NATURAL SELECTION. After the ea rth breame stocked \vith even a few comparatively simple forms, the selective principles in nature began to operate, resulting in the preservation of the fittest. The factor of natural selection, as stated by Wallace, is based first on "the enormous powers of increase in geometrical progression pos sessed by all organisms, and the inevitable strug gle for existence among them"; and, in the sec ond place, "the occurrence of much individual variation, combined with the hereditary transmis sion of such variations." Animals tend to increase in enormous numbers, though, owing to the destruction of eggs and young by animals of their own or other species, the earth's population is scarcely greater now than ages ago. When we consider that the cod lays a million of eggs, and that many other ani mals are nearly as prolific, the species yet being represented by a constant number of individuals, we see that the rate of embryo and infant mor tality is astonishingly great. What is called 'viability,' or the `prospect of life,' in man, is in the lower animals reduced to almost infinitesi mal proportions. A death-rate among us of more than 20 in a thousand excites alarm, but think of the death-rate in the cod, the bee, and most animals, where it reaches perhaps the figure of 909,908 out of I,000,000. All this life is not,
however, wasted. The young serve as food for other forms of life, and in this way the balance of nature is maintained, the too great increase in organic life is checked, and those that. survive and reach maturity are, so to speak, adequately fed and housed. See LONGEVITY.
In formulating his theory of natural selection, Darwin assumed a tendency to variation, the causes of which he did not discuss at length. This variation, by insensible gradations, is, he believed, fortuitous or 'chance,' this won] serv ing, Ile adds, "to acknowledge plainly our igno rance of the cause of each particular variation." The useful variations are those which survive. Natural selection, as Darwin claimed. "leads to divergence of character and to much extinction of the less-improved and intermediate forms of life," and he states: "It leads to the improvement of each creature in relation to its organic and in organic conditions of life, and consequently. in most cases, to what must be regarded as an ad vance in organization." See NATURAL SELEC TION.