VARIATION AND NATURAL SELECTION.
Everything, so far as in it lies, said Benedict Spinoza, tends to
persist in its own being. This is _the law of persistence_. It forms the
basis of Newton's First Law of Motion, which enunciates that, if a body
be at rest, it will remain so unless acted on by some external force;
or, if it be in motion, it will continue to move in the same straight
line and at a uniform velocity unless it is acted on by some external
force. Practically every known body is thus affected by external forces;
but the law of persistence is not thereby disproved. It only states what
would happen under certain exceptional or perhaps impossible
circumstances. To those ignorant of scientific procedure, it seems
unsatisfactory, if not ridiculous, to formulate laws of things, not as
they are, but as they might be. Many well-meaning but not very
well-informed people thus wholly misunderstand and mistake the value of
certain laws of political economy, because in those laws (which are
generalized statements of fact under narrowed and rigid conditions, and
do not pretend to be inculcated as rules of conduct) benevolence,
sentiment, even moral and religious duty, are intentionally excluded.
These laws state that men, under motives arising out of the pursuit of
wealth, will act in such and such a way, unless benevolence, sentiment,
duty, or some other motive, lead them to act otherwise. Such laws, which
hold good, not for phenomena in their entirety, but for certain isolated
groups of facts under narrowed conditions, are called _laws of the
factors_ of phenomena. And since the complexity of phenomena is such
that it is difficult for the human mind to grasp all the interlacing
threads of causation at a single glance, men of science have endeavoured
to isolate their several strands, and, applying the principle of
analysis, without which reasoning is impossible, to separate out the
factors and determine their laws. In this chapter we have to consider
some of the factors of organic progress, and endeavour to determine
their laws.
The law of heredity may be regarded as that of persistence exemplified
in a series of organic generations. When, as in the am[oe]ba and some
other protozoa, reproduction is by simple fission, two quite similar
organisms being thus produced, there would seem to be no reason why
(modifications by surrounding circumstances being disregarded)
hereditary persistence should not continue indefinitely. Where, however,
reproduction is effected by the detachment of a single cell from a
many-celled organism, hereditary persistence[M] will be complete only on
the condition that this reproductive cell is in some way in direct
continuity with the cells of the parent organism or the cell from which
that parent organism itself developed. And where, in the higher animals,
two cells from two somewhat different parents coalesce to give origin to
a new individual, the phenomena of hereditary persistence are still
further complicated by the blending of characters handed on in the ovum
and the sperm; still further complication being, perhaps, produced by
the emergence in the offspring of characters latent in the parent, but
derived from an earlier ancestor. And if characters acquired by the
parents in the course of their individual life be handed on to the
offspring, yet further complication will be thus introduced.
It is no matter for surprise, therefore, that, notwithstanding the law
of hereditary persistence, variations should occur in the offspring of
animals. At the same time, it must be remembered that the occurrence of
variations is not and cannot be the result of mere chance; but that all
such variations are determined by some internal or external influences,
and are thus legitimate and important subjects of biological
investigation. In the next chapter we shall consider at some length the
phenomena of heredity and the origin of variations. Here we will accept
them without further discussion, and consider some of their
consequences. But even here, without discussing their origin, we must
establish the fact that variations do actually occur.
Variations may be of many kinds and in different directions. In colour,
in size, in the relative development of different parts, in complexity,
in habits, and in mental endowments, organisms or their organs may vary.
Observers of mammals, of birds, and of insects are well aware that
colour is a variable characteristic. But these colour-variations are not
readily described and tabulated. In the matter of size the case is
different. In Mr. Wallace's recent work on "Darwinism" a number of
observations on size-variations are collected and tabulated. As this is
a point of great importance, I propose to illustrate it somewhat fully
from some observations I have recently made of the wing-bones of bats.
In carrying out these observations and making the necessary
measurements, I have had the advantage of the kind co-operation of my
friend Mr. Henry Charbonnier, of Clifton, an able and enthusiastic
naturalist.[N]
The nature of the bat's wing will be understood by the aid of the
accompanying figure (Fig. 12). In the fore limb the arm-bone, or
humerus, is followed by an elongated bone composed of the radius and
ulna. At the outer end of the radius is a small, freely projecting
digit, which carries a claw. This answers to the thumb. Then follow four
long, slender bones, which answer to the bones in the palm of our hand.
They are the metacarpals, and are numbered II., III., IV., and V. in the
tabulated figures in which the observations are recorded. The
metacarpals of the second and third digits run tolerably close together,
and form the firm support of the anterior margin of the wing. Those of
the third and fourth make a considerable angle with these and with each
other, and form the stays of the mid part of the wing. Beyond the
metacarpals are the smaller joints or phalanges of the digits, two or
three to each digit. The third digit forms the anterior point or apex of
the wing. The fourth and fifth digits form secondary points behind this.
Between these points the wing is scalloped into bays.
[Illustration: Fig. 12.--"Wing" of bat (Pipistrelle).
Hu., humerus, or arm-bone; Ul., conjoined radius and ulna, a bone in the
forearm; Po., pollex, answering to our thumb; II., III., IV., V.,
second, third, fourth, and fifth digits of the manus, or hand. The
figures are placed near the metacarpals, or palm-bones. These are
followed by the phalanges. Fe., femur or thigh-bone; Ti., tibia, the
chief bone of the shank. The digits of the pes, or foot, are short and
bear claws. Ca., calcar.]
From the point of the fifth or last digit the leathery wing membrane
sweeps back to the ankle. The bones of the hind limb are the femur, or
thigh-bone, and the tibia (with a slender, imperfectly developed
fibula). There are five toes, which bear long claws. From the ankle
there runs backward a long, bony and gristly spur, which serves to
support the membrane which stretches from the ankle to the tip (or near
the tip) of the tail.
Thus the wing of the bat consists of a membrane stretched on the
expanded or spread fingers of the hand, and sweeping from the point of
the little finger to the ankle. Behind the ankle there is a membrane
reaching to the tip of the tail. This forms a sort of net in which some
bats, at any rate, as I have myself observed, can catch insects.
I have selected the wing of the bat to exemplify variation, (1) because
the bones are readily measured even in dried specimens; (2) because they
form the mutually related parts of a single organ; and (3) because they
offer facilities for the comparison of variations, not only among the
individuals of a single species, but also among several distinct
species.
The method employed has been as follows: The several bones have been
carefully measured in millimetres,[O] and all the bones tabulated for
each species. Such tables of figures are here given in a condensed form
for three species of bats.
Bat-Measurements (in Millimetres).
--------------------------------------------------------------------------
|R | P | 2nd | Third | Fourth | Fifth |T |
|a | o |Digit.| Digit. | Digit. | Digit. |i |
|d U | l |------|--------------|--------------|------------|b |
|i l | l | M | M | P | P | M | P | P | M | P | P |i |
|u n | e | e | e | h | h | e | h | h | e | h | h |a |
|s a | x | t | t | a | a | t | a | a | t | a | a |. |
| . | . | a | a | l | l | a | l | l | a | l | l | |
|a | | c | c | a | a | c | a | a | c | a | a | |
|n | | a | a | n | n | a | n | n | a | n | n | |
|d | | r | r | g | g | r | g | g | r | g | g | |
| | | p | p | e | e | p | e | e | p | e | e | |
| | | a | a | | s | a | | s | a | | s | |
| | | l | l | 1 | | l | 1 | | l | 1 | | |
| | | . | . | . | 2,| . | . | 2,| . | . | 2,| |
| | | | | | 3 | | | 3 | | | 3 | |
| | | | | | . | | | . | | | . | |
-------------------------------------------------------------------------
Hairy-armed bat (_Vesperugo leisleri_).
|41 |6.5| 38 |40 |16 |19 | 38 |14 | 7 | 32 | 8 | 7 |16|[Male]
|41 |6 | 38 |40 |16 |19 | 39 |15.5| 7 | 33 | 8 |6.5|16| "
|41 |6 | 39 |40 |16 |18 | 39 |16 | 6.5| 33 | 8 | 7 |16| "
|41.5|5 | 39 |40.5|17 |20 | 39 |16 | 7 | 33 | 8 | 7 |15| "
|40 |6 | 39 |37 |15.5|18 | 37 |14.5| 7 | 32 | 8 |6.5|15|[Fem.]
|41 |5.5| 38.5 |39 |16.5|20 | 39 |15 | 7.5| 33 | 8 |7.5|17| "
|41 |6 | 39 |40 |15.5|20.5| 39 |15.5| 7 | 33 | 8 | 7 |16| "
Horseshoe bat (_Rhinolophus ferri-equinum_).
|51 | 5 | 39 |36 | 19 | 29 | 40 | 11 | 18 | 40 |13 |15 |22|[Male]
|54 | 5 | 40 |36 | 18 | 32 | 40 | 11 | 19 | 40 |14 |16 |28|[Fem.]
|52 | 5 | 39 |36 | 18 | 31 | 39 | 10 | 19 | 40 |13 |14 |23| "
|54 | 5 | 39 |36 | 18 | 32 | 40 | 11 | 17 | 40 |13 |13 |25| "
|46 | 5 | 36 |34 | 16 | 29 | 36 | 10 | 19 | 36 |13 |17 |22| ?
Lesser horseshoe bat (_Rhinolophus hipposideros_).
|34 | 4 | 25 |23 | 12 | 17 | 26 | 6.5| 12 | 26 | 9 |13 |17|[Male]
|37 | 3 | 26 |24 | 13 | 20 | 28 | 8 | 13 | 28 | 9 |14 |17| "
|35 | 3 | 26 |24.5| 13 | 17 | 27 | 7 | 12 | 26 |10 |12 |15| "
-------------------------------------------------------------------------
It would be troublesome to the reader to pick out the meaning from these
figures. I have, therefore, plotted in the measurements for four other
species of bats in tabular form (Figs. 13, 14, 15, 16).
Fig. 13, for example, deals with the common large noctule bat, which may
often be seen flying high up on summer evenings. Now, the mean length of
the radius and ulna in eleven individuals was 51.5 millimetres. Suppose
all the eleven bats had this bone (for the two bones form practically
one piece) of exactly the same length. There would then be no variation.
We may express this supposed uniformity by the straight horizontal line
running across the part of the figure dealing with the radius and ulna.
Practically the eleven bats measured did not have this bone of the same
length; in some of them it was longer, in others it was shorter than the
mean. Let us run through the eleven bats (which are represented by the
numbers at the head of the table) with regard to this bone. The first
fell below the average by a millimetre and a half, the length being
fifty millimetres. This is expressed in the table by placing a dot or
point three quarters of a division below the mean line. Each division on
the table represents two millimetres, or, in other words, the distance
between any two horizontal lines stands for two millimetres measured.
Half a division, therefore, is equivalent to one measured millimetre; a
quarter of a division to half a millimetre. The measurements are all
made to the nearest half-millimetre. The second bat fell short of the
mean by one millimetre. The bone measured 50.5 millimetres. The third
exceeded the mean by a millimetre and a half; the fourth, by three
millimetres and a half. The fifth was a millimetre and a half above the
mean; and the sixth and seventh were both half a millimetre over the
mean. The eighth fell short by half a millimetre; the ninth and tenth by
a millimetre and a half; and the eleventh by two millimetres and a half.
The points have been connected together by lines, so as to give a curve
of variation for this bone.
[Illustration: Fig 13.--The noctule (_Vesperugo noctula_).]
[Illustration: Fig. 14.--The long-eared bat (_Plecotus auritus_).]
[Illustration: Fig. 15.--The pipistrelle (_Vesperugo pipistrellus_).]
[Illustration: Fig. 16.--The whiskered bat (_Vespertilio mystacinus_).]
The other curves in these four tables are drawn in exactly the same way.
