how it is that low forms of life continue to exist, what kind of
circumstances are most favourable to the formation of new species, and,
lastly, to what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to consider some of
the more important objections and difficulties which have been advanced
by eminent naturalists.
FOOTNOTES:
[Footnote 37: _Origin of Species_, p. 71.]
[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
77.]
[Footnote 39: _Origin of Species_, p. 89.]
[Footnote 40: _Nature_, vol. xxx. p. 30.]
CHAPTER VI
DIFFICULTIES AND OBJECTIONS
Difficulty as to smallness of variations - As to the right
variations occurring when required - The beginnings of important
organs - The mammary glands - The eyes of flatfish - Origin of the
eye - Useless or non-adaptive characters - Recent extension of the
region of utility in plants - The same in animals - Uses of
tails - Of the horns of deer - Of the scale-ornamentation of
reptiles - Instability of non-adaptive characters - Delboeuf's
law - No "specific" character proved to be useless - The swamping
effects of intercrossing - Isolation as preventing
intercrossing - Gulick on the effects of isolation - Cases in
which isolation is ineffective.
In the present chapter I propose to discuss the more obvious and often
repeated objections to Darwin's theory, and to show how far they affect
its character as a true and sufficient explanation of the origin of
species. The more recondite difficulties, affecting such fundamental
questions as the causes and laws of variability, will be left for a
future chapter, after we have become better acquainted with the
applications of the theory to the more important adaptations and
correlations of animal and plant life.
One of the earliest and most often repeated objections was, that it was
difficult "to imagine a reason why variations tending in an
infinitesimal degree in any special direction should be preserved," or
to believe that the complex adaptation of living organisms could have
been produced "by infinitesimal beginnings." Now this term
"infinitesimal," used by a well-known early critic of the _Origin of
Species_, was never made use of by Darwin himself, who spoke only of
variations being "slight," and of the "small amount" of the variations
that might be selected. Even in using these terms he undoubtedly
afforded grounds for the objection above made, that such small and
slight variations could be of no real use, and would not determine the
survival of the individuals possessing them. We have seen, however, in
our third chapter, that even Darwin's terms were hardly justified; and
that the variability of many important species is of considerable
amount, and may very often be properly described as large. As this is
found to be the case both in animals and plants, and in all their chief
groups and subdivisions, and also to apply to all the separate parts and
organs that have been compared, we must take it as proved that the
average _amount_ of variability presents no difficulty whatever in the
way of the action of natural selection. It may be here mentioned that,
up to the time of the preparation of the last edition of _The Origin of
Species_, Darwin had not seen the work of Mr. J.A. Allen of Harvard
University (then only just published), which gave us the first body of
accurate comparisons and measurements demonstrating this large amount of
variability. Since then evidence of this nature has been accumulating,
and we are, therefore, now in a far better position to appreciate the
facilities for natural selection, in this respect, than was Mr. Darwin
himself.
Another objection of a similar nature is, that the chances are immensely
against the right variation or combination of variations occurring just
when required; and further, that no variation can be perpetuated that is
not accompanied by several concomitant variations of dependent
parts - greater length of a wing in a bird, for example, would be of
little use if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong one so long
as it was supposed that variations occurred singly and at considerable
intervals; but it ceases to have any weight now we know that they occur
simultaneously in various parts of the organism, and also in a large
proportion of the individuals which make up the species. A considerable
number of individuals will, therefore, every year possess the required
combination of characters; and it may also be considered probable that
when the two characters are such that they always _act_ together, there
will be such a correlation between them that they will frequently _vary_
together. But there is another consideration that seems to show that
this coincident variation is not essential. All animals in a state of
nature are kept, by the constant struggle for existence and the survival
of the fittest, in such a state of perfect health and usually
superabundant vigour, that in all ordinary circumstances they possess a
surplus power in every important organ - a surplus only drawn upon in
cases of the direst necessity when their very existence is at stake. It
follows, therefore, that _any_ additional power given to one of the
component parts of an organ must be useful - an increase, for example,
either in the wing muscles or in the form or length of the wing might
give _some_ increased powers of flight; and thus alternate
variations - in one generation in the muscles, in another generation in
the wing itself - might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals. On either
supposition, however, this objection appears to have little weight if we
take into consideration the large amount of coincident variability that
has been shown to exist.
_The Beginnings of Important Organs._
We now come to an objection which has perhaps been more frequently urged
than any other, and which Darwin himself felt to have much weight - the
first beginnings of important organs, such, for example, as wings, eyes,
mammary glands, and numerous other structures. It is urged, that it is
almost impossible to conceive how the first rudiments of these could
have been of any use, and, if not of use they could not have been
preserved and further developed by natural selection.
