VIII. The Factors of Organic Evolution.
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.
[1.] Jordan: Footnotes of Evolution; New York, 1898.