darwin also refers to the phosphorescent, or luminous, organs as a
supposed case of difficulty for his theory.
“The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the
body, offer, under our present state of ignorance, a difficulty almost
exactly parallel with that of the electric organs.”
In this case also, as in that of the electric organs, the structures
appear in entirely different parts of the body of the insect in
different species, so that their occurrence in this group cannot be
accounted for on a common descent. In whatever way they have arisen,
they must have evolved independently in different species. Darwin
advances no explanation of the origin of the luminous organs, but states
that they “offer under our present state of ignorance a difficulty
almost exactly parallel with that of the electric organs.” It will be
noticed that the difficulty referred to rests on the assumption that
since the organs are well developed they must have some important use!
We may next consider “organs of little apparent importance as affected
by natural selection.” Darwin says:—
“As natural selection acts by life and death,—by the survival of the
fittest, and by the destruction of the less well-fitted individuals,—I
have sometimes felt great difficulty in understanding the origin or
formation of parts of little importance; almost as great, though of a
very different kind, as in the case of the most perfect and complex
organs.”
His answers to this difficulty are: (1) we are too ignorant “in regard
to the whole economy of any one organic being to say what slight
modifications would be of importance or not,”—thus such apparently
trifling characters as the down on fruit, or the colors of the skin and
hair of quadrupeds, which from being correlated with constitutional
differences or from determining the attacks of insects might be acted on
by natural selection; (2) organs now of trifling importance have in some
cases been of high importance to an early progenitor; (3) the changed
conditions of life may account for some of the useless organs; (4)
reversion accounts for others; (5) the complex laws of growth account
for still others, such as correlation, compensation of the pressure of
one part on another, etc.; (6) the action of sexual selection is
responsible for many characters not to be explained by natural
selection. Admitting that there may be cases that can be accounted for
on one or the other of these six possibilities, yet there can be no
doubt that there are still a considerable number of specific characters
that cannot be explained in any of these ways. I do not think that
Darwin has by any means met this objection, even if all these six
possibilities be admitted as generally valid.
Amongst the “miscellaneous objections” to his theory that Darwin
considers we may select the most important cases. The following
paragraph has been sometimes quoted by later writers to show that Darwin
saw, to a certain extent, the insufficiency of fluctuating variations as
a basis for selection. What he calls here “spontaneous variability”
refers to sudden and extensive variations, or what we may call
discontinuous variations. “In the earlier editions of this work I
underrated, as it now seems probable, the frequency and importance of
modifications due to spontaneous variability. But it is impossible to
attribute to this cause the innumerable structures which are so well
adapted to the habits of life of each species. I can no more believe in
this, that the well-adapted form of a race-horse or greyhound, which
before the principle of selection by man was well understood, excited so
much surprise in the minds of the older naturalists, can thus be
explained.”
Darwin appears to mean by the latter part of this statement, that he
cannot believe that such sudden and great variations as have caused a
peach tree to produce nectarines can account for the wonderful
adaptations of organisms; but it is not really necessary to suppose that
this would often occur, for the same result could be reached by several
stages, even if the discontinuous variations had been small, and had
appeared in many individuals simultaneously. After showing that in a
number of flowers, especially of the Compositæ and Umbelliferæ, the
individual flowers in the closely crowded heads are sometimes formed on
a different type, Darwin concludes: “In these several cases, with the
exception of that of the well-developed ray-florets, which are of
service in making the flowers conspicuous to insects, natural selection
cannot, as far as we can judge, have come into play, or only in a quite
subordinate manner. All these modifications follow from the relative
position and interaction of the parts; and it can hardly be doubted that
if all the flowers and leaves on the same plant had been subjected to
the same external and internal condition, as are the flowers and leaves
in certain positions, all would have been modified in the same manner.”
Further on we meet with the following remarkable statement: “But when,
from the nature of the organism and of the conditions, modifications
have been induced which are unimportant for the welfare of the species,
they may be, and apparently often have been, transmitted in nearly the
same state to numerous, otherwise modified, descendants. It cannot have
been of much importance to the greater number of mammals, birds, or
reptiles, whether they were clothed with hair, feathers, or scales; yet
hair has been transmitted to almost all mammals, feathers to all birds,
and scales to all true reptiles. A structure, whatever it may be, which
is common to many allied forms, is ranked by us as of high systematic
importance, and consequently is often assumed to be of high vital
importance to the species. Thus, as I am inclined to believe,
morphological differences, which we consider as important,—such as the
arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc.,—first appeared in many cases
as fluctuating variations, which sooner or later became constant through
the nature of the organism and of the surrounding conditions, as well as
through the intercrossing of distinct individuals, but not through
natural selection; for as these morphological characters do not affect
the welfare of the species, any slight deviations in them could not have
been governed or accumulated through this latter agency. It is a strange
result which we thus arrive at, namely, that characters of slight vital
importance to the species are the most important to the systematist;
but, as we shall hereafter see when we treat of the genetic principle of
classification, this is by no means so paradoxical as it may at first
appear.”
If all this be granted, it is once more evident that the only variations
that come under the action of selection are the limited number that are
of vital importance to the organism. How little the theory of natural
selection can be used to explain the origin of species will be apparent
from the above quotation. This is, of course, not an argument against
the theory itself, which would still be one of vast importance if it
explained adaptive characters alone; but enough has been said, I think,
to show that it is improbable that the origin of adaptive and
non-adaptive characters are to be explained by entirely different
principles.
In reply to a criticism of Mivart, Darwin makes the further admission as
to the insufficiency of the theory of natural selection: “When
discussing special cases, Mr. Mivart passes over the effects of the
increased use and disuse of parts, which I have always maintained to be
highly important, and have treated in my ‘Variation under Domestication’
at greater length than, as I believe, any other writer. He likewise
often assumes that I attribute nothing to variation, independent of
natural selection, whereas in the work just referred to I have collected
a greater number of well-established cases than is to be found in any
other work known to me.” If this is admitted, and if it can be shown
that the evidence in favor of the inheritance of acquired characters is
very doubtful at best, may we not conclude that Mivart’s criticisms have
sometimes hit the mark?
