species is the third part of the struggle for existence, and it is often
unsurpassed in its ferocity. When two lion cubs of the same litter begin
to shift for themselves, they must naturally compete in the same
territory, and their contest is keener than that which involves either of
them and a young lion born ten or fifteen miles away. The seeds of one
parent plant falling in a restricted area will be engaged in a competitive
struggle for existence that is much more intense than many other parts of
nature's warfare. In brief, the intensity of the competition will be
directly proportional to the similarity of two organisms in constitution
and situation, and to the consequent similarity of vital welfare. The
interests of the white man and the Indian ran counter to each other a few
hundred years ago, and the more powerful colonists won. The assumption of
the white man's burden too often demonstrates the natural effect of
diversity of interest, and the domination of the stronger over the weaker.
In any civilized community the manufacturer, farmer, financier, lawyer,
and doctor must struggle to maintain themselves under the conditions of
their total inorganic and social environments; and in so far as the object
of each is to make a living for himself, they are competitors. But the
contest becomes more absorbing when it involves broker and broker, lawyer
and lawyer, financier and magnate, because in each case the contestants
are striving for an identical need of success.
Although the severity of the conflict imposed by nature is somewhat
modified in the case of social organisms, where community competes with
community and nation with nation, no form of social organization has yet
been developed where the individual contest carried on by the members of
one community has been done away with. It is an inexorable law of nature
that all living things must fight daily and hourly for their very lives,
because so many are brought into the world with each new generation that
there is not sufficient room for all. No organism can escape the struggle
for existence except by an unconditional surrender that results in death.
Everywhere we turn to examine the happenings of organic life we can find
nothing but a wearisome warfare in which it is the ultimate and cruel lot
of every contestant to admit defeat.
* * * * *
What now are the results of variation, over-multiplication, and
competition? Since some must die because nature cannot support all that
she produces, since only a small proportion of those that enter upon life
can find a foothold or successfully meet the hordes of their enemies,
which will be the ones to survive? Surely those that have even the
slightest advantage over their fellows will live when their companions
perish. It is impossible that the result could be otherwise; it must
follow inevitably from what has been described before. The whole process
has its positive and its negative aspects: the survival of the fittest and
the elimination of the unfit. Perhaps it would be more correct to say the
more real element is the negative one, for those which are least capable
of meeting their living foes and the decimating conditions of inorganic
nature are the first to die, while the others will be able to prolong the
struggle for a longer or shorter period before they too succumb. Thus the
destruction of the unfit leaves the field to the better adapted, that is,
to those that vary in such a way as to be completely or at least partially
adapted to carry on an efficient life. In this way Darwinism explains the
universal condition of organic adjustment, showing that it exists because
there is no place in nature for the incompetent.
* * * * *
Finally we come to the process of inheritance as viewed by Darwin, and its
part in the production and perfection of new species. In every case,
Darwin said, the efficiency or inefficiency of an animal depends upon its
characteristics of an inherited or congenital nature. Variations in these
qualities provide the array of more or less different individuals from
which impersonal nature selects the better by throwing out first the
inferior ones. An organism can certainly change in direct response to
environmental influence or by the indirect results of use and disuse, but
not unless it is so constituted by heredity as to be able to change
adaptively. Therefore the final basis of success in life must be sought in
the inherited constitutions of organic forms.
For the reason that the qualities which preserve an animal's existence are
already congenital, they are already transmissible, as Darwin contended.
Since his time much has been learned about the course of inheritance and
its physical basis, and the new discoveries have confirmed the essential
truth of Darwin's statement that the congenital characters only possess a
real power in the evolution of species.
We must devote some time to the subject of inheritance at a later
juncture, but before leaving the matter an additional point must be
established here; the selective process deals immediately with congenital
results, as the heritable characters that make for success or failure in
life, but by doing this it really selects the group of congenital factors
behind and antecedent to their effects. For example, an ape that survives
because of its superior cunning, does so because it varies congenitally in
an improved direction; and the factors that have made it superior are
indirectly but no less certainly preserved through the survival of their
results in the way of efficiency. Hereditary strains are thus the ultimate
things selected through the organic constitutions that they determine and
produce.