Now, the first remark to be made on objections of this nature is, that
they are really outside the question of the origin of all existing
species from allied species not very far removed from them, which is all
that Darwin undertook to _prove_ by means of his theory. Organs and
structures such as those above mentioned all date back to a very remote
past, when the world and its inhabitants were both very different from
what they are now. To ask of a new theory that it shall reveal to us
exactly what took place in remote geological epochs, and how it took
place, is unreasonable. The most that should be asked is, that some
probable or possible mode of origination should be pointed out in some
at least of these difficult cases, and this Mr. Darwin has done. One or
two of these may be briefly given here, but the whole series should be
carefully read by any one who wishes to see how many curious facts and
observations have been required in order to elucidate them; whence we
may conclude that further knowledge will probably throw light on any
difficulties that still remain.[41]
In the case of the mammary glands Mr. Darwin remarks that it is admitted
that the ancestral mammals were allied to the marsupials. Now in the
very earliest mammals, almost before they really deserved that name, the
young may have been nourished by a fluid secreted by the interior
surface of the marsupial sack, as is believed to be the case with the
fish (Hippocampus) whose eggs are hatched within a somewhat similar
sack. This being the case, those individuals which secreted a more
nutritious fluid, and those whose young were able to obtain and swallow
a more constant supply by suction, would be more likely to live and come
to a healthy maturity, and would therefore be preserved by natural
selection.
In another case which has been adduced as one of special difficulty, a
more complete explanation is given. Soles, turbots, and other flatfish
are, as is well known, unsymmetrical. They live and move on their sides,
the under side being usually differently coloured from that which is
kept uppermost. Now the eyes of these fish are curiously distorted in
order that both eyes may be on the upper side, where alone they would be
of any use. It was objected by Mr. Mivart that a sudden transformation
of the eye from one side to the other was inconceivable, while, if the
transit were gradual the first step could be of no use, since this would
not remove the eye from the lower side. But, as Mr. Darwin shows by
reference to the researches of Malm and others, the young of these fish
are quite symmetrical, and during their growth exhibit to us the whole
process of change. This begins by the fish (owing to the increasing
depth of the body) being unable to maintain the vertical position, so
that it falls on one side. It then twists the lower eye as much as
possible towards the upper side; and, the whole bony structure of the
head being at this time soft and flexible, the constant repetition of
this effort causes the eye gradually to move round the head till it
comes to the upper side. Now if we suppose this process, which in the
young is completed in a few days or weeks, to have been spread over
thousands of generations during the development of these fish, those
usually surviving whose eyes retained more and more of the position into
which the young fish tried to twist them, the change becomes
intelligible; though it still remains one of the most extraordinary
cases of degeneration, by which symmetry - which is so universal a
characteristic of the higher animals - is lost, in order that the
creature may be adapted to a new mode of life, whereby it is enabled the
better to escape danger and continue its existence.
The most difficult case of all, that of the eye - the thought of which
even to the last, Mr. Darwin says, "gave him a cold shiver" - is
nevertheless shown to be not unintelligible; granting of course the
sensitiveness to light of some forms of nervous tissue. For he shows
that there are, in several of the lower animals, rudiments of eyes,
consisting merely of pigment cells covered with a translucent skin,
which may possibly serve to distinguish light from darkness, but nothing
more. Then we have an optic nerve and pigment cells; then we find a
hollow filled with gelatinous substance of a convex form - the first
rudiment of a lens. Many of the succeeding steps are lost, as would
necessarily be the case, owing to the great advantage of each
modification which gave increased distinctness of vision, the creatures
possessing it inevitably surviving, while those below them became
extinct. But we can well understand how, after the first step was taken,
every variation tending to more complete vision would be preserved till
we reached the perfect eye of birds and mammals. Even this, as we know,
is not absolutely, but only relatively, perfect. Neither the chromatic
nor the spherical aberration is absolutely corrected; while long-and
short-sightedness, and the various diseases and imperfections to which
the eye is liable, may be looked upon as relics of the imperfect
condition from which the eye has been raised by variation and natural
selection.
These few examples of difficulties as to the origin of remarkable or
complex organs must suffice here; but the reader who wishes further
information on the matter may study carefully the whole of the sixth
and seventh chapters of the last edition of _The Origin of Species_, in
which these and many other cases are discussed in considerable detail.