The following objection appears to be a veritable stumbling-block to the
theory. Flatfishes and soles lie on one side, and do not stand in a
vertical position as do other fish. Some species lie on one side and
some on the other, and some species contain both right-sided and
left-sided individuals. In connection with this unusual habit we find a
striking change in the structure. The eye that would be on the under
side has shifted, so that it has come to lie on the upper side of the
head, _i.e._ both eyes lie on the same side,—a condition found in no
other vertebrate. As a result of the shifting of the eye, the bones of
the skull have also become profoundly modified. The young fish that
emerge from the egg swim at first upright, as do ordinary fish, and only
after they have led a free existence for some time do they turn to one
side and sink to the bottom. Unless the under eye moved to the upper
side it would be of no use to the flatfish, and might even be a source
of injury. Mivart points out that a sudden, spontaneous transformation
in the position of eye is hardly conceivable, and to this Darwin, of
course, assents. Mivart adds: “If the transit was gradual, then how such
transit of one eye a minute fraction of the journey towards the other
side of the head could benefit the individual is, indeed, far from
clear. It seems even that such an incipient transformation must rather
have been injurious.” Darwin’s reply is characteristic:—
“We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to the
individual and to the species, of endeavoring to look upwards with both
eyes, whilst resting on one side at the bottom. We may also attribute to
the inherited effects of use the fact of the mouth in several kinds of
flatfish being bent towards the lower surface, with the jaw-bones
stronger and more effective on this, the eyeless side of the head, than
on the other side, for the sake, as Dr. Traquair supposes, of feeding
with ease on the ground. Disuse, on the other hand, will account for the
less developed condition of the whole inferior half of the body,
including the lateral fins; though Yarrell thinks that the reduced size
of these fins is advantageous to the fish, as ‘there is so much less
room for their action, than with the larger fins above.’ Perhaps the
lesser number of teeth in the proportion of four to seven in the upper
halves of the two jaws of the plaice, to twenty-five to thirty in the
lower halves, may likewise be accounted for by disuse. From the
colorless state of the ventral surface of most fishes and of many other
animals, we may reasonably suppose that the absence of color in flatfish
on the side, whether it be the right or left, which is undermost, is due
to the exclusion of light.”
By falling back on the theory of inheritance of acquired characters
Darwin tacitly admits the incompetence of natural selection to explain
the evolution of the flatfish. If the latter theory prove incorrect, it
must then be admitted that the evolution of the flatfishes cannot be
accounted for by either of the two main theories on which Darwin relies.
Mivart further points out that the beginning stages of the mammary
glands cannot be explained by Darwin’s theory. To which Darwin replies,
that an American naturalist, Mr. Lockwood, believes from what he has
seen of the development of the young of the pipe-fish (Hippocampus) that
“they are nourished by a secretion from the cutaneous glands of the sac”
in which the young are enclosed. This can scarcely be said to be a
satisfactory reply; for, if it is true that this is the case for the
pipe-fish,—and I cannot find on inquiry that this statement has been
confirmed,—it is still rather speculative to suppose that the ancestral
mammals nourished their young by secreting a fluid into the marsupial
sac around the embryos.
Darwin deals with instincts of animals in the same way as he deals with
their structures. After pointing out that instincts are variable, and
that the variations are hereditary, he proceeds to show how selection
may act by picking out those individuals possessing the more favorable
instincts. In other words, the theory of natural selection is applied to
functions, as well as to structure. Darwin makes use here also of the
Lamarckian factor of inheritance, and concludes that “in most cases
habit and selection have probably both occurred.”
A few examples will sufficiently serve to illustrate Darwin’s meaning.
The first case given is that of the cuckoo, which lays its eggs in the
nests of other birds, where they are hatched and the young reared by
their foster-parents. The starting-point for such a perversion of the
ordinary habits of birds is to be found, he thinks, in the occasional
deposition of eggs in the nests of other birds, which has at times been
observed for a number of species. For instance, this has been seen in
the American cuckoo, which ordinarily builds a nest of its own. It is
recorded and believed to be true that the young English cuckoo, when
only two or three days old, ejects from the nest the offspring of its
foster-parents, and this “strange and odious instinct” is supposed by
Darwin to have been acquired in order that the young cuckoo might get
more food, and that the young bird has acquired during successive
generations the strength and structure necessary for the work of
ejection. This is of course largely speculative, and it is by no means
obvious that it was a greater benefit to the cuckoo to have other birds
rear its young than to do so itself. We can equally well imagine, since
this is the turn the argument takes, that the occasional instinct to
deposit eggs in the nests of other birds would be disadvantageous, and
could not have been acquired by the selection of a fluctuating instinct
of this sort. We have no right to assume, that because a new habit has
been acquired, that it is a more advantageous one than the one that has
been lost. All that we can legitimately infer is, that, although the
normal instinct has been changed into another, the race has still been
able to remain in existence. The same conclusion applies to the case of
_Molothrus bonariensis_, cited by Darwin, and is here even more
obvious:—
“Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of
their instincts. The sexes of _Molothrus badius_ are stated by an
excellent observer, Mr. Hudson, sometimes to live promiscuously together
in flocks, and sometimes to pair. They either build a nest of their own,
or seize on one belonging to some other bird, occasionally throwing out
the nestlings of the stranger. They either lay their eggs in the nest
thus appropriated, or oddly enough build one for themselves on the top
of it. They usually sit on their own eggs and rear their own young; but
Mr. Hudson says it is probable that they are occasionally parasitic, for
he has seen the young of this species following old birds of a distinct
kind and clamoring to be fed by them. The parasitic habits of another
species of Molothrus, the _M. bonariensis_, are much more highly
developed than those of the last, but are still far from perfect. This
bird, as far as is known, invariably lays its eggs in the nest of
strangers; but it is remarkable that several together sometimes commence
to build an irregular untidy nest of their own, placed in singularly
ill-adapted situations, as on the leaves of a large thistle. They never,
however, as far as Mr. Hudson has ascertained, complete a nest for
themselves. They often lay so many eggs—from fifteen to twenty—in the
same foster-nest, that few or none can possibly be hatched. They have,
moreover, the extraordinary habit of pecking holes in the eggs, whether
of their own species or of their foster-parents, which they find in the
appropriated nests. They drop also many eggs on the bare ground, which
are thus wasted.”