The mean length is stated; and the amount by which a bone in any bat
exceeds or falls short of the mean can be seen and readily estimated by
means of the horizontal lines of the table. Any one can reconvert the
tables into figures representing our actual measurements.
Now, it may be said that, since some bats run larger than others, such
variation is only to be expected. That is true. But if the bones of the
wing all varied equally, _all the curves would be similar_. That is
clearly not the case. The second metacarpal is the same length in 5 and
6. But the third metacarpal is two millimetres shorter in 6 than in 5.
In 10 the radius and ulna are _longer_ than in 11; but the second
metacarpal is _shorter_ in 10 than in 11. A simple inspection of the
table as a whole will show that there is a good deal of _independent_
variation among the bones.
The amount of variation is itself variable, and in some cases is not
inconsiderable. In the long-eared bats 4 and 5 in Fig. 14, the phalanges
of the third digit measured 26.5 millimetres in 4, and 34 millimetres in
5--a difference of more than 28 per cent. This is unusually large, and
it is possible that there may have been some slight error in the
measurements.[P] A difference of 10 or 12 per cent. is, however, not
uncommon.
In any case, the observations here tabulated show (1) that variations of
not inconsiderable amount occur among the related bones of the bat's
wing; and (2) that these variations are to a considerable extent
independent of each other.
So far we have compared a series of individuals of the same species of
bat, each table in Figs. 13-16 dealing with a distinct species. Let us
now compare the different species with each other. To effect such a
comparison, we must take some one bone as our standard, and we must
level up our bats for the purposes of tabulation. I have selected the
radius and ulna as the standard. In both the noctule and the greater
horseshoe bats the mean length of this bone is 51.5 millimetres. The
bones of each of the other bats have been multiplied by such a number as
will bring them up to the level of size in these two species. Mr.
Galton, in his investigations on the variations of human stature, had to
take into consideration the fact that men are normally taller than
women. He found, however, that the relation of man to woman, so far as
height is concerned, is represented by the proportion 108 to 100. By
multiplying female measurements by 1.08, they were brought up to the
male standard, and could be used for purposes of comparison. In the same
way, by multiplying in each case by the appropriate number, I have
brought all the species in the table (Fig. 17) up to the standard of the
noctule. When so multiplied, the radius and ulna (selected as the
standard of comparison) has the same length in all the species, and is
hence represented by the horizontal line in the table.
[Illustration: Fig. 17.--Variations adjusted to the standard of the
noctule.]
Compared with this as a standard, the mean length of the second
metacarpal in the seven species is forty-three millimetres; that of the
third metacarpal, forty-four millimetres; and so on. The amount by which
each species exceeds or falls short of the mean is shown on the table,
and the points are joined up as before. Here, again, the table gives the
actual measurements in each case. For example, if the mean length of the
third metacarpal of the greater horseshoe bat be required, it is seen by
the table to fall short of the mean by four horizontal divisions and a
quarter, that is to say, by eight millimetres and a half. The length is
therefore (44 - 8-1/2) 35.5 millimetres.
Now, it will be seen from the table that the variation in the mean
length of the bones in different species is much greater than the
individual variations in the members of the same species. The table also
brings out in an interesting way the variation in the general character
of the wing. The noctule, for example, is especially strong in the
development of the second and third metacarpals, the phalanges of the
third digit being also a little above the average. Reference to the
figure of the bat's wing on p. 64 will show that these excellences give
length to the wing. It fails, however, in the metacarpal and phalanges
of the fifth digit, and in the length of the hind leg as represented by
the tibia. On consulting the figure of the wing, it is seen that these
are the bones which give breadth to the wing. Here the noctule fails.
Its wing is, therefore, long and narrow. It is a swallow among bats.
On the other hand, the horseshoe bats fail conspicuously in the second
and third metacarpals, though they make up somewhat in the corresponding
digits. On the whole, the wing is deficient in length. But the phalanges
of the fourth and fifth digits, and the length of the hind limb
represented by the tibia, give a corresponding increase of breadth. The
wing is, therefore, relatively short and broad. The long-eared bat,
again, has the third metacarpal and its digits somewhat above the mean,
and therefore a somewhat more than average length. But it has the fifth
metacarpal with its digit and also the tibia decidedly above the mean,
and therefore more than average breadth. Without possessing the great
length of the noctule's wing, or the great breadth of that of the
horseshoe, it still has a more than average length and breadth.
The total wing-areas are very variable, the females having generally an
advantage over the males. I do not feel that our measurements are
sufficiently accurate to justify tabulation. Taking, however, the radius
and ulna as the standard for bringing the various species up to the same
level, the greater horseshoe seems to have decidedly the largest
wing-area; the noctule stands next; then come the lesser horseshoe and
the long-eared bat; somewhat lower stands the hairy-armed bat; while the
pipistrelle and the whiskered bat (both small species) stand lowest.[Q]
Sufficient has now been said in illustration of the fact that variations
in the lengths of the bones in the bat's wing do actually occur in the
various individuals of one species; that the variations are independent;
and that the different species and genera have the character of the wing
determined by emphasizing, so to speak, variations in special
directions. I make no apology for having treated the matter at some
length. Those who do not care for details will judiciously exercise
their right of skipping.
As before mentioned, Mr. Wallace has collected and tabulated other
observations on size and length variations. And in addition to such
variations, there are the numerous colour-variations that do not admit
of being so readily tabulated. Mr. Cockerell tells us that among
snail-shells, taking variations of banding alone, he knows of 252
varieties of _Helix nemoralis_ and 128 of _H. hortensis_.[R]
That variations do occur under nature is thus unquestionable. And it is
clear that all variations necessarily fall under one of three
categories. Either they are of advantage to the organism in which they
occur; or they are disadvantageous; or they are neutral, neither
advantageous nor disadvantageous to the animal in its course through
life.
We must next revert to the fact to which attention was drawn in the last
chapter, that every species is tending, through natural generation, to
increase in numbers. Even in the case of the slow-breeding elephant, the
numbers tend to increase threefold in each generation; for a single pair
of elephants give birth to three pairs of young. In many animals the
tendency is to increase ten, twenty, or thirtyfold in every generation;
while among fishes, amphibians, and great numbers of the lower
organisms, the tendency is to multiply by a hundredfold, a thousandfold,
or even in some cases ten thousandfold. But, as before noticed, this is
only a tendency. The law of increase is a law of one factor in life's
phenomena, the reproductive factor. In any area, the conditions of which
are not undergoing change, the numbers of the species which constitute
its fauna remain tolerably constant. They are not actually increasing in
geometrical progression. There is literally no room for such increase.
The large birth-rate of the constituent species is accompanied by a
proportionate death-rate, or else the tendency is kept in check by the
prevention of certain individuals from mating and bearing young.[S]
Now, the high death-rate is, to a large extent among the lower organisms
and in a less degree among higher animals, the result of indiscriminate
destruction. When the ant-bear swallows a tongue-load of ants, when the
Greenland whale engulfs some hundreds of thousands of fry at a gulp,
when the bear or the badger destroys whole nests of bees,--in such cases
there is wholesale and indiscriminate destruction. Those which are thus
destroyed are nowise either better or worse than those which escape. At
the edge of a coral reef minute, active, free-swimming coral embryos are
set free in immense numbers. Presently they settle down for life. Some
settle on a muddy bottom, others in too great a depth of water. These
are destroyed. The few which take up a favourable position survive. But
they are no better than their less fortunate neighbours. The destruction
is indiscriminate. So, too, among fishes and the many marine forms which
produce a great number of fertilized eggs giving rise to embryos that
are from an early period free-swimming and self-supporting. Such embryos
are decimated by a destruction which is quite indiscriminate. And again,
to take but one more example, the liver-fluke, whose life-history was
sketched in the last chapter, produces its tens or hundreds of thousands
of ova. But the chances are enormously against their completing their
life-cycle. If the conditions of temperature and moisture are not
favourable, the embryo is not hatched or soon dies; even if it emerges,
no further development takes place unless it chances to come in contact
with a particular and not very common kind of water-snail. When it
emerges from the intermediate host and settles on a blade of grass, it
must still await the chance of that blade being eaten by a sheep or
goat. It is said that the chances are eight millions to one against it,
and for the most part its preservation is due to no special excellence
of its own. The destruction is to a large extent, though not entirely,
indiscriminate.
Even making all due allowance, however, for this indiscriminate
destruction--which is to a large extent avoided by those higher
creatures which foster their young--there remain more individuals than
suffice to keep up the normal numbers of the species. Among these there
arises a struggle for existence, and hence what Darwin named _natural
selection_.
"How will the struggle for existence"--I quote, with some omissions, the
words of Darwin--"act in regard to variation? Can the principle of
selection, which is so potent in the hands of man, apply under nature? I
think that we shall see that it can act most efficiently. Let the
endless number of slight variations and individual differences be borne
in mind; as well as the strength of the hereditary tendency. Let it also
be borne in mind how infinitely complex and close-fitting are the mutual
relations of all organic beings to each other and to their physical
conditions of life; and consequently what infinitely varied diversities
of structure might be of use to each being under changing conditions of
life. Can it, then, be thought improbable, seeing that variations useful
to man have undoubtedly occurred, that other variations, useful in some
way to each being in the great and complex battle of life, should occur
in the course of many successive generations? If such do occur, can we
doubt (remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight, over
others, would have the best chance of surviving and of procreating their
kind? On the other hand, we may feel sure that any variation in the
least degree injurious would be rigidly destroyed. This preservation of
favourable individual differences and variations, and the destruction of
those which are injurious, I have called Natural Selection, or the
Survival of the Fittest. Variations neither useful nor injurious would
not be affected by natural selection, and would be left either a
fluctuating element, or would ultimately become fixed, owing to the
nature of the organism and the nature of the conditions."[T]
"The principle of selection," says Darwin, elsewhere, "may conveniently
be divided into three kinds. _Methodical selection_ is that which guides
a man who systematically endeavours to modify a breed according to some
predetermined standard. _Unconscious selection_ is that which follows
from men naturally preserving the most valued and destroying the less
valued individuals, without any thought of altering the breed. Lastly,
we have _Natural selection_, which implies that the individuals which
are best fitted for the complex and in the course of ages changing
conditions to which they are exposed, generally survive and procreate
their kind."[U]
I venture to think that there is a more logical division than this. A
man who is dealing with animals or plants under domestication may
proceed by one of two well-contrasted methods. He may either select the
most satisfactory individuals or he may reject the most unsatisfactory.
We may term the former process _selection_, the latter _elimination_.
Suppose that a gardener is dealing with a bed of geraniums. He may
either pick out first the best, then the second best, then the third,
and so on, until he has selected as many as he wishes to preserve. Or,
on the other hand, he may weed out first the worst, then in succession
other unsatisfactory stocks, until, by eliminating the failures, he has
a residue of sufficiently satisfactory flowers. Now, I think it is clear
that, even if the ultimate result is the same (if, that is to say, he
selects the twenty best, or eliminates all but the twenty best), the
method of procedure is in the two cases different. Selection is applied
at one end of the scale, elimination at the other. There is a difference
in method in picking out the wheat-grains (like a sparrow) and
scattering the chaff by the wind.
Under nature both methods are operative, but in very different degrees.
Although the insect may select the brightest flowers, or the hen-bird
the gaudiest or most tuneful mate, the survival of the fittest under
nature is in the main the net result of the slow and gradual process of
the elimination of the unfit.[V] The best-adapted are not, save in
exceptional cases, selected; but the ill-adapted are weeded out and
eliminated. And this distinction seems to me of sufficient importance to
justify my suggestion that _natural selection_ be subdivided under two
heads--_natural elimination_, of widespread occurrence throughout the
animal world; and _selection proper_, involving the element of
individual or special choice.
The term "natural elimination" for the major factor serves definitely to
connect the natural process with that struggle for existence out of
which it arises. The struggle for existence is indeed the reaction of
the organic world called forth by the action of natural elimination.