_Useless or non-adaptive Characters._
Many naturalists seem to be of opinion that a considerable number of the
characters which distinguish species are of no service whatever to their
possessors, and therefore cannot have been produced or increased by
natural selection. Professors Bronn and Broca have urged this objection
on the continent. In America, Dr. Cope, the well-known palaeontologist,
has long since put forth the same objection, declaring that non-adaptive
characters are as numerous as those which are adaptive; but he differs
completely from most who hold the same general opinion in considering
that they occur chiefly "in the characters of the classes, orders,
families, and other higher groups;" and the objection, therefore, is
quite distinct from that in which it is urged that "specific characters"
are mostly useless. More recently, Professor G.J. Romanes has urged this
difficulty in his paper on "Physiological Selection" (_Journ. Linn.
Soc._, vol. xix. pp. 338, 344). He says that the characters "which serve
to distinguish allied species are frequently, if not usually, of a kind
with which natural selection can have had nothing to do," being without
any utilitarian significance. Again he speaks of "the enormous number,"
and further on of "the innumerable multitude" of specific peculiarities
which are useless; and he finally declares that the question needs no
further arguing, "because in the later editions of his works Mr. Darwin
freely acknowledges that a large proportion of specific distinctions
must be conceded to be useless to the species presenting them."
I have looked in vain in Mr. Darwin's works to find any such
acknowledgment, and I think Mr. Romanes has not sufficiently
distinguished between "useless characters" and "useless specific
distinctions." On referring to all the passages indicated by him I find
that, in regard to specific characters, Mr. Darwin is very cautious in
admitting inutility. His most pronounced "admissions" on this question
are the following: "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_" (_Origin_, p. 175). The words I have here italicised
clearly show that such characters are usually not "specific," in the
sense that they are such as distinguish species from each other, but are
found in numerous allied species. Again: "Thus a large yet undefined
extension may safely be given to the direct and indirect results of
natural selection; but I now admit, after reading the essay of Nägeli on
plants, and the remarks by various authors with respect to animals, more
especially those recently made by Professor Broca, that in the earlier
editions of my _Origin of Species_ I perhaps attributed too much to the
action of natural selection or the survival of the fittest. I have
altered the fifth edition of the _Origin_ so as to confine my remarks to
adaptive changes of structure, _but I am convinced, from the light
gained during even the last few years, that very many structures which
now appear to us useless, will hereafter be proved to be useful, and
will therefore come within the range of natural selection_. Nevertheless
I did not formerly consider sufficiently the existence of structures
which, _as far as we can at present judge_, are neither beneficial nor
injurious; and this I believe to be one of the greatest oversights as
yet detected in my work." Now it is to be remarked that neither in these
passages nor in any of the other less distinct expressions of opinion on
this question, does Darwin ever admit that "specific characters" - that
is, the particular characters which serve to distinguish one species
from another - are ever useless, much less that "a large proportion of
them" are so, as Mr. Romanes makes him "freely acknowledge." On the
other hand, in the passage which I have italicised he strongly expresses
his view that much of what we suppose to be useless is due to our
ignorance; and as I hold myself that, as regards many of the supposed
useless characters, this is the true explanation, it may be well to give
a brief sketch of the progress of knowledge in transferring characters
from the one category to the other.
We have only to go back a single generation, and not even the most acute
botanist could have suggested a reasonable use, for each species of
plant, of the infinitely varied forms, sizes, and colours of the
flowers, the shapes and arrangement of the leaves, and the numerous
other external characters of the whole plant. But since Mr. Darwin
showed that plants gained both in vigour and in fertility by being
crossed with other individuals of the same species, and that this
crossing was usually effected by insects which, in search of nectar or
pollen, carried the pollen from one plant to the flowers of another
plant, almost every detail is found to have a purpose and a use. The
shape, the size, and the colour of the petals, even the streaks and
spots with which they are adorned, the position in which they stand, the
movements of the stamens and pistil at various times, especially at the
period of, and just after, fertilisation, have been proved to be
strictly adaptive in so many cases that botanists now believe that all
the external characters of flowers either are or have been of use to the
species.
It has also been shown, by Kerner and other botanists, that another set
of characteristics have relation to the prevention of ants, slugs, and
other animals from reaching the flowers, because these creatures would
devour or injure them without effecting fertilisation. The spines,
hairs, or sticky glands on the stem or flower-stalk, the curious hairs
or processes shutting up the flower, or sometimes even the extreme
smoothness and polish of the outside of the petals so that few insects
can hang to the part, have been shown to be related to the possible
intrusion of these "unbidden guests."[42] And, still more recently,
attempts have been made by Grant Allen and Sir John Lubbock to account
for the innumerable forms, textures, and groupings of leaves, by their
relation to the needs of the plants themselves; and there can be little
doubt that these attempts will be ultimately successful. Again, just as
flowers have been adapted to secure fertilisation or
cross-fertilisation, fruits have been developed to assist in the
dispersal of seeds; and their forms, sizes, juices, and colours can be
shown to be specially adapted to secure such dispersal by the agency of
birds and mammals; while the same end is secured in other cases by
downy seeds to be wafted through the air, or by hooked or sticky
seed-vessels to be carried away, attached to skin, wool, or feathers.