Can we possibly be expected to believe that it has been to the advantage
of this species to give up its original regular method of incubating its
own eggs, and acquire such a haphazard, new method? Does not the
explanation prove too much, rather than give support to Darwin’s
hypothesis? Is it not better to conclude, that despite the disadvantages
entailed by a change in the original instincts, the species is still
able to remain in existence?
Darwin points out, in the case of the slave-making ants, that the
slave-making instinct may have arisen in the first instance by ants
carrying pupæ, that they have captured, into their own nests. Later this
habit might become fixed, and, finally, after passing through several
stages of development, the ants might become absolutely dependent on
their slaves. It is also supposed that those colonies in which this
instinct was better developed would survive in competition with other
colonies of the same species on account of the supposed advantage of
owning slaves. In this way natural selection steps in and perfects the
process.
It is far from proven, or even made probable, that a species of ant that
becomes gradually dependent on its slaves is more likely to survive than
other colonies that are not so dependent. All we can be certain of is
that with slaves they have still been able to maintain their own.
Moreover, we must not forget that it is not enough to show that a
particular habit might be useful to a species, but it should also be
shown that it is of sufficient importance, at every stage of its
evolution, to give a decisive advantage in the “struggle for existence.”
For unless a life and death struggle takes place between the different
colonies, natural selection is powerless to bring about its supposed
results. And who will be bold enough to affirm that the presence of
slaves in a nest will give victory to that colony in competition with
its neighbors? Has the history of mankind taught us that the
slave-making countries have exterminated the countries without slaves?
Is the question so simple as this? May not the degeneration of the
masters more than compensate for the acquirement of slaves, and may not
the loss of life in obtaining slaves more than counterbalance the
advantage of the slaves after they are captured? In the face of these
possibilities it is not surprising to find that Darwin, when summing up
the chapter, makes the following admission: “I do not pretend that the
facts in this chapter strengthen in any degree my theory; but none of
the cases of difficulty, to the best of my judgment, annihilate it.”
Darwin, with his usual frankness, adds:—
“No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,—cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate
gradations are known to exist; cases of instincts of such trifling
importance, that they could hardly have been acted on by natural
selection; cases of instincts almost identically the same in animals so
remote in the scale of nature, that we cannot account for their
similarity by inheritance from a common progenitor, and consequently
must believe that they were independently acquired through natural
selection. I will not here enter on these several cases, but will
confine myself to one special difficulty, which at first appeared to me
insuperable, and actually fatal to the whole theory. I allude to the
neuters or sterile females in insect communities; for these neuters
often differ widely in instinct and in structure from both the males and
fertile females, and yet, from being sterile, they cannot propagate
their kind.
“The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can be
shown that some insects and other articulate animals in a state of
nature occasionally become sterile; and if such insects had been social,
and it had been profitable to the community that a number should have
been annually born capable of work, but incapable of procreation, I can
see no especial difficulty in this having been effected through natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax, and
in being destitute of wings and sometimes of eyes, and in instinct. As
far as instinct alone is concerned, the wonderful difference in this
respect between the workers and the perfect females, would have been
better exemplified by the hive-bee. If a working ant or other neuter
insect had been an ordinary animal, I should have unhesitatingly assumed
that all its characters had been slowly acquired through natural
selection; namely, by individuals having been born with slight
profitable modifications, which were inherited by the offspring; and
that these again varied and again were selected, and so onwards. But
with the working ant we have an insect differing greatly from its
parents, yet absolutely sterile; so that it could never have transmitted
successively acquired modifications of structure or instinct to its
progeny. It may well be asked, how is it possible to reconcile this case
with the theory of natural selection?”
Darwin’s answer is that the differences of structure are correlated with
certain ages and with the two sexes, but this is obviously only shifting
the difficulty, not meeting it. He concludes, “I can see no great
difficulty in any character becoming correlated with the sterile
condition of certain members of the insect communities, the difficulty
lies in understanding how such correlated modifications of structure
could have been slowly accumulated by natural selection.” “This
difficulty, though appearing insuperable, is lessened, or, as I believe,
disappears, when it is remembered that selection may be applied to the
family, as well as to the individual, and may thus give the desired
end.”
Darwin did not fail to see that there is a further difficulty even
greater than the one just mentioned. He says: “But we have not as yet
touched on the acme of the difficulty; namely, the fact that the neuters
of several ants differ, not only from the fertile females and males, but
from each other, sometimes to an almost incredible degree, and are thus
divided into two or even three castes. The castes, moreover, do not
commonly graduate into each other, but are perfectly well defined; being
as distinct from each other as are any two species of the same genus, or
rather as any two genera of the same family. Thus in Eciton, there are
working and soldier neuters, with jaws and instincts extraordinarily
different: in Cryptocerus, the workers of one caste alone carry a
wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they
have an enormously developed abdomen which secretes a sort of honey,
supplying the place of that excreted by the aphides, or the domestic
cattle as they may be called, which our European ants guard and
imprison.”
“It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate the theory. In the simpler
case of neuter insects all of one caste, which, as I believe, have been
rendered different from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary variations, that
the successive, slight, profitable modifications did not first arise in
all the neuters in the same nest, but in some few alone; and that by the
survival of the communities with females which produced most neuters
having the advantageous modification, all the neuters ultimately came to
be thus characterized. According to this view we ought occasionally to
find in the same nest neuter insects, presenting gradations of
structure; and this we do find, even not rarely, considering how few
neuter insects out of Europe have been carefully examined.”
From this the conclusion is reached:—
“With these facts before me, I believe that natural selection, by acting
on the fertile ants or parents, could form a species which should
regularly produce neuters, all of large size with one form of jaw, or
all of small size with widely different jaws; or lastly, and this is the
greatest difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and
structure;— a graduated series having first been formed, as in the case
of the driver ant, and then the extreme forms having been produced in
greater and greater numbers, through the survival of the parents which
generated them, until none with an intermediate structure were produced.
