When the farmer or the stock-breeder selects his seeds or his animals
for propagating purposes he has an eye to a great natural law,
heredity. He knows that scrub animals and the seeds of degenerate
plants will stamp their inferiority upon their descendants. He has
learned that like produces like, therefore he selects such seeds and
animals as will produce those best suited to his purposes. He is not
only guided by his knowledge of a great law of Nature, but in the main
he is following her method in preserving and improving the breed. This
method of changing and improving plants and animals under domestication
is called artificial selection.
Plants and animals in the natural state are capable of multiplying at
so enormous a rate that there is an incessant struggle for existence
going on among them. If it were not for this fierce struggle with
one another and with their enemies and other environmental forces in
which the vast majority die early, the world would be overstocked.
There is hardly a species of animal of which a single pair would not
choke out all other animal life in a few generations by filling the
world with its descendants if all of the one species were permitted
to reach old age. If a pair of elephants (one of the slowest-breeding
species of animals) should bring forth only six young ones, and all
should live to be one hundred years old, their descendants in 750 years
would number nineteen million--a number so great that they would form
a closely-packed herd occupying forty-one square miles. The codfish
produces nine million eggs a year. If each egg should develop into a
mature fish, half of them females, in ten years the sea would be a
solid mass of codfish.
This tendency to increase at so tremendous a rate, and the fact that
no two individuals are exactly alike, supply the material and the
conditions for the great law of natural selection to operate. Each
individual must of necessity compete so fiercely with other individuals
of the same species, and of allied species, and with enemies, with
climate and changing conditions that out of the struggling many only a
few live to propagate. Those individuals that vary from the mass in the
right direction live; the others die, leaving few descendants, or none.
Though like produces like there are no two individuals exactly alike.
If all of the same species were alike there would be no opportunity
for the law of natural selection to operate. Nothing but chance would
determine in the struggle for existence which individuals would live to
propagate their species and which would die without descendants. Nor
would it matter, so far as the species is concerned. But individuals
do differ in their traits in some degree--Wallace has shown that
variation usually amounts from ten to twenty, sometimes twenty-five,
per cent of the varying part--and this variation, even if small, often
spells the difference between an early death with no descendants and a
long life with a numerous progeny.
By the law of heredity the descendants are endowed in a greater or less
degree with the same life-saving characteristics of their parents.
Each generation being subjected to this weeding-out process, and only
a few of the best fitted individuals being selected to preserve the
species, we can easily see that, as the generations come and go, those
essential, life-saving characters or traits are being developed to a
greater and greater degree. And this means change, modification of
species.
With horn or tooth or claw or hoof or sting or poison or odor, among
all the wild creatures that swim or crawl or run or climb or fly, the
struggle for life and food and mate goes on today just as it has gone
on for millions of years. Other factors in the struggle are alertness,
agility, cunningness, sharpness of vision and hearing and smell,
protective color of covering, fleetness of foot and wing, the degree of
heat, the amount of moisture and the food supply.
The traits that survive are of course those that are the most useful in
a given environment. (By environment is meant surroundings, all outside
influences, the not-me of each individual.) A change of environment, of
the conditions of life, calls for a re-adjustment of the traits most
vital to the individual in the new environment. In one environment
one highly developed trait would be most useful while in another
some other character or set of characters would be the saving factor,
because a change of environment means a change of weather conditions,
of the food supply and the means of getting it, of enemies, etc.
From the foregoing brief outline it would seem that only a little
exercise of reason is required of any one to see that natural
selection operating in unlike environments would, in the course of
many generations, produce from the same species types of animals (or
plants) very unlike--not only unlike each other but unlike their parent
species. At first these types derived from a common ancestor would be
only varieties, but varieties are incipient species. Given sufficient
time, and the intervention of natural barriers to prevent the crossing
of extremes, and the creation of new species would be the natural
result.
As a vivid illustration of how the law of natural selection works, and
of how great is the sum of the results of its operation accumulated
through many generations, let us observe one of many modifications
an animal undergoes in its struggle for existence. To the question,
“Why are animals of the arctic regions white?” we ought to be able,
with what we already know of natural selection, to give the answer.
Imagine those lands of snow and ice originally inhabited by animals of
all colors from white to black, or even of all shades of one color,
brown. The animals of what colors or shades, among the flesh-eaters,
would have the least difficulty in stealthily approaching their prey,
and so, be most apt to survive in the struggle against starvation?
And which among the animals preyed on would run the least risk of
detection and so be most likely to escape destruction? The answer
to both questions, of course, would be, “Those whose colors most
nearly conformed to the snowy background.” Imagine this process of
culling out the darker colors continuing for many generations and
we can understand that the whiteness of all arctic animals would be
the inevitable result. We can also see why most wild animals of our
regions are of colors that best harmonize with the brown earth and dead
leaves. Let us keep in mind the fact that while we were observing the
modification of one character, all the favorable variations, however
slight, in every other character that could be of the least advantage
to those creatures were being added up as fast as they appeared.
After several generations, if for any cause the environment were
undergoing a radical change, or if a species had migrated from a widely
different one, a greatly modified animal would be the result. However,
there can be no modification in an animal perfectly adapted to its
environment--provided the environment does not change.
Every organ, and every other part of the body, internal and external,
of every creature in the natural state is subject to modification by
the law of natural selection, just as was the color of the hair or the
feathers of polar animals.
Man’s method of improving plants and animals--artificial
selection--interferes with the work of natural selection. For one
thing, man makes a radical change in the environment of every plant and
animal he domesticates; for another, man selects for other purposes
than the one Nature has in view--that is, if it can be said that Nature
has a purpose.
From the same parent stock man breeds one strain of cattle for beef,
another for milk and butter; one variety of the horse for the saddle
and another for draft purposes. Nature adapts the species to the
environment--never the environment to the species--sacrificing those
individuals that do not measure up to her standard. She seems to care
only for the species, not for the individuals, or only for those
individuals that give promise of a better species.
This secret wrung from Nature (natural selection as a factor in
progressive development) is man’s most precious truth, for it is the
key to his further progress.
To summarize: organisms tend to increase at a great rate; this
intensifies the struggle for existence; organisms vary; in the struggle
for existence the vast majority die early, leaving those that vary
in the favorable direction to live and reproduce; this means change,
progress. This is natural selection.
There is also De Vries’ “Mutations theory” which some scientists
believe to have been an important factor in evolution. It is the
theory that at times a species may progress by “jumps”--that is, that
occasionally individuals come into existence that vary extremely from
the mass, and that they may become the parents of a new species. It is
on account of the relative value attached by some scientists to this
theory and to natural selection that they disagree, and not in regard
to whether or not evolution is a fact.