Natural selection, as the whole of this intricate process, is simply trial
and error on a gigantic scale. Nature is such that thousands of varying
individuals are produced in order that a mere handful or only one survivor
may be chosen to bear the burden of carrying on the species for another
generation. The effect of nature's process is judicial, as it were. We may
liken the many and varied conditions of life to as many jurymen, before
which every living thing must appear for judgment as to its fitness or
lack of it. A unanimous verdict of complete or partial approval must be
rendered, or an animal dies, for the failure to meet a single vital
condition results in sure destruction. Of course, we cannot regard
selection as involving anything like a primitive conscious choice. It is
because we individualize all of the complex totality of the world as
"Nature" with a capital N that so many people unconsciously come to think
of it as a human-like personality. He who would go further and hold that
all of nature is actually conscious and the dwelling-place of the
supernatural ultimate, must beware of the logical results of such a view.
What must we think of the ethical status of such a conscious power who
causes countless millions of creatures to come into the world and
ruthlessly compels them to battle with one another until a cruel and
tragic death ends their existence?
But that is a metaphysical matter, with which we need not concern
ourselves in this discussion; the important point is that among the
everyday happenings of life are processes that are quite competent to
account for the condition of adaptation exhibited by various animal forms.
These processes are real and natural, not imaginative or artificial, and
so they will remain even though it will become clear that much is still to
be learned about the causes of variation and the course of biological
inheritance. Darwin was the first to contend that natural selection is but
a part of nature's method of accomplishing evolution. As such it is
content to recognize variations and does not concern itself with the
origin of modifications; it accepts the obvious fact that congenital
variations are inherited, although it leaves the question as to how they
are inherited for further examination. Because the doctrine of natural
selection does not profess to answer all the questions propounded by
scientific inquisitiveness, it must not be supposed that it fails in its
immediate purpose of giving a natural explanation of how evolution may be
partly accounted for.
* * * * *
Before proceeding to the post-Darwinian investigations that have done so
much to amplify the account of natural evolution, let us consider the
contrasted explanation given by Lamarck and his followers. As we have
stated earlier, Lamarckianism is the name given to the doctrine that
modifications other than those due to congenital factors may enter into
the heritage of a species, and may add themselves to those already
combined as the peculiar characteristics of a particular species. Let us
take the giraffe and its long neck as a concrete example. The great length
of this part is obviously an adaptive character, enabling the animal to
browse upon the softer leafy shoots of shrubs and trees. The vertebral
column of the neck comprises just the same number of bones that are
present in the short-necked relatives of this form, so that we are
justified in accepting as a fact the evolution of the giraffe's long neck
by the lengthening of each one of originally shorter vertebræ. The
Lamarckian explanation of this fact would be that the earliest forms in
the ancestry of the giraffe as such stretched their necks as they fed, and
that this peculiar function with its correlated structural modification
became habitual. The slight increase brought about by any single
individual would be inherited and transmitted to the giraffes of the next
generation; in other words, an individually acquired character would be
inherited. The young giraffes of this next generation would then begin,
not where their parents did, but from an advanced condition. Thus, by
continued stretching of the neck and by continued transmission of the
elongated condition, the great length of this part of the body in the
modern giraffe would be attained.
The explanation of natural selection would be quite different. The
Darwinian would say that all the young giraffes of any one generation
would vary with respect to the length of the neck. Those with longer necks
would have a slight advantage over their fellows in the extended sphere of
their grazing territory. Being better nourished than the others, they
would be stronger and so they would be more able to escape from their
flesh-eating foes, like the lion. For the reason that their variation
would be congenital and therefore already transmissible, their offspring
would vary about the advanced condition, and further selection of the
longer necked individuals would lead to the modern result.
The Lamarckian explanation encounters one grave difficulty which is not
met by the second one, in so far as it demands some method by which a
bodily change may be introduced into the stream of inheritance. So far,
this difficulty has not been overcome, and the present verdict of science
is that the transmission of characters acquired as the result of other
than congenital factors is not proved. It would be unscientific to say
that it cannot be proved in the future, but there are good _a priori_
grounds for disbelief in the principle, while furthermore the results of
experiments that have been undertaken to test its truth have been entirely
negative. Rats and mice have had their tails cut off to see if this
mutilation would have its effect upon their young, and though this has
been done for more than one hundred successive generations the length of
the tail has not been altered. Quite unconscious of the scientific
problem, many human races have performed precisely similar experiments
through centuries of time. In some classes of Chinese, the feet of young
girls have been bound in such a way as to produce a small, malformed foot,
but this has not resulted in any hereditary diminution in the size of the
feet of Chinese females. Many other similar mutilations have been
practised, as for example, the flattening of the skull of some North
American Indians, but the deformity must be produced again with each
recurring generation. One after another, the cases that were supposed to
give positive evidence have been reinvestigated, with the result that has
been stated above. It would seem, therefore, that heredity and congenital
modification must play by far the greater part in the evolution of
species.