“I have now explained how, as I believe, the wonderful fact of two
distinctly defined castes of sterile workers existing in the same nest,
both widely different from each other and from their parents, has
originated. We can see how useful their production may have been to a
social community of ants, on the same principle that the division of
labor is useful to civilized man. Ants, however, work by inherited
instincts and by inherited organs or tools, whilst man works by acquired
knowledge and manufactured instruments. But I must confess, that, with
all my faith in natural selection, I should never have anticipated that
this principle could have been efficient in so high a degree, had not
the case of these neuter insects led me to this conclusion. I have,
therefore, discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and likewise
because this is by far the most serious special difficulty which my
theory has encountered. The case, also, is very interesting, as it
proves that with animals, as with plants, any amount of modification may
be effected by the accumulation of numerous, slight, spontaneous
variations, which are in any way profitable, without exercise or habit
having been brought into play. For peculiar habits confined to the
workers or sterile females, however long they might be followed, could
not possibly affect the males and fertile females, which alone leave
descendants. I am surprised that no one has hitherto advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.”
We may dissent at once from Darwin’s statement which, he thinks, “proves
that any amount of modification may be affected by the accumulation of
numerous slight variations which are in any way profitable without
exercise or habit having been brought into play”; we may dissent if for
no other reason than that this begs the whole point at issue, and is not
proven. It does not follow because in some colonies all intermediate
stages of neuters exist, that in other colonies, where no such
intermediate stages are present, these have been slowly weeded out by
natural selection, causing to disappear all colonies slightly below the
mark. It is this that begs the question. Because we can imagine that
intermediate stages between the different castes may have been present,
it neither follows that such fluctuating variations have been the basis
for the evolution of the more sharply defined types, nor that the
imagined advantage of such a change would have led through competition
to the extermination of the other colonies. However much we may admire
the skill with which Darwin tried to meet this difficulty, let us not
put down the results to the good of the theory, but rather repeat once
more Darwin’s own words at the end of this chapter, to the effect that
the facts do not strengthen the theory.
Sterility between Species
The care with which Darwin examined every bearing of his theory is
nowhere better exemplified than in his treatment of the question of
sterility between the individuals of different species. It would be so
obviously to the advantage of the selection theory if it were true that
sterility between species had been acquired by selection in order to
prevent intercrossing, that it would have been easy for a less cautious
thinker to have fallen into the error of supposing that sterility might
have been acquired in this way. Tempting as such a view appears, Darwin
was not caught by the specious argument, as the opening sentence in the
chapter of hybridism shows:—
“The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept
distinct had they been capable of freely crossing. The subject is in
many ways important for us, more especially as the sterility of species
when first crossed, and that of their hybrid offspring, cannot have been
acquired, as I shall show, by the preservation of successive profitable
degrees of sterility. It is an incidental result of differences in the
reproductive systems of the parent species.”
In dealing with this subject Darwin points out that we must be careful
to distinguish between “the sterility of species when first crossed, and
the sterility of hybrids produced from them.” In the former case, the
reproductive organs of each individual are in a perfectly normal
condition, while hybrids appear to be generally impotent owing to some
imperfection in the reproductive organs themselves. They are not
perfectly fertile, as a rule, either with each other, or with either of
the parent forms.
In striking contrast to the sterility between species is the fertility
of varieties. If, as Darwin believes, varieties are incipient species,
we should certainly expect to find them becoming less and less fertile
with other fraternal varieties, or with the parent forms in proportion
as they become more different. Yet experience appears to teach exactly
the opposite; but the question is not a simple one, and the results are
not so conclusive as appears at first sight. Let us first see how Darwin
met this obvious contradiction to his view.
In the first place, he points out that all species are not infertile
when crossed with other species. The sterility of various species, when
crossed, is so different in degree, and graduates away so insensibly,
and the fertility of pure species is so easily affected by various
circumstances, that it is most difficult to say where perfect fertility
ends and sterility begins. “It can thus be shown that neither sterility
nor fertility afford any certain distinction between species and
varieties.” Darwin cites several cases in plants in which crosses
between species have been successfully accomplished. The following
remarkable results are also recorded: “Individual plants in certain
species of Lobelia, Verbascum, and Passiflora can easily be fertilized
by pollen from a distinct species, but not by pollen from the same
plant, though this pollen can be proved to be perfectly sound by
fertilizing other plants or species. In the genus Hippeastrum, in
Corydalis as shown by Professor Hildebrand, in various orchids as shown
by Mr. Scott and Fritz Müller, all the individuals are in this peculiar
condition. So that with some species, certain abnormal individuals, and
in other species all the individuals, can actually be hybridized much
more readily than they can be fertilized by pollen from the same
individual plant!”[14]
Footnote 14:
A somewhat parallel case has recently been discovered by Castle for
the hermaphroditic ascidian _Ciona intestinalis_. In this case the
spermatozoa of any individual fail to fertilize the eggs of the same
individual, although they will fertilize the eggs of any other
individual.
In regard to animals, Darwin concludes that “if the genera of animals
are as distinct from each other as are the genera of plants, then we may
infer that animals more widely distinct in the scale of nature can be
crossed more easily than in the case of plants; but the hybrids
themselves are, I think, more sterile.”
The most significant fact in this connection is that the more widely
different two species are, so that they are placed in different
families, so much the less probable is it that cross-fertilization will
produce any result. From this condition of infertility there may be
traced a gradation between less different forms of the same genus to
almost complete, or even complete, fertility between closely similar
species. Darwin further points out that: “The hybrids raised from two
species which are very difficult to cross, and which rarely produce any
offspring, are generally very sterile; but the parallelism between the
difficulty of making a first cross, and the sterility of the hybrids
thus produced—two classes of facts which are generally confounded
together—is by no means strict. There are many cases, in which two pure
species, as in the genus Verbascum, can be united with unusual facility,
and produce numerous hybrid offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very
rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus,
for instance in Dianthus, these two opposite cases occur.”
