Home >> Encyclopedia Americana, Volume 8 >> Dance Of Death to Dean >> Darwinian Theory_P1

Darwinian Theory

species, varieties, selection, struggle, plants, life and races

Page: 1 2 3

DARWINIAN THEORY, the explanation of the working of natural selection in effecting specific changes in plants and animals. 'Dar winism' must not be confused with 'Evolution? The term Darwinism is applied to one particu lar interpretation of the mechanism of the uni verse, and is summarized in Darwin's great work, 'The crigin of Species by Means of Natural Selection.' Whatever may be the future development of our evolutionary ideas, the epoch-making importance of the Darwinian theory will be unaltered.

Outline of Origin of To gain an insight into the means of modification, Darwin begins with a study of the variation of plants and animals under domestication. Those who admit the unity of domestic races should be cautious in denying the unity of the wild ones. Domestic 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 successive variations, man accumu lates these, so making for himself useful breeds, and often (for example, in sheep, cattle, roses, dahlias) profoundly modifies their character even in a single human lifetime; so that in all the characteristics to which he pays attention they may differ more than the distinct species of the samegenera. Unconscious selection, which results from everyone trying to possess and breed the best animals, is even more im portant than conscious selection. Two flocks of Leicester sheep kept equally pure appear quite different varieties after 50 years. Such slowly accumulated change explains why we know so little of the origin of domestic races; and its absence in regions inhabited by uncivil ized man explains why these yield no plants worth immediate culture. Human selection is facilitated (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 No two blades of grass are alike, and far more marked dif ferences often occur, several strains•or varieties sometimes existing in the same species. Between these strains, and even between forms which systematic botanists and zoologists rank as true species, perfectly intermediate forms may occur.

No agreement about the definition of species (the amount of difference necessary togive any two forms specific rank) has ever reached It is an interesting fact, however, that closely allied species often differ in their chromosome number. Individual differences are of the high est importance as the first steps toward the slightest varieties worth recording; these in turn toward more distinct and permanent varieties; these varieties 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 dif fusion in their own territory, or which possess the greatest 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 existence, either of one individual with another of the same species, with the in dividuals of distinct species, or with the physical conditions of life; often with all these at once, and that more or less intensely throughout the whole duration of life. The checks which pre vent 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 stock of game on an estate depends chiefly on the destruction of vermin. Climate is important and periodic seasons of extreme cold and drought seem the most effective of all checks. Epidemics too may occur, especially where numbers have inordinately increased. The struggle for life is most severe among in dividuals and varieties of the same species and among the species 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.

Page: 1 2 3