NATURAL SELECTION, the doctrine advanced by Charles Darwin and almost coincidentally by Alfred Russel Wallace (q.v.), to account for the divergence of animal forms and their gradual separation into distinct species and groups, by a process akin to the selective mating practised by men in rearing and perpetu ating breeds of domestic animals. It forms the basis of Darwin's hypothesis of organic de velopment by descent, and depends upon the fact that variations constantly appear in ani mals, and may in some degree be perpetuated. These variations may be minute fluctuations on either side of a mean, a little more of one character and a little less of another; or they may be sudden steps of considerable magni tude; in other words, they may be continuous or discontinuous. They may visibly affect only one character at a time, or they may affect many parts of the organism at once, as if there were a general movement to a new position of organic equilibrium.
(2) Living creatures are involved in a mani fold and intricate struggle for existence, vary ing greatly in its form and in its intensity, and due to a variety of causes. It is necessitated especially by two facts: first, that two parents usually produce many more than a pair of off spring, and that the population tends to outrun the means of subsistence; and, secondly, that organisms are at the best only relatively well adapted to the external conditions of their life, which moreover are variable. The °struggle" may be for food or foothold, for mates or prop erty, for self-preservation or for the welfare of the young, including much more than an inter necine scramble around the platter which con tains the necessaries of life; the phrase is appli cable as regards relative length of life, vigor or constitution, success in having offspring, and so on.
(3) In this struggle for existence the rela tively less fit organisms are weeded out or elim inated, and sometimes only a small proportion of those born survive to become adults or re productive. But it must be clearly understood that elimination does not necessarily involve sudden death or no offspring; it may simply in volve, in the first instance, a slightly shorter, less successful life, or a smaller, less vigorous family. Yet whether the eliminative process be gentle or severe, the result is the same — that the relatively more fit variants tend to survive; and since many variations are demonstrably transmissible from generation to generation, and may, through the pairing of similar or suitable mates, or in other ways, gradually increase in amount, the eliminative or selective process works toward the establishment of new adapta tions and new species.
The three steps in the argument are thus: (1) The occurrence of transmissible germinal variations is a fact of life; (2) the struggle for existence is a fact of life; and (3) the elimina tion of the relatively less fit is a fact of life. The result has been, and is, the rise and prog ress of new adaptations, new varieties, new species, new types.
A formidable objection to the selection the ory, first clearly stated by Prof. Fleemina
Jenkin, is that variations of small amount and sparse occurrence would tend to be swamped out by intercrossing. In human or (artificial" selection. the breeder takes measures to orevent this by pairing similar or suitable forms, but what in nature corresponds to this action of the breeder? Various suggestions have been made in answer to this objection. Thus Weismann says: "The necessary variations from which transformations arise must in all cases be ex hibited, over and over again, by many indi viduals," and in his ingenious theory of Ger minal Selection he has suggested the internal mechanism by which this result may come about.
But the answer at present most relied on is that worked out by Romanes, Gulick and others—the theory of isolation (q.v.). The theory of isolation emphasizes the great variety of ways in which, in the ordinary course of nature, the range of intercrossing may be re stricted, for example, by geographical barriers, by differences in habit, by psychical likes and dislikes, and by those remarkable reproductive variations which cause mutual sterility between two sections of a species living on a common area.
Another point that has been brought out by De Vries (q.v.) is that variations are not necessarily minute at the start, and that new species may spring into existence at once by what he calls a mutation. Of course, the later history of such a species is conditioned by nat ural selection.
We have given a statement of the theory of natural selection very much as it might have been given in 1859, when Charles Darwin and Alfred Russel Wallace enriched biology by their independent exposition of the selection idea; but since then our knowledge 0' the nature and origin of variations has greatly in creased, the analysis of the various modes of inheritance has become much more precise, the difficulty of proving any instance of the trans mission of "an acquired character') or direct somatic modification is generally acknowledged, and we have recognized the value of a second directive factor in evolution, namely, isolation. It may be said that the theory of natural selec tion is now being subjected to more severe and more dispassionate criticism than it had to en counter in the early Darwinian days, when the validity of the general evolution idea was the central subject of discussion.
Thus there is a demand for some serious attempt to measure the intensity of the struggle for existence in typical cases, and for evidence that the absence of a particular variation in cer tain members of a stock does really determine their elimination. In other words, evolutionists have awakened to the necessity of testing nat ural selection in relation to actual cases.
Lastly it should be noted that the doctrines of Lamarck, which were that use-and-disuse, inheritance of acquired characters, and other factors were more potent than natural selection, have been revived and strengthened by a school of naturalists who insist that they must at least be held to have had an important share in the phenomena of biology.