In regard to reciprocal crosses Darwin makes the following important
statements: “The diversity of the result in reciprocal crosses between
the same two species was long ago observed by Kölreuter. To give an
instance: _Mirabilis jalapa_ can easily be fertilized by the pollen of
_M. longiflora_, and the hybrids thus produced are sufficiently fertile;
but Kölreuter tried more than two hundred times, during eight following
years, to fertilize reciprocally _M. longiflora_ with the pollen of _M.
jalapa_, and utterly failed.”
A formal interpretation of this difference can be easily imagined. The
infertility in one direction may be due to some physical difficulty met
with in penetrating the stigma, or style. For instance, the tissue in
one species may be too compact, or the style too long. Pflüger, who
carried out a large number of experiments by cross-fertilizing different
species of frogs, reached the conclusion that the spermatozoa having
small and pointed heads could cross-fertilize more kinds of eggs, than
could the spermatozoa with large blunt heads. This is probably due to
the ability of the smaller spermatozoa to penetrate the jelly around the
eggs, or the pores in the surface of the egg itself. But there are also
other sides to this question, as recent results have shown, for, even if
a foreign spermatozoon can enter an egg, it does not follow that the
development of the egg will take place. Here the difficulty is due to
some obscure processes in the egg itself. Now that we know more of the
nicely balanced combinations that take place during fertilization of the
egg, and during the process of cell division, we can easily see that if
the processes were in the least different in the two species it might be
impossible to combine them in a single act.
“Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together?”
“The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences in their
reproductive systems; the differences being of so peculiar and limited a
nature, that, in reciprocal crosses between the same two species, the
male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction.”
Does Darwin give here a satisfactory answer to the difficulty that he
started out to explain away? On the whole, the reader will admit, I
think, that he has fairly met the situation, in so far as he has shown
that there is no absolute line of demarcation between the power of
intercrossing of varieties and races, and of species. It is also
_extremely important to have found that the difficulties increase, so to
speak, even beyond the limits of the species_; since species, belonging
to different genera, are as a rule more difficult to intercross than
when they belong to the same genus. The further question, as to whether
there are differences in respect to the power of intercrossing between
different kinds of varieties, such as those dependent on selection of
fluctuating variations, of local conditions, of mutations, etc., is far
from being settled at the present time.
That this property of species is useful to them, in the somewhat unusual
sense that it keeps them from freely mingling with other species, is
true; but, as has been said, this would be a rather peculiar kind of
adaptation. If, however, it be claimed that this property is useful to
species, as Darwin himself claims, then, as he also points out, it is a
useful acquirement that cannot have arisen through natural selection. It
is not difficult to show why this must be so. If two varieties were to
some extent at the start less fertile, _inter se_, than with their own
kind, the only way in which they could become more infertile through
selection would be by selecting those individuals in each generation
that are still more infertile, but the forms of this sort would, _ex
hypothese_, become less numerous than the descendants of each species
itself, which would, therefore, supplant the less fertile ones.
Darwin’s own statement in regard to this point is as follows:—
“At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared in
certain individuals of one variety when crossed with those of another
variety. For it would clearly be advantageous to two varieties or
incipient species, if they could be kept from blending, on the same
principle that, when man is selecting at the same time two varieties, it
is necessary that he should keep them separate.
“In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps from
slightly lessened fertility to absolute sterility. It may be admitted
that it would profit an incipient species, if it were rendered in some
slight degree sterile when crossed with its parent form or with some
other variety; for thus fewer bastardized and deteriorated offspring
would be produced to commingle their blood with the new species in
process of formation. But he who will take the trouble to reflect on the
steps by which this first degree of sterility could be increased through
natural selection to that high degree which is common with so many
species, and which is universal with species which have been
differentiated to a generic or family rank, will find the subject
extraordinarily complex. After mature reflection it seems to me that
this could not have been effected through natural selection. Take the
case of any two species which, when crossed, produced few and sterile
offspring; now, what is there which could favor the survival of those
individuals which happened to be endowed in a slightly higher degree
with mutual infertility, and which thus approached by one small step
toward absolute sterility? Yet an advance of this kind, if the theory of
natural selection be brought to bear, must have incessantly occurred
with many species, for a multitude are mutually quite barren.”
Darwin points out the interesting parallel existing between the results
of intercrossing, and those of grafting together parts of different
species.
“As the capacity of one plant to be grafted or budded on another is
unimportant for their welfare in a state of nature, I presume that no
one will suppose that this capacity is a _specially_ endowed quality,
but will admit that it is incidental on differences in the laws of
growth of the two plants. We can sometimes see the reason why one tree
will not take on another, from differences in their rate of growth, in
the hardness of their wood, in the period of the flow or nature of their
sap, etc.; but in a multitude of cases we can assign no reason whatever.
Great diversity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other deciduous, and adapted to
widely different climates, do not always prevent the two grafting
together. As in hybridization, so with grafting, the capacity is limited
by systematic affinity, for no one has been able to graft together trees
belonging to quite distinct families; and, on the other hand, closely
allied species, and varieties of the same species, can usually, but not
invariably, be grafted with ease. But this capacity, as in
hybridization, is by no means absolutely governed by systematic
affinity. Although many distinct genera within the same family have been
grafted together, in other cases species of the same genus will not take
on each other. The pear can be grafted far more readily on the quince,
which is ranked as a distant genus, than on the apple, which is a member
of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties
of the apricot and peach on certain varieties of the plum.”
“We thus see, that although there is a clear and great difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a rude
degree of parallelism in the results of grafting and of crossing of
distinct species. And we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other as
incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of first
crosses are incidental on unknown differences in their reproductive
systems.... The facts by no means seem to indicate that the greater or
lesser difficulty of either grafting or crossing various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.”