* * * * *
The doctrine of natural selection took form in the mind of Darwin mainly
on account of three potent influences; these were, first, the geological
doctrine of uniformitarianism proposed by Lyell, second, his own
observations of wild life in many lands and his analysis of the breeder's
results with domesticated animals, and third, the writings of Malthus
dealing with overpopulation. As Darwin had read the works of Buffon,
Lamarck, and Erasmus Darwin, his grandfather, who had written a famous
treatise under the title of "Zoonomia," he was familiar with the evidences
known in his student days tending to prove that organic evolution was a
real natural process. Lyell's doctrine of uniform geological history made
an early and deep impression upon his mind, and it led him to ask himself
whether the efficient causes of past evolution might not be revealed by an
analysis of the present workings of nature. As naturalist of the "Beagle"
during its four years' cruise around the world, Darwin saw many new lands
and observed varied circumstances under which the organisms of the tropics
and other regions lived their lives. The fierce struggle for existence
waged by the denizens of the jungle recalled to him the views of Malthus
regarding overpopulation and its results. These and other influences led
him to begin the remarkable series of note-books, from which it is
interesting indeed to learn how the doctrine of natural selection began to
assume a definite and permanent form in his mind, as year followed year,
and evidence was added to evidence. And it is a valuable lesson to the
student of science that for twenty-five years Darwin devoted all his time
to the acquisition of facts before he gave his doctrine to the world in
the famous "Origin of Species."
Darwin was particularly impressed by the way mankind has dealt with the
various species of domesticated animals, and he was the first naturalist
to point out the correspondence between the breeder's method of
"artificial selection," and the world-wide process of natural selection.
As every one knows, the breeder of race horses finds that colts vary much
in their speed; discarding the slower animals, he uses only the swifter
for breeding purposes, and so he perfects one type of horse. With other
objects in view, the heavy draught horse, the spirited hackney, and the
agile polo pony have been severally bred by exactly the same method. Among
cattle many kinds occur, again the products of an artificial or human
selection; hornless breeds have been originated, as well as others with
wide-spreading or sharply curved horns; the Holstein has been bred for an
abundant supply of milk as an object, while Jerseys and Alderneys excel in
the rich quality of their milk. Various kinds of domesticated sheep and
rabbits and cats also owe their existence to the employment of the
selfsame method, unconsciously copied by man from nature; for men have
found variations arising naturally among their domesticated animals, and
they have simply substituted their practical purposes or their fancy for
nature's criterion of adaptive fitness, preserving those that they wish to
perfect and eliminating those unfitted to their requirements or ideas.
In the case of many of these and other examples, wild forms still occur
which seem to be like the ancestral stock from which the domesticated
forms have been produced. All the varied forms of dogs--from mastiff to
toy-terrier, and from greyhound to dachshund and bulldog--find their
prototypes in wild carnivora like the wolf and jackal. In Asia and
Malaysia the jungle fowl still lives, while its domesticated descendants
have altered under human direction to become the diverse strains of the
barnyard, and even the peculiar Japanese product with tail feathers
sometimes as long as twenty feet. That far-reaching changes can be brought
about in a relatively short time is proved by the history of the game
cock, which has nearly doubled in height since 1850, while at the same
time its slender legs, long spurs, and other qualities have been perfected
for the cruel sport for which it has been bred. Again, the wild rock
pigeon seems to be the ancestral form from which the fantail and pouter
and carrier-pigeon with their diverse characters have taken their origin.
It is true that some biologists have urged certain technical objections to
the employment of domesticated animals and their history as analogies to
the processes and results in wild nature. To my mind, however, artificial
selection is truly a part of the whole process of natural selection. Man
is but one element of the environment of tame forms, and his fancy or need
is therefore one of the varied series of external criteria that must be
met if survival is to be the result; failing this, elimination follows as
surely as under the conditions of an area uninhabited or uninfluenced by
mankind. Congenital variation is real, selection is real and the heredity
of the more fit modification is equally real. Surely Darwin was right in
contending that the facts of this class amplify the conception of natural
selection developed on the basis of an analysis of wild life.
* * * * *
Knowing the elements of the selective process, it is possible to analyze
and to understand many significant phenomena of nature, and to gain a
clearer conception of the results of the struggle for existence,
especially when the human factor is involved. Let us see how much is
revealed when the foregoing results are employed in a further study of
some of nature's vital situations.