Organisms are tending to increase in geometrical ratio. There is not
room or subsistence for the many born. The tendency is therefore held in
check by elimination, involving the struggle for existence. And the
factors of elimination are three: first, elimination through the action
of surrounding physical or climatic conditions, under which head we may
take such forms of disease as are not due to living agency; secondly,
elimination by enemies, including parasites and zymotic diseases; and
thirdly, elimination by competition. It will be convenient to give some
illustrative examples of each of these.
Elimination through the action of surrounding physical conditions, taken
generally, deals with the very groundwork or basis of animal life. There
are certain elementary mechanical conditions which must be fulfilled by
every organism however situated. Any animal which fails to fulfil these
conditions will be speedily eliminated. There are also local conditions
which must be adequately met. Certain tropical animals, if transferred
to temperate or sub-Arctic regions, are unable to meet the requirements
of the new climatic conditions, and rapidly or gradually die. Fishes
which live under the great pressure of the deep sea are killed by the
expansion of the gases in their tissues when they are brought to the
surface. Many fresh-water animals are killed if the lake in which they
live be invaded by the waters of the sea. If the water in which corals
live be too muddy, too cold, or too fresh--near the mouth of a great
river on the Australian coast, for example--they will die off. During
the changes of climate which preceded and followed the oncoming of the
glacial epoch, there must have been much elimination of this order. Even
under less abnormal conditions, the principle is operative. Darwin tells
us that in the winter of 1854-5 four-fifths of the birds in his grounds
perished from the severity of the weather, and we cannot but suppose
that those who were thus eliminated were less able than others to cope
with or stand the effects of the inclement climatic conditions. My
colleague, Mr. G. Munro Smith, informs me that, in cultivating microbes,
certain forms, such as _Bacillus violaceus_ and _Micrococcus
prodigiosus_, remain in the field during cold weather when other less
hardy microbes have perished. The insects of Madeira may fairly be
regarded as affording another instance. The ground-loving forms allied
to insects of normally slow and heavy flight have in Madeira become
wingless or lost all power of flight. Those which attempted to fly have
been swept out to sea by the winds, and have thus perished; those which
varied in the direction of diminished powers of flight have survived
this eliminating process. On the other hand, among flower-frequenting
forms and those whose habits of life necessitate flight, the Madeira
insects have stronger wings than their mainland allies. Here, since
flight could not be abandoned without a complete change of life-habit,
since all must fly, those with weaker powers on the wing have been
eliminated, leaving those with stronger flight to survive and procreate
their kind.[W] In Kerguelen Island Mr. Eaton has found that all the
insects are incapable of flight, and most of them in a more or less
wingless condition.[X] Mr. Wallace regards the reduction in the size of
the wing in the Isle of Man variety of the small tortoiseshell butterfly
as due to the gradual elimination of larger-winged individuals.[Y] These
are cases of elimination through the direct action of surrounding
physical conditions. Even among civilized human folk, this form of
elimination is still occasionally operative--in military campaigns, for
example (where the mortality from hardships is often as great as the
mortality from shot or steel), in Arctic expeditions, and in arduous
travels. But in early times and among savages it must be a more
important factor.
Elimination by enemies needs somewhat fuller exemplification. Battle
within battle must, throughout nature, as Darwin says, be continually
recurring with varying success. The stronger devour the weaker, and wage
war with each other over the prey. In the battle among co-ordinates the
weaker are eliminated, the stronger prevail. When the weaker are preyed
upon by the stronger and a fair fight is out of the question, the slow
and heavy succumb, the agile and swift escape; stupidity means
elimination, cunning, survival; to be conspicuous, unless it be for some
nasty or deleterious quality, is inevitably to court death: the
sober-hued stand at an advantage. In these cases, if there be true
selection at work, it is the selection of certain individuals, the
plumpest and most toothsome to wit, for destruction, not for survival.
This mode of elimination has been a factor in the development of
protective resemblance and so-called mimicry, and we may conveniently
illustrate it by reference to these qualities. If the hue of a creature
varies in the direction of resemblance to the normal surroundings, it
will render the animal less conspicuous, and therefore less liable to be
eliminated by enemies. This is well seen in the larvæ or caterpillars of
many of our butterflies and moths. It is not easy to distinguish the
caterpillar of the clouded yellow, so closely does its colour assimilate
to the clover leaves on which it feeds, nor that of the Lulworth skipper
on blades of grass. I would beg every visitor to the Natural History
Museum at South Kensington to look through the drawers containing our
British butterflies and moths and their larvæ, in the further room on
the basement, behind the inspiring statue of Charles Darwin. Half an
hour's inspection will serve to bring home the fact of protective
resemblance better than many words.
It may, however, be remarked that not all the caterpillars exhibit
protective resemblance; and it may be asked--How have some of these
conspicuous larvæ, that of the magpie moth, for example, escaped
elimination? What is sauce for the Lulworth goose should be sauce for
the magpie gander. How is it that these gaudy and variable caterpillars,
cream-coloured with orange and black markings, have escaped speedy
destruction? Because they are so nasty. No bird, or lizard, or frog, or
spider would touch them. They can therefore afford to be
bright-coloured. Nay, their very gaudiness is an advantage, and saves
them from being the subject of unpleasant experiments in the matter.
Other caterpillars, like the palmer-worms, are protected by barbed hairs
that are intensely irritating. They, too, can afford to be conspicuous.
But a sweet and edible caterpillar, if conspicuous, is eaten, and thus
by the elimination of the conspicuous the numerous dull green or brown
larvæ have survived.
A walk through the Bird Gallery in the National collection will afford
examples of protective resemblance among birds. Look, for example, at
the Kentish plover with its eggs and young--faithfully reproduced in our
frontispiece--and the way in which the creature is thus protected in
early stages of its life will be evident. The stone-curlew, the
ptarmigan, and other birds illustrate the same fact, which is also seen
with equal clearness in many mammals, the hare being a familiar example.
Many oceanic organisms are protected through general resemblance. Some,
like certain medusæ, are transparent. The pellucid or transparent sole
of the Pacific (_Achirus pellucidus_), a little fish about three inches
long, is so transparent that sand and seaweed can be seen distinctly
through its tissues. The salpa is transparent save for the intestine and
digestive gland, which are brown, and look like shreds of seaweed. Other
forms, like the physalia, are cærulean blue. The exposed parts of
flat-fish are brown and sandy coloured or speckled like the sea-bottom;
and in some the sand-grains seem to adhere to the skin. So, too, with
other fish. "Looking _down_ on the dark back of a fish," says Mr. A. R.
Wallace, "it is almost invisible, while to an enemy looking _up_ from
below, the light under surface would be equally invisible against the
light of clouds and sky." Even some of the most brilliant and gaudiest
fish, such as the coral-fish (_Chætodon, Platyglossus_, and others), are
brightly coloured in accordance with the beautiful tints of the
coral-reefs which form their habitat; the bright-green tints of some
tropical forest birds being of like import. No conception of the range
of protective resemblance can be formed when the creatures are seen or
figured isolated from their surroundings. The zebra is a sufficiently
conspicuous animal in a menagerie or a museum; and yet Mr. Galton
assures us that, in the bright starlight of an African night, you may
hear one breathing close by you, and be positively unable to see the
animal. A black animal would be visible; a white animal would be
visible; but the zebra's black and white so blend in the dusk as to
render him inconspicuous.
To cite but one more example, this time from the invertebrates.
Professor Herdman found in a rock-pool on the west coast of Scotland "a
peculiarly coloured specimen of the common sea-slug (_Doris
tuberculata_). It was lying on a mass of volcanic rock of a dull-green
colour, partially covered with rounded spreading patches of a purplish
pink nullipore, and having numerous whitish yellow _Spirorbis_ shells
scattered over it--the general effect being a mottled surface of dull
green and pink peppered over with little cream-coloured spots. The upper
surface of the Doris was of precisely the same colours arranged in the
same way.... We picked up the Doris, and remarked the brightness and the
unusual character of its markings, and then replaced it upon the rock,
when it once more became inconspicuous."[Z]
Then, too, there are some animals with variable protective
resemblance--the resemblance changing with a changing environment. This
is especially seen in some Northern forms, like the Arctic hare and fox,
which change their colour according to the season of the year, being
brown in summer, white and snowy in winter. The chamæleon varies in
colour according to the hue of its surroundings through the expansion
and contraction of certain pigment-cells; while frogs and cuttle-fish
have similar but less striking powers. Mr. E. B. Poulton's[AA] striking
and beautiful experiments show that the colours of caterpillars and
chrysalids reared from the same brood will vary according to the colour
of their surroundings.
[Illustration: Fig. 18.--Caterpillar of a moth (_Ennomos tiliaria_) on
an oak-spray. (From an exhibit in the British Natural History Museum.)]
If this process of protective resemblance be carried far, the general
resemblance in hue may pass into special resemblance to particular
objects. The stick-insect and the leaf-insect are familiar
illustrations, though no one who has not seen them in nature can realize
the extent of the resemblance. Most of us have, at any rate, seen the
stick-caterpillars, or loopers (Fig. 18), though, perhaps, few have
noticed how wonderful is the protective resemblance to a twig when the
larva is still and motionless, for the very reason that the resemblance
is so marked that the organism at that time escapes, not only casual
observation, but even careful search. Fig. 19 gives a representation of
a locust with special protective resemblance to a leaf--not a perfect
leaf, but a leaf with fungoid blotches. This insect and the
stick-caterpillar may be seen in the insect exhibits on the basement at
South Kensington, having been figured from them by the kind permission
of Professor Flower.
[Illustration: Fig. 19.--A locust (_Cycloptera speculata_) which closely
resembles a leaf. (From an exhibit in the British Natural History
Museum.)]
Perhaps one of the most striking instances of special protective
resemblance is that of the Malayan leaf-butterfly (_Kallima paralecta_).
So completely, when the wings are closed, does this insect resemble a
leaf that it requires a sharp eye to distinguish it. These butterflies
have, moreover, the habit of alighting very suddenly. As a recent
observer (Mr. S. B. T. Skertchly) remarks, they "fly rapidly along, as
if late for an appointment, suddenly pitch, close their wings, and
become leaves. It is generally done so rapidly that the insect seems to
vanish."[AB] Instances might be multiplied indefinitely. Mr. Guppy thus
describes a species of crab in the Solomon Islands: "The light purple
colour of its carapace corresponds with the hue of the coral at the base
of the branches, where it lives; whilst the light red colour of the big
claws, as they are held up in their usual attitude, similarly imitates
the colour of the branches. To make the guise more complete, both
carapace and claws possess rude hexagonal markings which correspond
exactly in size and appearance with the polyp-cells of the coral."[AC]
When the special protective resemblance is not to an inanimate object,
but to another organism, it is termed mimicry. It arises in the
following way:--
Many forms, especially among the invertebrates, escape elimination by
enemies through the development of offensive weapons (stings of wasps
and bees), a bitter taste (the Heliconidæ among butterflies), or a hard
external covering (the weevils among beetles). The animals which prey
upon these forms learn to avoid these dangerous, nasty, or indigestible
creatures; and the avoidance is often instinctive. It thus becomes an
advantage to other forms, not thus protected, to resemble the animals
that have these characteristics. Such resemblance is termed mimicry,
concerning which it must be remembered that the mimicry is unconscious,
and is reached by the elimination of those forms which do not possess
this resemblance. Thus the _Leptalis_, a perfectly sweet insect, closely
resembles the _Methona_, a butterfly producing an ill-smelling yellow
fluid. The quite harmless _Clytus arietis_, a beetle, resembles, not
only in general appearance, but in its fussy walk, a wasp. The
soft-skinned _Doliops_, a longicorn, resembles the strongly encased
_Pachyrhyncus orbifex_, a weevil. The not uncommon fly _Eristalis tenax_
(Fig. 20), is not unlike a bee, and buzzes in an unpleasantly suggestive
manner.[AD]
Mimicry is not confined to the invertebrates. A harmless snake, the
eiger-eter of Dutch colonists at the Cape, subsists mainly or entirely
on eggs. The mouth is almost or quite toothless; but in the throat
hard-tipped spines project into the gullet from the vertebræ of the
column in this region. Here the egg is broken, and there is no fear of
losing the contents. Now, there is one species of this snake that
closely resembles the berg-adder. The head has naturally the elongated
form characteristic of the harmless snakes. But when irritated, this
egg-eater flattens it out till it has the usual viperine shape of the
"club" on a playing-card. It coils as if for a spring, erects its head
with every appearance of anger, hisses, and darts forward as if to
strike its fangs into its foe, in every way imitating an enraged
berg-adder. The snake is, however, quite harmless and inoffensive.[AE]
Here we have mimicry both in form and habit. Another case of imperfect
but no doubt effectual mimicry is given by Mr. W. Larden, in some notes
from South America.[AF] Speaking of the rhea, or South American ostrich,
he says, "One day I came across an old cock in a nest that it had made
in the dry weeds and grass. Its wings and feathers were loosely
arranged, and looked not unlike a heap of dried grass; at any rate, the
bird did not attract my attention until I was close on him. The long
neck was stretched out close along the ground, the crest feathers were
flattened, and an appalling hiss greeted my approach. It was a
pardonable mistake if for a moment I thought I had come across a huge
snake, and sprang back hastily under this impression."