Weismann’s Germinal Selection
We cannot do better, in bringing this long criticism of the Darwinian
theory to an end, than by considering the way in which Weismann has
attempted in his paper on “Germinal Selection” to solve one of the
“patent contradictions” of the selection theory. He calls attention, in
doing so, to what he regards as a vital weakness of the theory in the
form in which it was left by Darwin himself. Weismann says:—
“The basal idea of the essay—the existence of Germinal Selection—was
propounded by me some time since,[15] but it is here for the first time
fully set forth and tentatively shown to be the necessary complement of
the process of selection. Knowing this factor, we remove, it seems to
me, the patent contradiction of the assumption that the general fitness
of organisms, or the adaptations _necessary_ to their existence, are
produced by _accidental_ variations—a contradiction which formed a
serious stumbling-block to the theory of selection. Though still
assuming that the _primary_ variations are ‘accidental,’ I yet hope to
have demonstrated that an interior mechanism exists which compels them
to go on increasing in a definite direction, the moment selection
intervenes. _Definitely directed variation exists_, but not predestined
variation, running on independently of the life conditions of the
organism, as Nägeli, to mention the most extreme advocate of this
doctrine, has assumed; on the contrary, the variation is such as is
elicited and controlled by those conditions themselves, though
indirectly.”
Footnote 15:
_Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer_,
Jena, 1895.
“The real aim of the present essay is to rehabilitate the principle of
selection. If I should succeed in reinstating this principle in its
emperilled rights, it would be a source of extreme satisfaction to me;
for I am so thoroughly convinced of its indispensability as to believe
that its demolition would be synonymous with the renunciation of all
inquiry concerning the causal relation of vital phenomena. If we could
understand the adaptations of nature, whose number is infinite, only
upon the assumption of a teleological principle, then, I think, there
would be little inducement to trouble ourselves about the causal
connection of the stages of ontogenesis, for no good reason would exist
for excluding teleological principles from this field. Their
introduction, however, is the ruin of science.”[16]
Footnote 16:
Translated by J. McCormack. The Open Court Publishing Company. The
following quotations are also taken from this translation.
Weismann states that those critics who maintain that selection cannot
create, but only reject, “fail to see that precisely through this
rejection its creative efficacy is asserted.” There is raised here,
though not for the first time, a point that is of no small importance
for both Darwinians and anti-Darwinians to consider; for, without
further examination, it is by no means self-evident, as Weismann
implies, that by exterminating all variations that are below the average
the standard of successive generations could ever be raised beyond the
most extreme fluctuating variation. At least this appears to be the case
if individual, fluctuating variations be the sort selected, and it is to
this kind of variation to which Weismann presumably refers. Without
discussing this point here, let us examine further what Weismann has to
say. He thinks that while in each form there may be a very large number
of possible variations, yet there are also impossible variations as
well, which do not appear. “The cogency, the irresistible cogency as I
take it, of the principle of selection is precisely its capacity of
explaining why fit structures always arise, and this certainly is the
great problem of life.” Weismann points out that it is a remarkable fact
that to-day, after science has been in possession of this principle for
something over thirty years, “during which time she has busily occupied
herself with its scope, the estimation in which the theory is held
should be on the decline.” “It would be easy to enumerate a long list of
living writers who assign to it a subordinate part only in evolution, or
none at all.” “Even Huxley implicitly, yet distinctly, intimated a doubt
regarding the principle of selection when he said: ‘Even if the
Darwinian hypothesis were swept away, evolution would still stand where
it is.’ Therefore he, too, regarded it as not impossible that this
hypothesis should disappear from among the great explanatory principles
by which we seek to approach nearer to the secrets of nature.”
Weismann is not, however, of this opinion, and believes that the present
depression is only transient, because it is only a reaction against a
theory that had been exalted to the highest pinnacle. He thinks that the
principle of selection is not overestimated, but that naturalists
imagined too quickly that they understood its workings. “On the
contrary, the deeper they penetrated into its workings the clearer it
appeared that something was lacking, that the action of the principle,
though upon the whole clear and representable, yet when carefully looked
into encountered numerous difficulties, which were formidable, for the
reason that we were unsuccessful in tracing out the actual details of
the individual process, and, therefore, in _fixing_ the phenomenon as it
actually occurred. We can state in no single case how great a variation
must be to have selective value, nor how frequently it must occur to
acquire stability. We do not know when and whether a desired useful
variation really occurs, nor on what its appearance depends; and we have
no means of ascertaining the space of time required for the fulfilment
of the selective processes of nature, and hence cannot calculate the
exact number of such processes that do and can take place at the same
time in the same species. Yet all this is necessary if we wish to follow
out the precise details of a given case.
“But perhaps the most discouraging circumstance of all is, that we can
assert in scarcely a single actual instance in nature whether an
observed variation is useful or not—a drawback that I distinctly
emphasized some time ago. Nor is there much hope of betterment in this
respect, for think how impossible it would be for us to observe all the
individuals of a species in all their acts of life, be their habitat
ever so limited—and to observe all this with a precision enabling us to
say that this or that variation possessed selective value, that is, was
a decisive factor in determining the existence of the species.”
“And thus it is everywhere. Even in the most indubitable cases of
adaptation as, for instance, in that of the striking protective coloring
of many butterflies, the sole ground of inference that the species on
the whole is adequately adapted to its conditions of life, is the simple
fact that the species is, to all appearances, preserved undiminished,
but the inference is not at all permissible that just this protective
coloring has selective value for the species, that is, if it were
lacking, the species would necessarily have perished.”
Few opponents of Darwinism could give a more pessimistic account of the
accomplishments of the theory of natural selection than this, by one of
the leaders of the modern school: “Discouraging, therefore, as it may be
that the control of nature in her minutest details is here gainsaid us,
yet it were equivalent to sacrificing the gold to the dross, if simply
from our inability to follow out the details of the individual case we
should renounce altogether the principle of selection, or should
proclaim it as only subsidiary, on the ground that we believe the
protective coloring of the butterfly is not a protective coloring, but a
combination of colors inevitably resulting from internal causes. The
protective coloring remains a protective coloring whether at the time in
question it is or is not necessary for the species; and it arose as
protective coloring—arose not because it was a constitutional necessity
of the animal’s organism that here a red and there a white, black, or
yellow spot should be produced, but because it was advantageous, because
it was necessary for the animal. There is only one explanation possible
for such patent adaptations, and that is selection. What is more, no
other natural way of their originating is conceivable, for we have no
right to assume teleological forces in the domain of natural phenomena.”
Weismann states that he does not accept Eimers’s view that the markings
of the wings of the butterflies of the genus Papilio are due to a
process of evolution in a direct line, independent of external causes.
