DARWINIAN THEORY, the explana tion of the working of natural selection in effecting specific changes in plants and animals. "Darwinism" must not be con fused with "Evolution." Darwinism is restricted to one particular interpretation of the mechanism of the universe, and is essentially stated in Darwin's great work, "The Origin of Species by Means of Nat ural Selection." Outline of Origin of Species.—To gain insight into the means of modification, Darwin begins with a study of the varia tion of plants and animals under domes tication. Those who admit the unity of domestic races should be cautious in denying the unity of the wild ones. Do mestic races all exhibit adaptations to man's use or fancy, rather than to their own good. The key to this is man's power of selection. Nature gives succes sive variations, man accumulates these, so making for himself useful breeds, and often (e. g., in sheep, cattle, roses, dah lias) profoundly modifies their character even in a single human lifetime; so that in all characters to which he attends, they may differ more than the distinct species of the same genera. Unconscious selection, which results from everyone trying is possess and breed the best ani mals, s even more important than con scious selection. Two flocks of Leicester sheep kept equally pure appear of quite different varieties after 50 years. Such slowly accumulated change explains why we know so little of the origin of do mestic races; and its absence in regions inhabited by uncivilized man explains why these yield no plants worth immedi ate culture. Human selection is facil itated (1) by the keeping of large numbers, since variations will be more frequent; and (2) by preventing free intercrossing. Some species vary more than others.
Variation Under Nature.—No two blades of grass are alike, and far more marked differences often occur, several strains or varieties sometimes existing in the same species. Between these strains, and much more frequently between forms which systematic botanists and zoologists rank as true species, perfectly intermedi ate forms may occur. No agreement about the definition of species (the amount of difference necessary to give any two forms specific rank) has ever been reached. Individual differences are of the highest importance, as the first steps toward the slightest varieties worth re cording; these in turn toward more dis tinct and permanent varieties; these vari eties again toward sub-species, and in the next stage to species, though extinction may often arrest the process. The species which present most varieties are those which have the greatest geographical range, or the widest diffusion in their own territory, or which possess the great est number of individuals.
Struggle for Existence.—All organic beings tend to increase with extreme rapidity, so that if they were not kept down, the earth would soon be covered by the progeny of a single pair. Since organisms are reproducing themselves so rapidly, and not all their offspring can escape their enemies, get food and live, much less leave progeny in turn, there must in every case be a struggle for ex istence, either of one individual with an other of the same species, with the indi viduals of distinct species or with the physical conditions of life; often with all these at once, and that more or less in tensely throughout the whole duration of life. The checks which prevent increase are more obscure, and vary in each case. In all cases the amount of food gives the limit. The youngest organisms generally suffer most. The struggle for life is most severe among individuals and varieties of the same species, and among the spe cies of the same genus, since these tend to fill the same place in the economy of nature. The structure of every being is related to that of the others with which it competes, or from which it seeks to escape, or on which it preys.
Natural Selection.—The preservation of favorable variations, and the destruc tion of injurious ones, is termed by Dar win "Natural Selection," or less figur atively by Spencer, the "Survival of the Fittest." Human selection acts only for man's own good, on mere external and visible characters, and irregularly throughout a short period; natural selec III—Cyc tion acts for the good of the being itself, on the whole machinery of its whole life, and incessantly on the species, through out almost infinite time. The circum stances favorable to the production of new forms are great variability; large numbers of individuals; the complex ef fects of intercrossing; isolation in small areas; also extension over continental ones, especially if these vary in altitude; and considerable lapse of time. Rare species are shown to be in process of ex tinction. The divergence of character in domestic breeds, largely due to the fact that "fanciers do not, and will not, ad mire a medium standard, but like ex tremes," applies throughout nature from the circumstance that the more diversi fied the descendants from any one species become in structure, constitution and habits, by so much will they be better en abled to seize on many and widely diver sified places in nature, and so to increase in numbers. A carnivorous animal which has reached the maximum numbers its territory can support, can succeed in in creasing only by its varying descendants seizing places hitherto occupied by other animals. This must hold equally of all species, and is separately demonstrated for plants.