Protective resemblance and mimicry have been considered at some length
because, on the hypothesis of natural selection, they admirably
illustrate the results which may be reached through long-continued
elimination by enemies.
Sufficient has now been said to show that this form of elimination is an
important factor. We are not at present considering the question how
variations arise, or why they should take any particular direction. But
granting the fact that variations may and do occur in all parts of the
organism, it is clear that, in a group of organisms surrounded by
enemies, those individuals which varied in the direction of swiftness,
cunning, inconspicuousness,[AG] or resemblance to protected forms,
would, other things being equal, stand a better chance of escaping
elimination.
Elimination by competition is, as Darwin well points out, keenest
between members of the same group and among individuals of the same
species, or between different groups or different species which have, so
to speak, similar aims in life. While enemies of various kinds are
preying upon weaker animals, and thus causing elimination among them,
they are also competing one with another for the prey. While the slower
and stupider organisms are succumbing to their captors, and thus leaving
more active and cunning animals in possession of the field, the slower
and stupider captors, failing to catch their cunning and active prey,
are being eliminated by competition. While protective resemblance aids
the prey to escape elimination by enemies, a correlative resemblance,
called by Mr. Poulton aggressive resemblance, in the captors aids them
in stealing upon their prey, and so gives advantage in competition. Thus
the hunting spider closely resembles the flies upon which he pounces,
even rubbing his head with his fore legs after their innocent fashion.
As in the case of protective resemblance, so, too, in its aggressive
correlative, the resemblance may be general or special, or may reach the
climax of mimicry. And since the same organism is not only a would-be
captor, but sometimes an unwilling prey, the same resemblance may serve
to protect it from its enemies and to enable it to steal upon its prey.
The mantis, for example, gains doubly by its resemblance to the
vegetation among which it lives. Certain spiders, described by Mr. H. O.
Forbes, in Java, closely resemble birds'-droppings. This may serve to
protect them from elimination by birds; but it also enables them to
capture without difficulty unwary butterflies, which are often attracted
by such excreta. A parasitic fly (_Volucella bombylans_) closely
resembles (Fig. 20) a bumble-bee (_Bombus muscorum_), and is thus
enabled to enter the nest of the bee without molestation. Its larvæ feed
upon the larvæ of the bee. The cuckoo bee _Psithyrus rupestris_, an idle
quean, who collects no pollen, and has no pollen-baskets, steals into
the nest of the bumble-bee _Bombus lapidarius_, and lays her eggs there.
The resemblance between the two is very great, and it not only enables
the mother bee to enter unmolested, but the young bees, when they are
hatched, to escape. Another bee (_Nomada solidaginis_), which plays the
cuckoo on _Halictus cylindricus_, does not resemble this bee, but is
wasp-like, and thus escapes molestation, not because it escapes notice,
but because it looks more dangerous than it really is.[AH]
Many are the arts by which, in keen competition, organisms steal a march
upon their congeners--not, be it remembered, through any conscious
adaptation, but through natural selection by elimination. Mr. Poulton
describes an Asiatic lizard (_Phrynocephalus mystaceus_) in which the
"general surface resembles the sand on which it is found, while the fold
of their skin at each angle of the mouth is of a red colour, and is
produced into a flower-like shape exactly resembling a little red flower
which grows in the sand. Insects, attracted by what they believe to be
flowers, approach the mouth of the lizard, and are, of course,
captured."[AI] The fishing frog, or angler-fish, is possessed of
filaments which allure small fry, who think them worms, into the
neighbourhood of the great mouth in which they are speedily engulfed;
and certain deep-sea forms discovered during the _Challenger_ expedition
have the lure illumined by phosphorescent light.
[Illustration: Fig. 20.--Mimicry of bees by flies.
a, b, _Bombus muscorum_; c, d, _Volucella bombylans_; e, _Eristalis
tenax_; f, _Apis mellifica_. The underwings of the hive bee (f) were
invisible in the photograph from which the figure was drawn. (From an
exhibit in the British Natural History Museum.)]
We need say no more in illustration of the resemblances which have
enabled certain organisms to escape elimination by competition. Once
more, be it understood that we are not at present considering _how_ any
of these resemblances have been brought about; we are merely indicating
that, given certain resemblances, advantageous either for captor or
prey, those organisms which possess them not will have to suffer
elimination--elimination by enemies, or else elimination by competition.
The interaction between these two kinds of elimination is of great
importance. Hunters and hunted are both, so to speak, playing the game
of life to the best of their ability. Those who fail on either side are
weeded out; and elimination is carried so far that those who are only as
good as their ancestors are placed at a disadvantage as compared with
their improving congeners. The standard of efficiency is thus improving
on each side; and every improvement on the one side entails a
corresponding advance on the other. Nor is there only thus a competition
for subsistence, and arising thereout a gradual sharpening of all the
bodily and mental powers which could aid in seeking or obtaining food;
there is also in some cases a competition for mates, reaching
occasionally the climax of elimination by battle. There is, indeed,
competition for everything which can be an object of appetence to the
brute intelligence; and, owing to the geometrical tendency in
multiplication--the law of increase--the competition is keen and
unceasing.
Such, then, in brief, are the three main modes of elimination:
elimination by physical and climatic conditions; elimination by enemies;
elimination by competition. Observe that it is a differentiating
process. Unlike the indiscriminate destruction before alluded to, the
incidence of which is on all alike, good, bad, and indifferent, it
separates the well-adapted from the ill-adapted, dooming the latter to
death, and allowing the former to survive and procreate their kind. The
destruction is not indiscriminate, but differential.
Let us now turn to cases of selection, properly so called, where Nature
is in some way working at the other end of the scale; where her method
is not the elimination of the unfit, but the selection of the fit. Such
a case may be found on Darwin's principles in brightly coloured flowers
and fruits. "Flowers," he says, "rank amongst the most beautiful
productions of nature; but they have been rendered conspicuous in
contrast with the green leaves, and, in consequence, at the same time
beautiful, so that they may be easily observed by insects. I have come
to this conclusion from finding it an invariable rule that, when a
flower is fertilized by the wind, it never has a gaily coloured corolla.
Several plants habitually produce two kinds of flowers--one kind open
and coloured, so as to attract insects; the other closed, not coloured,
destitute of nectar, and never visited by insects. Hence we may conclude
that, if insects had not been developed on the face of the earth, our
plants would not have been decked with beautiful flowers, but would have
produced only such poor flowers as we see on our fir, oak, nut, and ash
trees, on grasses, spinach, docks, and nettles, which are all fertilized
through the agency of the wind. A similar line of argument holds good
with fruits; that a ripe strawberry or cherry is as pleasing to the eye
as to the palate; that the gaily coloured fruit of the spindle-wood
tree, and the scarlet berries of the holly, are beautiful objects,--will
be admitted by every one. But this beauty serves merely as a guide to
birds and beasts, in order that the fruit may be devoured and manured
seeds disseminated: I infer that this is the case from having as yet
found no exception to the rule that seeds are always thus disseminated
when embedded within a fruit of any kind (that is, within a fleshy or
pulpy envelope), if it be coloured of any brilliant tint, or rendered
conspicuous by being white or black."[AJ]
Here we have a case of the converse of elimination--a case of genuine
selection under nature. But even here the process of elimination also
comes into play, for the visitations of flowers by insects involve
cross-fertilization. The flowers of two distinct individuals of the same
species of plants in this manner fertilize each other; and the act of
crossing, as Darwin firmly believed, though it is doubted by some
observers nowadays, gives rise to vigorous seedlings, which consequently
would have the best chance of flourishing and surviving--would best
resist elimination by competition. So that we here have the double
process at work; the fairest flowers being selected by insects, and
those plants which failed to produce such flowers being eliminated as
the relatively unfit.
If we turn to the phenomena of what Darwin termed sexual selection, we
find both selection and elimination brought into play. By the law of
battle, the weaker and less courageous males are eliminated so far as
the continuation of their kind is concerned. By the individual choice of
the females (on Darwin's view, by no means universally accepted), the
finer, bolder, handsomer, and more tuneful wooers are selected.
Let us again hear the voice of Darwin himself. "Most male birds," he
says, "are highly pugnacious during the breeding season, and some
possess weapons especially adapted for fighting with their rivals. But
the most pugnacious and the best-armed males rarely or never depend for
success solely on their power to drive away or kill their rivals, but
have special means for charming the female. With some it is the power of
song, or of emitting strange cries, or of producing instrumental music;
and the males in consequence differ from the females in their vocal
organs or in the structure of certain feathers. From the curiously
diversified means for producing various sounds, we gain a high idea of
the importance of this means of courtship. Many birds endeavour to charm
the females by love-dances or antics, performed on the ground or in the
air, and sometimes at prepared places. But ornaments of many kinds, the
most brilliant tints, combs and wattles, beautiful plumes, elongated
feathers, top-knots, and so forth, are by far the commonest means. In
some cases, mere novelty appears to have acted as a charm. The ornaments
of the males must be highly important to them, for they have been
acquired in not a few cases at the cost of increased danger from
enemies, and even at some loss of power in fighting with their
rivals[AK].... What, then, are we to conclude from these facts and
considerations? Does the male parade his charms with so much pomp and
rivalry for no purpose? Are we not justified in believing that the
female exerts a choice, and that she receives the addresses of the male
who pleases her most?"[AL]
Here again, then, we have the combined action of elimination
and selection. And now we may note that selection involves
intelligence--involves the play of appetence and choice. Hence it is
that, when we come to consider the evolution of human-folk, the
principle of elimination is so profoundly modified by the principle of
selection. Not only are the weaker eliminated by the inexorable pressure
of competition, but we select the more fortunate individuals and heap
upon them our favours. This enables us also to soften the rigour of the
blinder law; to let the full stress of competitive elimination fall upon
the worthless, the idle, the profligate, and the vicious; but to lighten
its incidence on the deserving but unfortunate.
