The animal kingdom represents one of the two grand branches of the
organic universe. It has been evolved—evolved in a manner as
simple and straightforward as it is revolting. It has all been
brought about by _partiality_ or _selection_. Generations of beings
have come into existence. The individual members of each generation
have differed from each other—differed in size, strength, speed,
colour, shape, sagacity, luck, and likelihood of life. No two
beings, not even those born from the same womb, are in all respects
identical. Hardships have come. They have come from the inanimate
universe in the form of floods, fires, frosts, accidents, diseases,
droughts, storms, and the like; from other species, who were
competitors or enemies; and from unbrotherly members of the same
species. Some have survived, but the great majority have perished.
Only a fraction, and generally an appallingly small fraction, of
each generation of a species have lived to maturity. The lobster
lays 10,000 eggs in a season, yet the mortality is such that the
number of lobsters do not increase from one year to another. The
elephant is the slowest breeder of all animals, yet, if they should
all live, the offspring of a single pair in 750 years would,
according to Darwin, number nearly 19,000,000. It has been shown
that at the normal rate of increase of English sparrows, if none
were to die save of old age, it would take but twenty years for a
single pair to give one sparrow to every square inch in the State of
Indiana.[1] A single cyclops (one of the humbler crustaceans) may
have 5,000,000 descendants in a season. One aphis will produce 100
young, and these young will reproduce in like manner for ten
generations in a season, when, if they should all live, there would
be a quintillion of young. A female white ant, when adult, does
nothing but lie in a cell and lay eggs. She lays 80,000 eggs a day
regularly for several months. An oyster lays 2,000,000 eggs in a
season, and if all these eggs came to maturity a few dozen oysters
might supply the markets of the world. The tapeworm is said to
produce the incredible number of 1,000,000,000 ova, and some of the
humbler plants three times this number of spores. If each egg of the
codfish should produce an adult, a single pair in twenty-five years
would produce a mass of fish larger than the earth. Lower forms of
life are even more prolific than the higher. Maupas said that
certain microscopic infusorians which he studied multiplied so
rapidly that, if they should continue to multiply for thirty-eight
days, and all of them should live, any one of them would produce a
mass of protoplasm as big as the sun.
Those of each generation that have died have been inferior, or
unfitted to the environment in which they found themselves. Those
that have survived have been superior, superior in
something—bigness, cunning, courage, virtue, vitality, strength,
speed, littleness, or ferocity—something that has related them
advantageously to surrounding conditions. The surviving remnant of
each generation have become the progenitors of the next generation,
and have transmitted, or tended to transmit, to their offspring the
qualities of their superiority. This winnowing has gone on in each
generation of living beings during many millions of years—almost
ever since life commenced to be on the earth. Some have continued
themselves, and others have died childless. The environment of each
species has been an immense sieve, and only the superior have gone
through it. Different environments have emphasised different
qualities of structure and disposition, and have thus given rise to
permanent varieties in survival. These varieties, through the
accumulated effects of many generations of selection, have diverged
into species; species, after a still longer series of selections,
have evolved into genera; genera have evolved into families;
families into orders; and so on. In this simple, terrible manner
have all the branches of organic beings (thanks to the horrors of a
million ages) been brought into existence.
_Variation_, therefore, which furnishes variety in offspring;
_Heredity_, which tends to perpetuate peculiarities by causing
offspring to resemble more or less the characters of their parents;
and _Environment_, which determines the character of the selections,
are the three factors, and the only three factors, in organic
evolution.