As a consequence of the many-sided struggle for existence, the
interrelations of a series of species will approach a condition of
equilibrium in an area where the natural circumstances remain relatively
undisturbed for a long time. For example, among the field-mice of one
generation, just as many individuals will survive as will be able to find
food and to escape hereditary foes such as cats and snakes and owls. The
number of owls, in their turn, will be determined by the number of
available mice and other food organisms, as well as by the severity of the
adverse circumstances that cause elimination of the less fit among the
fledglings brought into the world. The vital chain of connections is
sometimes astonishingly long and intricate. One remarkable illustration is
given by Fiske, as an elaboration of an example cited by Darwin. He points
out that the fine quality of the traditional roast beef of England is
directly determined by the number of elderly spinsters in that country.
The chain of circumstances is as follows: the quality of the clover
fields, furnishing the best food for cattle, depends largely upon the
visits to the clover-blossoms by wild bees, that accomplish the
fertilization of the flowers by carrying pollen upon their bodies from one
plant to another. Field-mice devour the young in the nests of these bees,
so if there are few field-mice there will be many bees, and consequently
better grazing for the cattle. The number of field-mice will vary
according to the abundance of cats, and so the number of these domestic
animals will exert an influence upon the whole foregoing chain of forms.
But, as Fiske points out, cats are the favorite companions of elderly
spinsters; therefore, if there are many of the latter, there will be more
cats, fewer field-mice, more bees, richer clover fields, and finer cattle!
Each link is real and the whole chain is a characteristic example of the
countless ways that the natural destinies of living things are
interrelated and intertwined.
The reality of such organic interrelationships is revealed with wonderful
clearness in the numerous instances where some disturbing factor has
altered one or another element of the balanced system. The invasion of the
new world by Europeans has directly led to the partial or complete
extinction of the tribes of Indians to whom the land formerly belonged;
they have disappeared almost entirely from our state of New York, together
with the bear and wolf and many other species of animals that formerly
existed here. Wild horses and bison have also vanished before the advances
of civilization and the alteration of their homes. Sometimes the
extermination of one pest has resulted in an increase in the number of
another through human interference with nature's equilibrium. In some of
our Western states, a bounty was offered for the scalps of wolves, so as
to lessen the number of these predatory foes of sheep. But when the wolves
were diminished in number, their wild food-animals, the prairie dogs,
found their lot much bettered, and they have multiplied so rapidly that in
some places they have become even more destructive than the wolves.
One of the most remarkable illustrations is that of the rabbits introduced
into Australia. This island continent was cut off from the surrounding
lands long before the higher mammals evolved in far distant regions, so
that the balance of nature was worked out without reference to animals
like the rabbit. When the first of these were introduced they found a
territory without natural enemies where everything was favorable. They
promptly multiplied so rapidly that within a few years their descendants
were numerous enough to eat up practically every green thing they could
reach. Two decades ago, the single province of Queensland was forced to
expend $85,000,000 in a vain effort to put down the rabbit plague. The
remarkable statement has been made that in some places nature has taken a
hand in causing a new type of rabbit to evolve. Finding the situation
desperate, some of the animals have begun to develop into tree-climbing
creatures. The animals exist in such numbers that the available food upon
the ground is insufficient for all, and so some elimination results. But
the young rabbits with longer claws, varying in this way on account of
congenital factors, have an advantage over their fellows because they can
climb some of the trees and so obtain food inaccessible to the others. If
the facts are correctly reported, and if the process of selection on the
basis of longer claws and the climbing habit is continued, the original
type of animal is splitting up into a form that will remain the same and
live upon the ground, and another that will be to all intents and purposes
a counterpart of our familiar squirrel. All the evidence goes to show that
squirrels have evolved from terrestrial rodents; if the data relating to
Australian rabbits are correct, nature is again producing a squirrel-like
animal by evolution in a region where the former natural situation has
been interfered with by man.
The laws of biological inheritance have received close and deep study by
numerous investigators of Darwinian and post-Darwinian times, because from
the first it was clearly recognized that a complete description of
nature's method of accomplishing evolution must show how species maintain
the same general characteristics from generation to generation, and also
how new qualities may be fixed in heredity as species transform in the
course of time. Before our modern era in biology, the fact of inheritance
was accepted as self-sufficient; now much is known that supplements and
extends the incomplete account given by natural selection of the way
evolution takes place.
It is not possible in the present brief outline to describe all the
results of recent investigations, but some of them are too important to be
passed over. Perhaps the most interesting one is that the laws of heredity
seem to be the same for man and other kinds of living creatures, as proved
by Galton and Pearson and many others who have dealt with such characters
as human stature, human eye color, and an extensive series of the
peculiarities of lower animals and even of plants.