Both selection and elimination occurring under nature, but elimination
having by far the wider scope, we may now inquire what will be their
effect as regards the three modes of variation--advantageous,
disadvantageous, and neutral. It must be remembered that these modes are
relative and dependent upon circumstances, so that variations, neutral
under certain conditions, may become relatively disadvantageous under
other conditions. Selection clearly leads to the preservation of
advantageous variations alone, and these variations are advantageous in
so far as they meet the taste of the selecting organism. For selection
depends upon individual choice; and uniformity of selection is entirely
dependent upon uniformity in the standard of taste. If, as Darwin
contends, the splendid plumage and tuneful notes of male birds are the
result of a selection of mates by the hens, there must be a remarkable
uniformity of taste among the hens of each particular species, since
there is a uniformity of coloration among the cock-birds. It may be said
that in all their mental endowments there is greater uniformity among
animals than among men; and it is true that individuation has not been
carried so far in them as in human-folk. Still, careful observers of
animals see in them many signs of individual character; and this
uniformity in the standard of taste in each species of birds seems to
many naturalists a real difficulty in the way of the acceptance of
sexual selection. We shall, however, return to this point. For the
present it is clear that selection chooses out advantageous variations,
that the advantage is determined by the taste of the selector, and that
uniform selection implies uniformity of taste.
Turning to elimination, it is clear that it begins by weeding out, first
the more disadvantageous, then the less disadvantageous variations. It
leaves both the advantageous and the neutral in possession of the field.
I imagine that many, perhaps most, of the variations tabulated by Mr.
Wallace and other observers belong to the neutral category. Their
fluctuating character seems to indicate that this is so. In any case,
they are variations which have so far escaped elimination. And I think
they are of great and insufficiently recognized importance. They permit,
through interbreeding, of endless experiments in the combination of
variations, some of which cannot fail to give favourable results.
It is just possible that it may be asked--If in natural elimination
there is nothing more than the weeding out of the unfit and the
suppression of disadvantageous variations, where is the possibility of
advance? The standard may thus be maintained, but where is the
possibility of progress? Such an objection would, however, imply
forgetfulness of the fact that all the favourable variations remain to
leaven the residual lump. Given a mean, with plus and minus variations:
if in any generations the minus variations are got rid of, the mixture
of the mean with the plus variations will give a new mean nearer the
plus or advantageous end of the scale than the old mean. By how much the
favourable variations tend to raise the mean standard, by so much will
the race tend to advance. But in this process I see no reason why the
neutral variations should be eliminated, except in so far as, in the
keen struggle for existence, they become relatively unfavourable.
It is clear, however, that the intercrossing and interbreeding which
occurs between average individuals on the one hand, and those possessing
favourable variations on the other, while it tends gradually to raise
the mean standard, tends also at the same time to reduce the
advantageous variations towards the mean. It must tend to check advance
by leaps and bounds, and to justify the adage, _Natura nil facit per
saltum_. At the same time, it will probably have a greater tendency to
reduce to a mean level neutral variations indefinite in direction than
advantageous variations definite in direction. Still, it is a most
important factor, and one not to be neglected. It tends to uniformity in
the species, and checks individualism. It may act as a salutary brake on
what we may figuratively term hasty and ill-advised attempts at
progress. And at the same time, it favours repeated new experiments in
the combination of variations, occasionally, we may suppose, with happy
results.
But it does more than this. It tends to check, and, if the offspring
always possessed the blended character of both parents, would be
absolutely fatal to, divergence of character within the interbreeding
members of a species. And yet no fact is more striking than this
divergence of character. It is seen in the diversified products of human
selection; for example, among pigeons. It is seen in the freedom of
nature. Mr. Wallace gives many examples. "Among our native species," he
says, "we see it well marked in the different species of titmice,
pipits, and chats. The great titmouse, by its larger size and stronger
bill, is adapted to feed on larger insects, and is even said sometimes
to kill small and weak birds. The smaller and weaker coal-titmouse has
adopted a more vegetarian diet, eating seeds as well as insects, and
feeding on the ground as well as among trees. The delicate little blue
titmouse, with its very small bill, feeds on the minutest insects and
grubs, which it extracts from crevices of bark and from the buds of
fruit trees. The marsh-titmouse, again, has received its name from the
low and marshy localities it frequents; while the crested titmouse is a
Northern bird, frequenting especially pine forests, on the seeds of
which trees it partially feeds. Then, again, our three common
pipits--the tree-pipit, the meadow-pipit, and the rock-pipit, or
sea-lark--have each occupied a distinct place in nature, to which they
have become specially adapted, as indicated by the different form and
size of the hind toe and claw in each species. So the stone-chat, the
whin-chat, and the wheat-ear are all slightly divergent forms of one
type, with modifications in the shape of the wing, feet, and bill
adapting them to slightly different modes of life."[AM] There is
scarcely a genus that does not afford examples of divergent species. The
question then naturally occurs--How have these divergent forms escaped
the swamping effects of intercrossing?
That perfectly free intercrossing, between any or all of the individuals
of a given group of animals, is, so long as the characters of the
parents are blended in the offspring, fatal to divergence of character,
is undeniable. Through the elimination of less favourable variations,
the swiftness, strength, and cunning of a race may be gradually
improved. But no form of elimination can possibly differentiate the
group into swift, strong, and cunning varieties, distinct from each
other, so long as all three varieties freely interbreed, and the
characters of the parents blend in the offspring. Elimination may and
does give rise to progress in any given group as a group; it does not
and cannot give rise to differentiation and divergence, so long as
interbreeding with consequent interblending of characters be freely
permitted. Whence it inevitably follows, as a matter of simple logic,
that where divergence has occurred, intercrossing and interblending must
in some way have been lessened or prevented.
Thus a new factor is introduced, that of _isolation_, or _segregation_.
And there is no questioning the fact that it is of great importance.[AN]
Its importance can, indeed, only be denied by denying the swamping
effects of intercrossing, and such denial implies the tacit assumption
that interbreeding and interblending are held in check by some form of
segregation. The isolation explicitly denied is implicitly assumed.
There are several ways in which isolation, or segregation, may be
effected. Isolation by geographical barriers is the most obvious. A
stretch of water, a mountain ridge, a strip of desert land, may
completely, or to a large extent, prevent any intercrossing between
members of a species on either side of the barrier. The animals which
inhabit the several islands of the Galapagos Archipelago are closely
allied, but each island has its particular species or well-marked
varieties. Intercrossing between the several varieties on the different
islands is prevented, and divergence is thus rendered possible and
proceeds unchecked. It is said that in the Zuyder Zee a new variety of
herrings, the fry of which are very small compared with open-sea
herrings, is being developed. And the salmon introduced into Tasmania
seem to be developing a fresh variety with spots on the dorsal fin and a
tinge of yellow on the adipose fin. In the wooded valleys of the
Sandwich Islands there are allied but distinct species of land-shells.
The valleys that are nearest each other furnish the most nearly related
forms, and the degree of divergence is roughly measured by the number of
miles by which they are separated. Here there is little or no
intercrossing between the slow-moving molluscs in adjoining valleys;
none at all between those at any distance apart.
But even if there are no well-marked physical barriers, the members of a
species on a continent or large island tend to fall into local groups,
between which, unless the animal be of a widely ranging habit, there
will be little intercrossing. Hence local varieties are apt to occur,
and varieties show the first beginnings of that divergence which, if
carried further and more deeply ingrained, results in the
differentiation of species. Geographically, therefore, we may have
either complete isolation or local segregation, and in both cases the
possibility of divergence.
Another mode of segregation arises also out of geographical conditions.
If variations of habits occur (and structure is closely correlated with
habit) such that certain individuals take to the mountains, others to
the plains or valleys; or that certain individuals take to the forests,
others to the open country; the probabilities are that the forest forms
will interbreed frequently with each other, but seldom with those in the
open, and so with the other varieties. The conditions of forest life or
mountain life being thus similar throughout a large area, and life being
through elimination slowly but surely adapted to its environment, there
might thus arise two distinct varieties scattered throughout the length
and breadth of the area, the one inhabiting the mountains, the other the
forests. In illustration of this mode of segregation, we may take the
case of two species of rats which have recently been found by Mr. C. M.
Woodford on one of the Solomon Islands. These two quite distinct species
are regarded by Mr. Oldfield Thomas as slightly modified descendants of
one parent species, the modifications resulting from the fact that of
this original species some individuals have adopted a terrestrial,
others an arboreal life, and their respective descendants have been
modified accordingly. Thus _Mus rex_ lives in trees, has broad
foot-pads, and a long rasp-like, probably semi-prehensile, tail; while
_Mus imperator_ lives on the ground, has smaller pads, and a short,
smooth tail. The segregation of these two species has probably been
effected by the difference of their mode of life, and each has been
adapted to its special environment through the elimination of those
individuals which were not in harmony with the condition of their life.
It is probable that this mode of segregation has been an important one.
And it is clear that in many cases competition would be a co-operating
factor in this process, weaker organisms being forced into otherwise
uncongenial habitats through the stress of competitive elimination, the
weaker forms not perishing, but being eliminated from more favoured
areas.
Protective coloration may also be a means of segregation. A species of
insects having no protective resemblance might vary in two
directions--in the direction of green tints, assimilating their hue to
that of vegetation; and in the direction of sandy or dull earthy
colours, assimilating them to the colour of the soil. In the one variety
elimination would weed out all but the green forms, and these would be
left to intercross. In the other variety, green forms would be
eliminated, dull-brown forms being left to interbreed. Stragglers from
one group into the other would stand a chance of elimination before
interbreeding was effected.[AO]
In the case of birds whose freedom of flight gives them a wide range,
sometimes almost a world-wide range, it would seem at first sight that
their facilities for interbreeding and intercrossing are so great that
divergence is well-nigh impossible. And yet the examples of divergence I
cited from Mr. Wallace were taken from birds, and it is well known that
divergence is particularly well shown in this class. But when the habits
of birds are studied attentively, it is found that, wide as is their
range, their breeding area is often markedly restricted. The sanderling
and knot range freely during the winter throughout the Northern
hemisphere; but their breeding area is restricted to the north polar
region. The interbreeding within this area keeps the species one and
homogeneous, notwithstanding its wide range, and, at the same time,
prevents intercrossing with allied species with different
breeding-grounds.
Another most important mode of segregation among animals arises out of
habitual or instinctive preferences. Where varieties are formed there is
a tendency for like to breed with like. In the Falkland Islands the
differently coloured herds of cattle, all descended from the same stock,
keep separate, and interbreed with each other, but not with individuals
outside their own colour-caste. If two flocks of merino sheep and heath
sheep be mixed together, they do not interbreed. In the Forest of Dean
and in the New Forest, the dark and pale coloured herds of fallow deer
have never been known to intermingle.[AP] Here we have a case of
selective _segregation through preferential mating_, and may find
therein the basis of sexual selection in its higher ranges as advocated
by Darwin.
The question of sexual selection will, however, be briefly considered in
the chapter on "Organic Evolution." At present what we have to notice is
that, through preferential mating, segregation is effected. The forms
that interbreed have a distinguishing colour. From this it is but a step
to the possession, not merely of a distinguishing colour, but of
distinguishing colour-markings. Hence, through preferential mating, may
arise those special markings which so frequently distinguish allied
species. They not only enable _us_ to recognize species as distinct, but
enable the species which possess them to recognize the members of their
own kind. Mr. Wallace calls these diacritical marks _recognition-marks_,
and gives many illustrative examples.[AQ] They are especially noticeable
in gregarious animals and in birds which congregate in flocks or which
migrate together. Mr. Wallace considers that they "have in all
probability been acquired in the process of differentiation for the
purpose of checking the intercrossing of allied forms;" for "one of the
first needs of a new species would be to keep separate from its nearest
allies, and this could be more readily done by some easily seen external
mark of difference." This language seems, however, to savour of
teleology (that pitfall of the evolutionist). The cart is placed before
the horse. The recognition-marks were, I believe, not produced to
prevent intercrossing, but intercrossing has been prevented because of
preferential mating between individuals possessing special
recognition-marks. To miss this point is to miss an important
segregation-factor. Undoubtedly, other tendencies co-operate in
maintaining the standard of the recognition-marks. Stragglers who failed
in the matter of recognition would get separated from their fellows, and
stand a greater chance of elimination by enemies; young who failed in
this respect would be in like condemnation. Still, I cannot doubt that
the foundations of recognition-marks were laid in preferential mating,
and that in this we have an important factor in segregation.