“On the contrary, I believe it can be clearly proved that the wing of
the butterfly is a tablet on which Nature has inscribed everything she
has deemed advantageous to the preservation and welfare of her
creatures, and nothing else; or, to abandon the simile, that these color
patterns have not proceeded from inward evolutional forces but are the
result of selection. At least in all places where we do understand their
biological significance these patterns are constituted and distributed
over the wing exactly as utility would require.”
Again: “I should be far from maintaining that the markings arose
unconformably to law. Here, as elsewhere, the dominance of law is
certain. But I take it, that the laws involved, that is, the
physiological conditions of the variation, here are without exception
subservient to the ends of a higher power—utility; and that it is
utility primarily that determines the kind of colors, spots, streaks,
and bands that shall originate, as also their place and mode of
disposition. The laws come into consideration only to the extent of
conditioning the quality of the constructive materials—the variations,
out of which selection fashions the designs in question. And this also
is subject to important restrictions, as will appear in the sequel.”
This conclusion contains all that the most ardent Darwinian could ask.
He rejects the idea that internal laws alone could have produced the
result, because:—
“If internal laws controlled the markings on butterflies’ wings, we
should expect that some general rule could be established, requiring
that the upper and under surfaces of the wings should be alike or that
they should be different, or that the fore wings should be colored the
same as or differently from the hind wings, etc. But in reality all
possible kinds of combinations occur simultaneously, and no rule holds
throughout. Or, it might be supposed that bright colors should occur
only on the upper surface or only on the under surface, or on the fore
wings or only on the hind wings. But the fact is they occur
indiscriminately, now here, now there, and no one method of appearance
is uniform throughout all the species. But the fitness of the various
distributions of colors is apparent, and the moment we apply the
principle of utility we know why in the diurnal butterflies the upper
surface alone is usually variegated and the under surface protectively
colored, or why in the nocturnal butterflies the fore wings have the
appearance of bark, of old wood, or of a leaf, whilst the hind wings,
which are covered when resting, alone are brilliantly colored. On this
theory we also understand the exceptions to these rules. We comprehend
why Danaids, Heliconids, Euploids, and Acracids, in fact all diurnal
butterflies offensive to the taste and smell, are mostly brightly marked
and equally so on both surfaces, whilst all species not thus exempt from
persecution have the protective coloring on the under surface and are
frequently quite differently colored there from what they are on the
upper.
“In any event, the supposed formative laws are not obligatory.
Dispensations from them can be issued and are issued _whenever utility
requires it_.”
Dispensations from the laws of growth! Does not a philosophy of this
sort seem to carry us back into the dark ages? Is this the best that the
Darwinian school can do to protect itself against the difficulties into
which its chief disciple confesses it has fallen?
Weismann lays great emphasis on the case of the Indian leaf-butterfly,
_Kallima inachis_; and points out that the leaf markings are executed
“in absolute independence of the other uniformities governing the wing.”
“The venation of the wing is utterly ignored by the leaf markings, and
its surface is treated as a _tabula rasa_ upon which anything
conceivable can be drawn. In other words, we are presented here with a
_bilaterally symmetrical_ figure engraved on a surface which is
essentially _radially symmetrical_ in its divisions.
“I lay unusual stress upon this point because it shows that we are
dealing here with one of those cases which cannot be explained by
mechanical, that is, by natural means, unless natural selection actually
exists and is actually competent to create new properties; for the
Lamarckian principle is excluded here _ab initio_, seeing that we are
dealing with a formation which is only passive in its effects: the leaf
markings are effectual simply by their existence and not by any function
which they perform; they are present in flight as well as at rest,
during the absence of a danger, as well as during the approach of an
enemy.
“Nor are we helped here by the assumption of _purely internal motive
forces_, which Nägeli, Askenasy, and others have put forward as
supplying a _mechanical_ force of evolution. It is impossible to regard
the coincidence of an Indian butterfly with the leaf of a tree now
growing in an Indian forest as fortuitous, as a _lusus naturæ_. Assuming
this seemingly mechanical force, therefore, we should be led back
inevitably to a teleological principle which produces adaptive
characters and which must have deposited the directive principle in the
very first germ of terrestrial organisms, so that after untold ages at a
definite time and place the illusive leaf markings should be developed.
The assumption of preëstablished harmony between the evolution of the
ancestral line of the tree with its prefigurative leaf, and that of the
butterfly with its imitating wing, is absolutely necessary here, as I
pointed out many years ago, but as is constantly forgotten by the
promulgators of the theory of internal evolutionary forces.”
Weismann concludes, therefore, that for his present purpose it suffices
to show “that cases exist wherein all natural explanations except that
of selection fail us,” and he then proceeds to point out that even the
natural selection of Darwin and of Wallace also fail to give us a
reasonable explanation of how, for example, the markings on the wings of
the Kallima butterfly have come about. The main reason that he gives to
show that this is the case rests on the difficulty of the assumption
that the right variations should always be present in the right place.
Here “is the insurmountable barrier for the explanatory power of the
principle [natural selection] for who, or what, is to be our guarantee
that the dark scales shall appear at the exact spots on the wing where
the midrib of the leaf must grow? And that later dark scales shall
appear at the exact spots to which the midrib must be prolonged? And
that still later such dark scales shall appear at the places whence the
lateral ribs start, and that here also a definite acute angle shall be
preserved.” Thus the philosopher in his closet multiplies and magnifies
the difficulties for which he is about to offer a panacea. Had the same
amount of labor been spent in testing whether the life of this butterfly
is so closely dependent on the exact imitation of the leaf, we might
have been spared the pains of this elaborate exordium. There are at
least some grounds for suspicion that the whole case of Kallima is “made
up.” If this should prove true, it will be a bad day for the Darwinians,
unless they fall back on Weismann’s statement that their theory is
insufficient to prove a single case!