Here, then, we have an enormous extension of the region of utility in
the vegetable kingdom, and one, moreover, which includes almost all the
specific characters of plants. For the species of plants are usually
characterised either by differences in the form, size, and colour of the
flowers, or of the fruits; or, by peculiarities in the shape, size,
dentation, or arrangement of the leaves; or by peculiarities in the
spines, hairs, or down with which various parts of the plant are
clothed. In the case of plants it must certainly be admitted that
"specific" characters are pre-eminently adaptive; and though there may
be some which are not so, yet all those referred to by Darwin as having
been adduced by various botanists as useless, either pertain to genera
or higher groups, or are found in some plants of a species only - that
is, are individual variations not specific characters.
In the case of animals, the most recent wide extension of the sphere of
utility has been in the matter of their colours and markings. It was of
course always known that certain creatures gained protection by their
resemblance to their normal surroundings, as in the case of white arctic
animals, the yellow or brown tints of those living in deserts, and the
green hues of many birds and insects surrounded by tropical vegetation.
But of late years these cases have been greatly increased both in number
and variety, especially in regard to those which closely imitate special
objects among which they live; and there are other kinds of coloration
which long appeared to have no use. Large numbers of animals, more
especially insects, are gaudily coloured, either with vivid hues or with
striking patterns, so as to be very easily seen. Now it has been found,
that in almost all these cases the creatures possess some special
quality which prevents their being attacked by the enemies of their kind
whenever the peculiarity is known; and the brilliant or conspicuous
colours or markings serve as a warning or signal flag against attack.
Large numbers of insects thus coloured are nauseous and inedible;
others, like wasps and bees, have stings; others are too hard to be
eaten by small birds; while snakes with poisonous fangs often have some
characteristic either of rattle, hood, or unusual colour, which
indicates that they had better be left alone.
But there is yet another form of coloration, which consists in special
markings - bands, spots, or patches of white, or of bright colour, which
vary in every species, and are often concealed when the creature is at
rest but displayed when in motion, - as in the case of the bands and
spots so frequent on the wings and tails of birds. Now these specific
markings are believed, with good reason, to serve the purpose of
enabling each species to be quickly recognised, even at a distance, by
its fellows, especially the parents by their young and the two sexes by
each other; and this recognition must often be an important factor in
securing the safety of individuals, and therefore the wellbeing and
continuance of the species. These interesting peculiarities will be more
fully described in a future chapter, but they are briefly referred to
here in order to show that the most common of all the characters by
which species are distinguished from each other - their colours and
markings - can be shown to be adaptive or utilitarian in their nature.
But besides colour there are almost always some structural characters
which distinguish species from species, and, as regards many of these
also, an adaptive character can be often discerned. In birds, for
instance, we have differences in the size or shape of the bill or the
feet, in the length of the wing or the tail, and in the proportions of
the several feathers of which these organs are composed. All these
differences in the organs on which the very existence of birds depends,
which determine the character of flight, facility for running or
climbing, for inhabiting chiefly the ground or trees, and the kind of
food that can be most easily obtained for themselves and their
offspring, must surely be in the highest degree utilitarian; although in
each individual case we, in our ignorance of the minutiae of their
life-history, may be quite unable to see the use. In mammalia specific
differences other than colour usually consist in the length or shape of
the ears and tail, in the proportions of the limbs, or in the length and
quality of the hair on different parts of the body. As regards the ears
and tail, one of the objections by Professor Bronn relates to this very
point. He states that the length of these organs differ in the various
species of hares and of mice, and he considers that this difference can
be of no service whatever to their possessors. But to this objection
Darwin replies, that it has been shown by Dr. Schöbl that the ears of
mice "are supplied in an extraordinary manner with nerves, so that they
no doubt serve as tactile organs." Hence, when we consider the life of
mice, either nocturnal or seeking their food in dark and confined
places, the length of the ears may be in each case adapted to the
particular habits and surroundings of the species. Again, the tail, in
the larger mammals, often serves the purpose of driving off flies and
other insects from the body; and when we consider in how many parts of
the world flies are injurious or even fatal to large mammals, we see
that the peculiar characteristics of this organ may in each case have
been adapted to its requirements in the particular area where the
species was developed. The tail is also believed to have some use as a
balancing organ, which assists an animal to turn easily and rapidly,
much as our arms are used when running; while in whole groups it is a
prehensile organ, and has become modified in accordance with the habits