We may here note, in passing, as also arising out of preference, how the
selection of flowers by insects may lead to segregation; for insects
seem often to have habitual or instinctive colour-preferences. Flowers
of similar colour would be thus cross-fertilized, but would not
intercross with those of different colour, whence colour-varieties might
arise. It is important to note that in these cases there is a
psychological factor in evolution.
We have so far assumed that intercrossing of parents and interblending
of their characters in the offspring always go together. This, we must
now notice, is not always the fact. If a blue-eyed Saxon marry a
dark-eyed Italian, the children will have blue eyes _or_ dark eyes, not
eyes of an intermediate tint. The characters do not interblend. The
_ancon_, or otter-sheep, a breed with a long body and short, bandy legs,
appeared in Massachusetts as a chance sport in a single lamb. The
offspring of this ram were either ancons or ordinary sheep. The ancon
characters did not blend. Hence for a time a definite breed was
maintained. We may call this mode of isolation _isolation by exclusive
inheritance_.
A further mode of isolation or segregation, for which Mr. Romanes[AR]
claims a foremost, indeed, the foremost, place, is _physiological
isolation_ as due to differential fertility. One among the many
variations to which organisms are subject is a variation in fertility,
which may reach the climax of absolute sterility. But it is clear that a
sterile variation carries with it its own death-warrant, since the
sterile individual leaves no descendants to inherit its peculiarity.
Relative infertility, too, unless it chances to be correlated with some
unusual excellence, would be no advantage, would be transmitted to few
descendants, and would tend to be extinguished. The same is not true,
however, of differential fertility. "It is by no means rare," said
Darwin,[AS] "to find certain males and females which will not breed
together, though both are known to be perfectly fertile with other males
and females." Mr. Romanes assumes, as a starting-point, the converse of
this, namely, that certain males and females will breed together, though
they are infertile with all other members of the species.
Suppose, then, a variety to arise which is perfectly fertile within the
limits of the varietal form, but imperfectly fertile or infertile with
the parent species. Such a variety would have to run the risks of those
ill effects which, as Darwin showed,[AT] are attendant upon close
interbreeding. But Mr. Wallace points out[AU] that these ill effects may
not be so marked under nature as they are under domestication. Suppose,
then, that it escapes these ill effects. In this case, Mr. Romanes
urges, it would neither be swamped by intercrossing nor die out on
account of sterility. But although it could not be swamped by
intercrossing, still, if it arose sporadically, here a case, there a
case, and so on, the chances would be enormously against the
perpetuation of the variety, unless some co-operating mode of
segregation aided in bringing together the varying individuals. If, for
example, there were a segregation of these variants in a particular
habitat--all the variants meeting in some definite locality for breeding
purposes; or if there were a further segregation through mutual
preferences; or if, again, there were a further segregation in time the
variety might obtain a firm footing. But without these co-operating
factors it is clear that if one male and one female in a hundred
individuals varied in this particular way, the chances would be at least
forty-nine to one against their happening to mate together.
It is interesting to note that almost the only particular example given
by Mr. Romanes in illustration of his theory is one that involves the
co-operation of one of these further segregation-factors. Suppose, he
says, the variation in the reproductive system is such that the season
of flowering or of pairing becomes either advanced or retarded. This
particular variation being inherited, the variety breeding, let us say,
in May, the parent species in July, there would arise two races, each
perfectly fertile within its own limits, but incapable of crossing with
the other. Thus is constituted "a barrier to intercrossing quite as
effectual as a thousand miles of ocean." Yes! a time-barrier instead of
a space-barrier. The illustration is faulty, inasmuch as it introduces a
mode of segregation other than that in question. I think it very
improbable that differential fertility alone, without the co-operation
of other segregation-factors, would give rise to separate varieties
capable of maintaining themselves as distinct species.
That distinct species are generally mutually infertile, or more
frequently still, that their male offspring are sterile, is, however, an
undoubted fact. But there are, exceptions. Fertile hybrids between the
sheep and the goat seem to be well authenticated. Of rats Darwin says
that "in some parts of London, especially near the docks, where fresh
rats are frequently imported, an endless variety of intermediate forms
may be found between the brown, black, and snake rat, which are all
three usually ranked as distinct species."[AV] Fertile hybrids have been
produced between the green-tinted Japanese and the long-tailed Chinese
pheasants. Mr. Thomas Moore, of Fareham, in Hants, has been particularly
successful in producing a hybrid breed between the golden pheasant
(_Thaumalia picta_), whose habitat is Southern and South-eastern China,
and the Amherst pheasant (_Thaumalia amherstiæ_), which is found in the
mountains of Yunnan and Thibet. In answer to my inquiries, Mr. Moore
kindly informs me that he "has bred the half-bred gold and Amherst
pheasant, crossed them again with gold, and recrossed them with
half-bred Amherst, and kept on crossing until only a strain of the gold
pheasant remained. The result is that the birds so produced are far
handsomer than either breed, since the feathers composing their tiplets
as well as those under the chin are of so beautiful a colour that they
beggar description. They all breed most freely, and are much more
vigorous than the pure gold or Amherst, and their tails reach a length
of over three feet. They are also exceedingly prolific. Out of a batch
of forty-two eggs, forty chickens were hatched out, of which
thirty-seven were reared to perfection."
Still, though there are exceptions, the general infertility of allied
species when crossed is a fact in strong contrast with the marked
fertility of varieties under domestication; concerning which, however,
it should be noted that our domesticated animals have been selected to a
very large extent on account of the freedom with which they breed in
confinement, and that domestication has probably a tendency to increase
fertility. The question, therefore, arises--Is the infertility between
species, and the general sterility of their male offspring, a secondary
effect of their segregation? or is their segregation the direct effect
of their differential fertility? The former is the general opinion; the
latter is held by Mr. Romanes. He contends that sterility is the primary
distinction of species, other specific characters being secondary, and
regards it as a pure assumption to say that the secondary differences
between species have been historically prior to the primary difference.
I do not propose to discuss this question. While it seems to me in the
highest degree improbable that differential fertility, apart from other
co-operating factors, has been or could be a practical mode of
segregation, it has probably been a not unimportant factor in
association with other modes of segregation or isolation. Suppose, for
example, two divergent local varieties were to arise in adjacent areas,
and were subsequently (by stress of competition or by geographical
changes) driven together into a single area: we are justified in
believing, from the analogy of the Falkland Island cattle, the Forest of
Dean deer, and other similar observed habits, that preferential
breeding, kind with kind, would tend to keep them apart. But, setting
this on one side, let us say they interbreed. If, then, their unions are
fertile, the isolation will be annulled by intercrossing--the two
varieties will form one mean or average variety. But if the unions be
infertile, the isolation will be preserved, and the two varieties will
continue separate. Suppose now, and the supposition is by no means an
improbable one, that this has taken place again and again in the
evolution of species: then it is clear that those varietal forms which
had continued to be fertile together would be swamped by intercrossing;
while those varietal forms which had become infertile would remain
isolated. Hence, in the long run, isolated forms occupying a common area
would be infertile. Or suppose, once more, that, instead of the unions
between the two varietal forms being infertile, they are fertile, but
give rise to sterile (mule) or degenerate offspring, as is said to be
the case in the unions of Japanese and Ainos: then it is clear that the
sterile or degenerate offspring of such unions would be eliminated, and
intercrossing, even though it occurred, would be inoperative while
breeding within the limits of the variety continued unchecked.
Sufficient has now been said concerning the modes of isolation and
segregation, geographical, preferential, and physiological. We must now
consider their effects. Where the isolated varieties are under different
conditions of life, there will be, through the elimination of the
ill-adapted in each case, differential adoption to these different
conditions. But suppose the conditions are similar: can there be
divergence in this case? The supposition is a highly hypothetical one,
because it postulates that all the conditions, climatal, environmental,
and competitive, are alike, which would seldom, if ever, be likely to
occur. Let us, however, make the supposition. Let us suppose that an
island is divided into two equal halves by the submersion of a stretch
of lowland running across it. Then the only possible causes of
divergence would lie in the organisms themselves[AW] thus divided into
two equal groups. We have seen that variations may be advantageous,
disadvantageous, or neutral. The neutral form a fluctuating, unfixed,
indefinite body. But they afford the material with which nature may
make, through intercrossing, endless experiments in new combinations,
some of which may be profitable. Such profitable variations would escape
elimination, and, if not bred out by intercrossing, would be preserved.
In any case, the variety would tend to advance through elimination as
previously indicated. But in the two equal groups we are supposing to
have become geographically isolated, the chances are many to one against
the same successful experiments in combination occurring in each of the
two groups. Hence it follows that the progress or advance in the two
groups, though analogous, would not be identical, and divergence would
thus be possible under practically similar conditions of life.
In his observations on the terrestrial molluscs of the Sandwich Islands,
Mr. Gulick notes that different forms are found in districts which
present essentially the same environment, and that there is no greater
divergence when the climatic conditions are dissimilar than there is
when those conditions are similar. As before noticed, the degree of
divergence is, roughly speaking, directly as the distance the varietal
forms are apart. Again, Darwin notes that the climate and environment in
the several islands of the Galapagos group are much the same, though
each island has a somewhat divergent fauna and flora. These facts lend
countenance to the view that divergence can and does occur under similar
conditions of life, if there be isolation. They seem, also, so far as
they go, to negative the view that the species is moulded directly by
the external conditions. For, if this factor were powerful, it would
override the effects of experimental combination of characters when the
conditions were similar, and would give rise to well-marked varietal
forms when the conditions were diverse.
If we admit preferential breeding as a segregation-factor (and arising
out of it sexual selection, in a modified form, as a determining one in
the evolution of the plumage of male birds), it is evident that the
standard of recognition-marks can only be maintained by a uniformity of
preference or taste. Still, the uniformity is not likely to be absolute.
In this matter, as in others, variations will occur, and after the lapse
of a thousand generations, in which elimination has been steadily at
work, it is hardly probable that the recognition standard would remain
absolutely unchanged. For, though there may not be any direct
elimination in this particular respect, there might well be
colour-eliminations in other (e.g. protective) respects, and the mental
nature would not remain quite unchanged. Moreover, we know that
secondary sexual characters are remarkably subject to variation, as may
be well seen in the case of ruffs (_Machetes pugnax_) in the British
Natural History Museum. In the case of our two islands with isolated
faunas, therefore, if they formed separate breeding-areas for birds, the
chances would be many to one against the change in the standard of
recognition-marks being identical in each area. Hence might arise those
minute but definite specific distinctions which are so noteworthy in
this class of the animal kingdom. Instance the Old and New World species
of teal, the Eastern and Western species of curlew and whimbrel, and
other cases numerous.[AX] This, in fact, is probably in many cases the
true explanation of the occurrence of representative species, slight
specific variations of the same form as it is traced across a continent
or through an archipelago of islands.