Weismann has used Kallima only as the most instructive illustration. The
objections that are here evident are found not only in the cases of
protective coloration, but “are applicable in all cases where the
process of selection is concerned. Take, for example, the case of
instincts that are called into action only once in life, as the pupal
performances of insects, the fabrication of cocoons, etc. How is it that
the useful variations were always present here?” Weismann concludes that
“something is still wanting to the selection theory of Darwin and
Wallace, which it is obligatory on us to discover, if we possibly can,
and without which selection as yet offers no complete explanation of the
phyletic processes of transformation.” Weismann’s first step in the
solution of the difficulty is contained in the following statement:—
“My inference is a very simple one: if we are forced by the facts on all
hands to the assumption that the useful variations which render
selection possible are always present, then, _some profound connection
must exist between the utility of a variation and its actual
appearance_, or, in other words, _the direction of the variation of a
part must be determined by utility_, and we shall have to see whether
facts exist that confirm our conjecture.”
Weismann finds the solution in the method by which the breeder has
obtained his results in artificial selection. For instance, the
long-tailed variety of the domestic cock of Japan owes its existence, it
is claimed, to skilful selection, and not at all to the circumstance
that, at some period of the race’s history, a cock with tail-feathers
six feet in length suddenly and spasmodically appeared.
Weismann continues: “Now what does this mean? Simply that the hereditary
diathesis, the germinal constitution (the _Anlage_) of the breed was
changed in the respect in question, and our conclusion from this and
numerous similar facts of artificial selection runs as follows: _by the
selection alone of the plus or minus variations of a character is the
constant modification of that character in the plus or minus direction
determined_. Obviously the hereditary _diminution_ of a part is also
effected by the simple selection of the individuals in each generation
possessing the smallest parts, as is proved, for example, by the tiny
bills and feet of numerous breeds of doves. We may assert, therefore, in
general terms: a definitely directed progressive variation of a given
part is produced by continued selection in that definite direction. This
is no hypothesis, but a direct inference from the facts and may also be
expressed as follows: _by a selection of the kind referred to the germ
is progressively modified in a manner corresponding with the production
of a definitely directed progressive variation of the part_.”
So far there is nothing essentially new offered, since Darwin often
tacitly recognized that the standard of variation could be raised in
this way, and in some places he has made definite statements that this
will take place. Weismann thinks that after each selection, fluctuation
will then occur around a higher average (mode). He says “that this is a
fact,” and is proved by the case of the Japanese cock. It need scarcely
be pointed out that it is an assumption, based on what is supposed to
have taken place in this bird, and is not a “fact.”
Weismann continues: “But the question remains, _why_ is this the fact?”
He believes his hypothesis of the existence of determinants in the germ
gives a satisfactory answer to this “why.” “According to this theory
every independent and hereditarily variable part is represented in the
germ by a _determinant_, whose size and power of assimilation
corresponds to the size and vigor of the part. These determinants
multiply as do all vital units by growth and division, and necessarily
they increase rapidly in every individual, and the more rapidly the
greater the quantity of the germinal cells the individual produces. And
since there is no more reason for excluding irregularities of passive
nutrition, and of the supply of nutriment in these minute,
microscopically invisible parts, than there is in the larger visible
parts of the cells, tissues, and organs, consequently the descendants of
a determinant can never all be exactly alike in size and capacity of
assimilation, but they will oscillate in this respect to and fro about
the maternal determinant as about their zero point, and will be partly
greater, partly smaller, and partly of the same size as that. In these
oscillations, now, the material for further selection is presented, and
in the inevitable fluctuations of the nutrient supply, I see the reason
why every step attained immediately becomes the zero point of new
fluctuations, and consequently why the size of a part can be augmented
or diminished by selection without limit, solely by the displacement of
the zero point of variation as the result of selection.”
The best illustration of this process of germinal selection is found,
Weismann believes, in the case of the degeneration of organs. “For in
most retrogressive processes _active_ selection in Darwin’s sense plays
no part, and advocates of the Lamarckian principle, as above remarked,
have rightly denied that active selection, that is, the selection of
individuals possessing the useless organ in its most reduced state, is
sufficient to explain the process of degeneration. I, for my part, have
never assumed this, and have on this very account enunciated the
_principle of panmixia_. Now, although this, as I have still no reason
for doubting, is a perfectly correct principle, which really does have
an essential and indispensable share in the process of retrogression,
still it is not _alone_ sufficient for a full explanation of the
phenomena. My opponents, in advancing this objection, were right, to the
extent indicated, and as I expressly acknowledge, although they were
unable to substitute anything positive in its stead or to render my
explanation complete. The very fact of the cessation of control over the
organ is sufficient to explain its _degeneration_, that is, its
deterioration, the disharmony of its parts, but not the fact which
actually and always occurs where an organ has become useless—viz., _its
gradual and unceasing diminution continuing for thousands and thousands
of years and culminating in its final and absolute effacement_.”
If then neither selection of persons nor the cessation of personal
selection can explain the phenomenon, we must look elsewhere for the
answer. This Weismann finds in the application of Roux’s hypothesis of
the struggle of the parts to obtain nourishment.
“The production of the long tail-feathers of the Japanese cock does not
repose solely on the displacement directly effected by personal
selection, of the zero point of variation upward, but that _it is also
fostered and strengthened by germinal selection_. Were that not so, the
phenomena of the transmutation of species, in so far as fresh growth and
the enlargement and complication of organs already present are
concerned, _would not be a whit more intelligible than they were
before_.”
Thus Weismann has piled up one hypothesis on another as though he could
save the integrity of the theory of natural selection by adding new
speculative matter to it. The most unfortunate feature is that the new
speculation is skilfully removed from the field of verification, and
invisible germs whose sole functions are those which Weismann’s
imagination bestows on them, are brought forward as though they could
supply the deficiencies of Darwin’s theory. This is, indeed, the old
method of the philosophizers of nature. An imaginary system has been
invented which attempts to explain all difficulties, and if it fails,
then new inventions are to be thought of. Thus we see where the theory
of the selection of fluctuating germs has led one of the most widely
known disciples of the Darwinian theory.
The worst feature of the situation is not so much that Weismann has
advanced new hypotheses unsupported by experimental evidence, but that
the speculation is of such a kind that it is, from its very nature,
unverifiable, and therefore useless. Weismann is mistaken when he
assumes that many zoologists object to his methods because they are
largely speculative. The real reason is that the speculation is so often
of a kind that cannot be tested by observation or by experiment.
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