EVOLUTION. 33] EVOLUTION. marked, but many gaps have in late years been closed. Scr Extinction of Species. Evidence from Geographical Distbibi now. The presenl distribution (q.v.) of plants :i n< i ani- mals can only be explained by reference to past geological changes in the shape and proportions of former continental masses and the resulting geO' logical extinction. We can in many cases only account for the apparently sudden appearance of groups of highly specialized animals in a given area by invoking past, migrations. Thus, the camel family (see Camelid.e) originated in western North America, where it since died out, lint is still surviving in South America and Asia. So with the ox family in Eurasia, and the elephants in northeastern Allien, the mammoth (q.V.) having migrated into North America by way of northeastern Asia. This interchange of forms between Eurasia and America, between Asia and Africa, between North and South Ameri- ca, and the changes of climate and other sur- roundings along the line of march, must have operated in inducing change of habits and varia- tion, and, more especially by isolation, have led to the origin not only of new species and climatic varieties, but to the beginning of new generic and family types. Australia is. in many respects, notably in its characteristic mammals, a Jurassic continent, while -Madagascar is a Tertiary island. The moa birds, peculiar to New Zealand, are the result of long ages of isolation and lack of corn- petit inn with predatory animals. The various modes of dispersal of organisms and their colonization in remote regions also throw light on the origin of species. The study of deep-sea life is instructive in this con- nection. It is now generally supposed that the abyssal or benthal fauna originated from shal- low-water forms, and that the characters in which these animals differ from those liv- ing near the coast are adaptations to life at great depths. Indeed, all the facts and conclu- sions of zoogeography converge toward the view that as the different types evidently originated from this or that centre of distribution, so they had common ancestral forms. Biological Environment. We owe to Darwin and to Wallace the facts and theories epitomized by the terms 'natural selection' (q.v.). 'struggle for existence,' and to Herbert Spencer the expres- sion 'survival of the fittest.' The competition con- tinually going on between the stronger and the weaker, between original stupidity and acquired wit and cunning, between the plant-eaters and the Ilesh-eaters, between parasites and their unwill- ing hosts, form a most important chapter in the story of evolution. As soon as, through the art ion of the primary factors of organic evolu- tion, the ocean began to be peopled with the low- est, most primitive organisms, and when the process of specialization began to operate, then competition between this and that form sprang tip. and the struggle for existence — for food, for place, for fixed abodes, or habitats, for a chance to live and multiply and dominate this or that area, and the rivalry of the sexes — set in. The result is natural selection, the elimination of the unadapted, of the 'unfit,' the weak and inept, and the surras in life of this or that form which be- came the founder of some one of the immensely numerous groups of organisms now peopling the globe. Vol. VII. — -22. Natural Selection, iiter the earth became stocked with even a few comparatively simple forms, i ho leleel a e pi im iple in aa1 ure to operate, resulting in the pre ervati I the lii test . the factor oi nat ura I select em, ;i - tated by Wallace, i, based firel on "the enorn powers of im rea e in gi omet i ica I progr< ion pos sessed by all organisms, and I he inevitabli gle for existence among them"; and, in the sec- ond plaee. "the occurrence of much individual variation, combined with the hereditary transm i sion of such variations." Animals tend to increase in enormous numbers, (hough, owing to the destruction of eggs and young by animals of their own or other -pecies, i he earth's population is scarcely greater now than ages ago. When we consider that the cod lays a million of eggs, and thai m:ni other ani- mals are ncarh a- prolific, the -peri,- yei being represented by a constant number of individual-. We see that the rate of embryo and infant mor- tality is astonishingly great. What is called 'viability,' or the 'prospect of lite," in man, is in the lower animals reduced to almost, infinitesi- mal proportions. A death rate ai ig us of nn 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 i he figure of 909,998 out of 1,000,000. All this life is not. however, wasted, The young serve a- 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 word serv- ing, he 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. Pisotective Mimicry. Much is said by Darwin, Wallace, Fritz Miiller, Bates, and others on this subject, ami natural selection appears to play an important part in bringing about protective resemblance. The initial causes of mimicry are the action of light, changes in temperature, etc.. which have brought about a variety of patterns of color in insects and other animals of different groups. But it is difficult to account for the re- semblance in form as well as coloration between the mimicker and the mimicked, unless we invoke the action of natural selection. The disguises of animals, danger signals, the bright spots, lines, bars, and other markings, primarily due, per- haps, to the action of light and shade, have been preserved and exaggerated by natural selection, the process resulting in the preservation of the species thus favored. For further facts and con- siderations relating to this phase of the subject, see Natural Selection; Mimicry; and Protec- tive Coloration.