The question has been raised, and of late a good deal discussed, whether
specific characters, those traits by which species are distinguishable,
are always of use to the species which possess them. Here it is
essential to define what is meant by utility. Characters may be of use
in enabling the possessor to resist elimination; or, like the colours of
flowers, they may be of use in attracting insects, and thus furthering
selection; or, like recognition-marks, they may be of use in effecting
segregation. This last form of utility is apt to be overlooked or lost
sight of. In speaking of humming-birds, the Duke of Argyll says that "a
crest of topaz is no better in the struggle for existence than a crest
of sapphire. A frill ending in spangles of the emerald is no better in
the battle of life than a frill ending in spangles of the ruby." But if
these characters be recognition-marks, they may be of use in
segregation. They are a factor in isolation. But it may be further
asked--What is the use of the segregation? Wherein lies the utility of
the divergence into two forms? This question, however, involves a
complete change of view-point. The question before us is whether
specific characters are of use to the species which possesses them. To
this question it is sufficient to answer that they are useful in
effecting or preserving segregation, without which the species, _as a
distinct species_, would cease to exist. We are not at present concerned
with the question whether divergence in itself is useful or
advantageous. If it be pressed, we must reply that, although divergence
is undoubtedly of immense advantage to life in general, enabling, as
Darwin said, its varying and divergent forms to become adapted to many
and highly diversified places in the economy of nature, still in many
individual cases it is neither possible nor in any respect necessary to
our conception of evolution to assign any grounds of utility or
advantage for the divergence itself.
In any case, we are dealing at present with the utility of specific
characters to the species which possess them; and under the head of
utility we are including usefulness in effecting or maintaining
segregation. Now, we have already seen that variations may be either
advantageous (useful), or neutral (useless), or disadvantageous (worse
than useless). The latter class we may here disregard; elimination will
more or less speedily dispose of them. With regard to neutral (useless)
variations, we must also note that they may be correlated with
variations of the other two classes. If correlated with disadvantageous
variations, they will be eliminated along with them; if correlated with
advantageous variations, they will escape elimination (or will be
selected) together with them. There remain neutral, or useless,
variations, not correlated with either of the other two classes. Are
these in any cases distinctive of species?
It is characteristic of specific distinctions that they are relatively
constant. Elimination, selection, or preferential breeding gives them
relative fixity. On the other hand, it is characteristic of neutral
variations that they are inconstant. There is nothing to give them
fixity. It is, of course, conceivable that all the migrants to a new
area were possessed of a useless neutral character, which those in the
mother area did not possess; or that such a useless character was in
them preponderant, and by intercrossing formed a less fluctuating,
useless character than their progenitors exhibited. Still, the extensive
occurrence of such neutral, or useless, characteristics would be in the
highest degree improbable. Our ignorance often prevents us from saying
in what particular way a character is useful. We must neither, on the
one hand, demand proof that this, that, or the other specific character
is useful, nor, on the other hand, demand negative evidence (obviously
impossible to produce) that it is without utilitarian significance; but
we may fairly request those who believe in the wide occurrence of
useless specific characters to tell us by what means these useless
characters have acquired their relative constancy and fixity. A
suggestion on this head will be found in the next chapter.
We must now pass on to consider briefly a most important factor in the
struggle for existence. Hitherto we have regarded this struggle as
uniform in intensity; we must now regard it as variable, with
alternations of good times and hard times, and indicate the causes to
which such variations are due.
With variations of climate, such as we know to occur from year to year,
or from decade to decade, there are variations in the productiveness of
the soil; and when we remember how closely interwoven are the web and
woof of life, we shall see that the increased or diminished
productiveness of any area will affect for good or ill all the life
which that area supports. The introduction of new forms of life into an
area, or their preponderance at certain periods owing to climatic or
other conditions particularly favourable to them as opposed to other
forms, may alter the whole balance of life in the district. We are often
unable to assign any reason for the sudden increase or diminution of the
numbers of a species; we can only presume that it is the result of some
favourable or unfavourable change of conditions. Thus Mr. Alexander
Becker[AY] has recently drawn attention to the fact that whereas for
several years various species of grasshoppers appeared in great numbers
in South-east Russia, there came then one year of sudden death for most
of them. They were sitting motionless on the grasses and dying. He gives
similar cases of butterflies for a while numerous, and then rare, and
states that a squirrel common near Sarepta suddenly disappeared in the
course of one summer, probably, he adds, succumbing to some contagious
disease. Such is the nice balance of life, that the partial
disappearance of a single form may produce remarkable and
little-expected effects. Darwin amusingly showed how the clover crops
might be beneficially affected by the introduction of a family of old
maids into a parish. The clover is fertilized by humble-bees, the bees
are preyed upon by mice; the relations between cats and mice, and
between old maids and cats, are well known and familiar: more old maids,
more cats; more cats, less mice; less mice, more humble-bees; more
humble-bees, better fertilization. A little thing may modify the balance
of life, and increase or diminish the struggle for existence, and the
rigour of the process of elimination.
But when we take a more extended view of the matter, and include secular
changes of climate, the possible range of variation in the struggle for
existence is seen to be enormously increased. It is well known to those
who have followed the progress of geology, that in early Kainozoic times
a mild climate extended to within the Arctic circle, while during the
glacial epoch much of the north temperate zone was fast locked in ice,
and the climate of the northern hemisphere was profoundly modified. The
animals in the north temperate zone were driven southwards.[AZ] Not only
was there much elimination from the severe climatic conditions, but the
migrants were driven southwards into areas already well stocked with
life, and the competition for means of subsistence in these areas must
have been rendered extremely severe. Elimination was at a maximum. Then
followed the withdrawal of glacial conditions. The increasing geniality
of the climate allowed an expansion of life within a given area, and the
withdrawal of snow and ice further and further north set free new areas
into which this expanding life could migrate and find subsistence. The
hard times of the glacial period were succeeded by good times of
returning warmth and an expanding area; and if, as some geologists
believe, there was an inter-glacial period (or more than one such
period) in the midst of the Great Ice Age, then hard times and good
times alternated during the glacial epoch.
Expansion and contraction of life-areas have also been effected again
and again in the course of geological history by elevations and
subsidences of the land. At the beginning of Mesozoic times much of
Europe was dry land. In Triassic and Rhætic times there were lakes in
England and in Germany, and a warm Mediterranean Sea to the south.
Subsidence of the European area brought with it a lessened land-area and
an increased sea-area: bad times and increased competition for land
animals; good times and a widening life-area for marine forms of life.
This continued, with minor variations, till its culmination in the
Cretaceous period. Then came the converse process: the land-areas
increased, the sea was driven back. A good time had come for terrestrial
life; the marine inhabitants of estuaries and inland seas felt the
pressure of increased competition in a lessening area. And so there
emerged the continental Europe of the beginning of the Kainozoic era.
And it is scarcely necessary to remind those who are in any degree
conversant with geology that during tertiary times there have been
alternate expansions and contractions of life-areas, marine and
terrestrial, the former bringing good times, the latter hard times and a
heightened struggle for existence.
Now, what would be the result of this alternation of good times and hard
times? During good times varieties, which would be otherwise unable to
hold their own, might arise and have time to establish themselves. In an
expanding area migration would take place, local segregation in the
colonial areas would be rendered possible, differential elimination in
the different migration-areas would produce divergence. There would be
diminished elimination of neutral variations, thus affording
opportunities for experimental combinations. In general, good times
would favour variation and divergence.
Intermediate between good times and hard times would come, in logical
order, the times in which there is neither an expansion nor a
contraction of the life-area. One may suppose that these are times of
relatively little change. There is neither the divergence rendered
possible by the expansion of life-area, nor the heightened elimination
enforced by the contraction of life-area.[BA] Elimination is steadily in
progress, for the law of increase must still hold good. Divergence is
still taking place, for the law of variation still obtains. But neither
is at its maximum. These are the good old-fashioned times of slow and
steady conservative progress. They are, perhaps, well exemplified by the
fauna of the Carboniferous period, and it is not at all improbable that
we are ourselves living in such a quiet, conservative period.
On the other hand, hard times would mean increased elimination. During
the exhibitions at South Kensington there were good times for rats. But
when the show was over, there followed times that were cruelly hard. The
keenest competition for the scanty food arose, and the poor animals were
forced to prey upon each other. "Their cravings for food," we read in
_Nature_, "culminated in a fierce onslaught on one another, which was
evidenced by the piteous cries of those being devoured. The method of
seizing their victims was to suddenly make a raid upon one weaker or
smaller than themselves, and, after overpowering it by numbers, to tear
it in pieces." Elimination by competition, passing in this way into
elimination by battle, would, during hard times, be increased. None but
the best organized and best adapted could hope to escape. There would be
no room for neutral variations, which, in the keenness of the struggle,
would be relatively disadvantageous. Slightly divergent varieties,
before kept apart through local segregation, would be brought into
competition. The weakest would in some cases be eliminated. In other
cases, the best-adapted individuals of each variety might survive. If
their experiments in intercrossing, should such occur, gave rise to
fertile offspring, more vigorous and better adapted than either
parent-race, these would survive, and the parent-forms would be
eliminated. But if such experiments in intercrossing gave rise to
infertile, weakly offspring, these would be eliminated. Thus sterility
between species would become fixed. Wherever, during the preceding good
times, divergence had taken place in two different directions of
adaptation, and some intermediate forms, fairly good in both directions,
had been able to escape elimination, the chances are that these
intermediates would be in hard times eliminated, and the divergent forms
left in possession of the field. Wherever, during good times, a species
had acquired or retained a habit of flexibility, that habit would stand
it in good stead in the midst of the changes wrought by hard times; but
when it had, on the other hand, acquired rigidity (like the proverbially
"inflexible goose"), it would be at a disadvantage in the stress of a
heightened elimination.
The alternation of good times and hard times may be illustrated by an
example taken from human life. The introduction of ostrich-farming in
South Africa brought good times to farmers. Whereupon there followed
divergence in two directions. Some devoted increased profits to
improvements upon their farms, to irrigation works which could not
before be afforded, and so forth. For others increased income meant
increased expenditure and an easier, if not more luxurious, mode of
life. Then came hard times. Others, in Africa and elsewhere, learnt the
secret of ostrich-farming. Competition brought down profits, and
elimination set in--of which variety need hardly be stated.
I believe that the alternation of good times and hard times, during
secular changes of climate and alternate expansions and contractions of
life-areas through geological upheavals and depression of the land, has
been a factor of the very greatest importance in the evolution of varied
and divergent forms of life, and in the elimination of intermediate
forms between adaptive variations. It now only remains in this chapter
to say a few words concerning convergence, adaptation, and progress.
Convergence, which is the converse of divergence, is brought about
through the adaptation of different forms of life to similar conditions
of existence. The somewhat similar form of the body and fin-like limbs
of fishes, of ancient reptiles (the ichthyosaurus and its allies), of
whales, seals, and manatees, is a case in point. Both birds, bats, and
pterodactyls have keeled breastbones for the attachment of the large
muscles for flight. A whole series of analogous adaptations, as the
result of analogous modes of life, are found in the placental mammals of
Europe and Asia, on the one hand, and the marsupial forms of Australia
on the other hand. The flying squirrel answers to the flying phalanger,
the fox to the vulpine phalanger, the bear to the koala, the badger to
the pouched badger, the rabbit to the bandicoot, the wolverine to the
Tasmanian devil, the weasel to the pouched weasel, the rats and mice to
the kangaroo rats and mice, and so on. A familiar example of convergence
is to be seen in our swallows and martins, on the one hand, and the
swifts on the other. Notwithstanding their superficial similarity in
external form and habits, they are now generally regarded as belonging
to distinct orders of birds.
These are examples of convergence.[BB] Animals of diverse descent and
ancestry have, through similarity of surrounding conditions or of habits
of life, become, in certain respects, assimilated. But some zoologists
go further than this. They maintain that the same genus or species may,
through adaptation to similar circumstances, be derived from dissimilar
ancestors. Some palæontologists, for example, believe that the horse has
been independently evolved along parallel lines in Europe and in
America. Professor Cope considers that in the one continent
_Protohippus_, and in the other _Hipparion_, was the immediate ancestor
of _Equus_. The probabilities are, however, so strongly against such a
view, that it cannot be accepted until substantiated by stronger
evidence than is yet forthcoming.
A special and particular form of convergence, at any rate in certain
obvious, if superficial, characters, has already been noticed in our
brief consideration of mimicry. In the first place, among a number of
closely related species of inedible butterflies, the tendency to
divergence is checked, so far as external markings and coloration are
concerned, that all may continue to profit by the resemblance, and that
the numbers tasted by young birds in gaining their experience (for the
avoidance seems to be at most incompletely instinctive) may be divided
amongst all the species, thus lessening the loss to each. Secondly,
there may be a convergence of certain genera of distantly related
inedible groups (e.g. among the Heliconidæ and the Danaidæ), which gain
by being apparently one species, since the loss from young birds is
shared between them. And lastly, there is the true mimicry of quite
distinct families of butterflies, not themselves inedible, but
sheltering themselves under the guise and sharing the bad reputation of
the mimicked forms. Such forms of convergence are in special adaptation
to a very special environment.
We must remember that in all cases adaptation is a matter of life and
environment. And these, we may now note, may be related in one or more
of three ways. In the first place, there is the adaptation of life to an
unchanging environment; for example, the adaptation of all forms of life
to the fixed and unchanging properties of inorganic matter. If we liken
life to a statue and the environment to a mould in which it is cast, we
have in this case a rigid mould and a plastic statue. Secondly, the
adaptation may be mutual, as, for example, when the structures of
insects and flowers are fitted each to the other, or when the speed of
hunters and hunted is steadily increased through the elimination of the
slow in either group. Here the mould and statue are both somewhat
plastic, and yield to each other's influence. Thirdly, the environment
may be moulded to life. This, again, is only relative, since life never
wholly loses its plasticity. The bird that builds a nest, the beaver
that constructs a dam, the insect that gives rise to a gall,--these, so
far, mould the environment to the needs of their existence. Man in
especial has the power, through his developed intelligence, of
manufacturing his own environment. Here the statue is relatively rigid,
and the mould plastic.
Progress may be defined as continuous adaptation. In modern phrase, this
is called evolution. The continuity makes the difference between
evolution and revolution. Both are natural. Both occur in the organic,
the social, and the intellectual sphere. Evolution is the orderly
progress of the organism or group of organisms, by which it becomes more
and more in harmony with surrounding conditions. If the conditions
become more and more complex, the organism will progress in complexity;
but if the conditions be more and more simple, progress (if such it may
still be called) will be towards simplicity of structure, unnecessary
complexity being eliminated, or, in any case, disappearing. Hence, in
parasites and some forms of life which live under simple conditions, we
have the phenomena of degeneration, or a passage from a more complex to
a more simple condition.
Revolution in organic life is the destruction of one organism or group
of organisms, and the replacement in its stead of a wholly different
organism or group of organisms. During hard times there may be much
revolution, or replacement of one set of organic forms by another set of
organic forms. It was by revolution that the dominant reptiles of the
Mesozoic epoch were replaced by the dominant mammals of Kainozoic times.
It was by revolution that pterodactyls were supplanted by birds.
Revolution has exterminated many a group in geological ages. On the
other hand, it was by evolution that the little-specialized Eocene
ungulates gave rise to the horse, the camel, and the deer; by divergent
evolution that the bears and dogs were derived from common ancestors.
Palæontology testifies both to evolution and revolution.[BC] That
history does the same, I need not stay to exemplify. The same laws also
apply to systems of thought. Darwinism has revolutionized our
conceptions of nature. Darwin placed upon a satisfactory basis a new
order of interpretation of the organic world. By it other
interpretations have been supplanted. And now this new conception is
undergoing evolution, not without some divergence.
In this chapter we have seen how evolution is possible under natural
conditions. If the law of increase be true, if more are born than can
survive to procreate their kind, natural selection is a logical
necessity. We must not blame our forefathers for not seeing this. Until
geology had extended our conception of time, no such conclusions could
be drawn. If organisms have existed but six or seven thousand years, and
if in the last thousand years little or no change in organic life has
occurred, the supposition that they could have originated by any such
process as natural selection is manifestly absurd. Lyell was the
necessary precursor of Darwin. Given, then, increase and elimination
throughout geological time, natural selection is a logical necessity. No
one who adequately grasps the facts can now deny it. It is an
unquestionable factor in organic evolution. Whether it is the sole
factor, is quite another matter, and one we will consider in the chapter
on "Organic Evolution."
NOTES
[M] Samuel Butler in England, and Ewald Hering in Prague, have
ingeniously likened this hereditary persistence to "organic
memory." What are ordinarily called memory, habit, instinct, and
embryonic reconstruction, are all referable to the memory of
organic matter. The analogy, if used with due caution, is a
helpful one, what we call memory being the psychical aspect (under
certain special organic and neural conditions) of what under the
physical aspect we call persistence.
[N] I have also to thank Mr. Edward Wilson for kindly giving me the
measurements of three or four bats in the Bristol Museum.
[O] A millimetre is about 1/25 of an inch, or more exactly .03937 inch.
[P] In nearly all cases the measurements were checked by comparing the
two wings. In one or two instances there were differences of as
much as two or three millimetres between the bones of the two
sides of the body, but in most cases they exactly corresponded.
[Q] We are anxious to extend our observations and to compare series of
bats from different localities. If any of my readers should feel
disposed to help us, by sending specimens (_with the locality duly
indicated_) to Mr. H. Charbonnier, 7, The Triangle South, Clifton,
Bristol, we shall be grateful.
[R] _Nature_, vol. xli. p. 393. The variation in molluscs is often
considerable. In one of the bays in the basement hall of the
Natural History Museum is a series showing the variation in size,
form, and sculpturing of _Paludomus loricatus_, which is found in
the streams of Ceylon. These varieties have in former times been
named as ten distinct species!
[S] More observations and fuller knowledge on this latter point and on
the relative numbers of the sexes in different species are much to
be desired. It is clear that the number of offspring mainly
depends upon the number of females. But if it be true that good
times and favourable conditions lead to an increased production of
females, while hard times and unfavourable conditions lead to a
relative increase of males, then it is evident that good times
will lead to a more rapid increase and hard times to a less rapid
increase of the species. Suppose, for example, in a particular
district food and other conditions were especially favourable for
frogs. Among the well-nourished tadpoles there would be a
preponderance of females. In the next generation the many females
would produce abundant offspring (for one male may fertilize the
ova laid by several females). There would be a greater number of
tadpoles to compete for the same amount of nutriment. They would
be less nourished. There would be less females; and in the
succeeding generation a diminished number of tadpoles. Thus to
some extent a balance between the number of tadpoles and the
amount of available nutrition would be maintained. These
conclusions are, perhaps, too theoretical to be of much value,
while the tendency here indicated would be but one factor among
many.
[T] "Origin of Species," pp. 62, 63.
[U] "Animals and Plants under Domestication," vol. ii. p. 177.
[V] I may here draw attention to the fact that the bats whose wing-bone
measurements were given above are those which have so far survived
and escaped such elimination as is now in progress.
[W] "Origin of Species," p. 109.
[X] "Darwinism," p. 106.
[Y] Ibid. p. 106.
[Z] Proceedings Liverpool Biological Society, 1889.
[AA] Since this chapter was written, Mr. Poulton has published his
interesting and valuable work on "The Colours of Animals," from
which I have contrived to insert one or two additional examples.
[AB] _Ann. and Mag. Nat. Hist._, September, 1889, p. 209, quoted by
Poulton, "Colours of Animals," p. 55.
[AC] Nature, vol. xxxv. p. 77.
[AD] Many other instances might be added. The hornet clear-wing moth
(_Sphecia apiformis_) mimics the hornet or wasp; the
narrow-bordered bee-hawk moth (_Sesia bombyliformis_) mimics a
bumble-bee. These insects may be seen in the lepidoptera drawers
in the Natural History Museum. But perhaps the most wonderful
instance of insect-mimicry is that observed by Mr. W. L. Sclater,
and given by Mr. E. B. Poulton, in his "Colours of Animals" (p.
252), where a (probably) homopterous insect mimics a leaf-cutting
ant, _together with its leafy burden_--a membranous expansion in
the mimic closely resembling the piece of leaf carried by the
particular kind of ant he resembles.
[AE] The late Mr. H. W. Oakley first drew my attention to this snake.
Since then Mr. Hammond Tooke has described the facts in _Nature_,
vol. xxxiv. p. 547.
[AF] _Nature_, vol. xlii. p. 115.
[AG] Since the above was written and sent to press, there has been
added, at the Natural History Museum, in the basement hall, a case
illustrating the adaptation of external colouring to the
conditions of life. All the animals, birds, etc., there grouped
were collected in the Egyptian desert, whence also the rocks,
stones, and sand on which they are placed were brought. Though
somewhat crowded, they exemplify protective resemblance very well.
[AH] I have to thank Mr. H. A. Francis for drawing my attention to
this, and showing me the insects in his cabinet.
[AI] "Colours of Animals," p. 73.
[AJ] "Origin of Species," p. 161.
[AK] "Descent of Man," summary of chap. xvi. pt. ii.
[AL] Ibid. chap. xiv.
[AM] "Darwinism," p. 108.
[AN] Its importance in artificial selection was emphasized by Darwin:
"The prevention of free crossing, and the intentional matching of
individual animals, are the corner-stones of the breeder's art"
("Animals and Plants under Domestication," ii. 62).
[AO] From the absence of interblending in some cases (to be considered
shortly), both brown _and_ green forms may be produced; and under
certain circumstances, even a power of becoming either brown _or_
green in the presence of appropriate stimuli.
[AP] Wallace, "Darwinism," p. 172, where other examples are cited.
[AQ] Ibid. pp. 217, _et seq._
[AR] _Journal of the Linnæan Society_, vol. xix. No. 115: "Zoology."
[AS] "Animals and Plants under Domestication," p. 145.
[AT] Ibid. chap. xvii.
[AU] "Darwinism," p. 326.
[AV] "Animals and Plants under Domestication," vol. ii. p. 65. For
Darwin's general conclusions on hybridism, see vol. ii. p. 162 of
the same work.
[AW] "In every case there are two factors, namely, the nature of the
organism and the nature of the conditions. The former seems to be
much the more important; for nearly similar variations sometimes
arise under, as far as we can judge dissimilar conditions; and, on
the other hand, dissimilar variations arise under conditions which
appear to be nearly uniform" ("Origin of Species," p. 6).
[AX] See "Evolution without Natural Selection," by Charles Dixon. This
author's facts are valuable; his theories are ill digested.
[AY] _Nature_, vol. xlii. p. 136.
[AZ] We may here note, in passing, the fact that the changes of
life-forms in a succession of beds points in nine cases out of ten
rather to substitution through migration than to transmutation.
Still, there are notable cases of transmutation, as in the
fresh-water _Planorbes_ of Steinhem, in Wittenberg (described,
after Hilgendorf, by O. Schmidt, "The Doctrine of Descent," p.
96).
[BA] I would ask historians whether there have not been, in English
history, good times of free and beneficial divergence exemplified
in diverse intellectual activity, hard times of rigorous
elimination, and intermediate times of placid, somewhat humdrum
conservatism.
[BB] Two more technical examples may be noticed in a note. (1)
Professor Haeckel has recently (_Challenger_ Reports, vol.
xxviii.) shown that the Siphonophora include two groups, closely
resembling each other, but of different ancestry: (_a_) The
Disconanthæ, traceable to trachomedusoid ancestors; (_b_) the
Siphonanthæ, traceable to anthomedusoid ancestors like Sarsia. (2)
M. Paul Pelseneer has been led to the conclusion that the pteropod
molluscs also include two groups resembling each other, but of
different ancestry: (_a_) The Thecosomes, traceable to tornatellid
ancestors; (_b_) the Gymnosomes, traceable to aphysiid ancestors.
In each case, the ancestral sea-slug has been modified for a
free-swimming life.
[BC] For evidence in copious abundance, see Nicholson's "Manual of
Palæontology," new edition, vol. i.: "Vertebrata," by R. Lydekker.