The mode of operation and the forms as sumed in the operation of the principle of isolation, has made a classification of the various, forms in which it manifests itself es sential and most of the remaining portion of this article will devote itself to the definition of these various manifestations of this prin cipie. 'Isolation itself has already been suf ficiently defined. The principle immediately reveals two forms: (1) Autonomic isolation; which stands in contrast' with heteronomie isolation, includes both endonomic isolation, produced by industrial, chronal and migrational isolation,' and reflexive isolation, produced by sexual and social instincts, by impregnational incompatibilities, and by institutional require ments; (2) Heteronomic isolation, is that form of isolation determined by conditions out side of the organic group, for example, by geological subsidence, or other causes resulting in the transportation to an isolated position. In the four forms of environal isolation (see below), namely in transportational, geological, fertilizatlonal and artificial isolation, hetero nomic influences prevail. Treated from another point of view one may distinguish the follow ing co-ordinate divisions of the principle of isolation: (a) Reflexive isolation, compre hending conjunctional, impregnational and in stitutional isolation (for • which see below); (b) environal isolation, where the relations of the and its environment are determined by conditions within the group. The import ance of this form lies in the fact that it often opens the way for the entrance of more funda mental forms of segregation; (c) regressive isolation, or the amalgamation of races; (d) indiscriminate isolation, which results in the divergence in the aptitudes • and -innate char acters of isolated groups, especially when at the time of the first setting apart, the group is represented by but one, or a few, individuals, and so.•roducing initial racial segregation. It usually attended with the loss of power to reproduce the average of the innate characters of the original stock.
Of the forms of reflexive isolation one may next distinguish: (1) Conjunctiohal isolation, which includes sexual and social isolation; (2) impregnational isolation due to the need of co ordination between the size, structure, sexual elements and functions of each sex and the related characters of the other sex, in any intergenerating group; (3) institutional isola tion, due to the differences of language, re ligion' and education, a prevention to free as sodiation, will also prevent intermingling of races.
Of the forms of environal isolation may likewise be distinguished co-ordinate with the three foregoing: (1) Endonomic isolation, an example of which is presented, when varieties of the same species of plant occupying the same areas are prevented from crossing by flowering at different seasons, and among ani mals, when there occurs the cyclical 'isolation between the broods of the periodical cicada even when they succeed each other in' the same district. And other forms of isolation so far as they are determined by the diversity of habits or instincts of the members of species, are forms of endonomic is'olation; (2)' herteronomic isolation, for which see above.
It remains no* to describe the following nearly co-ordinate sub-divisions of isolative influence.
The two forms of conjunctional isolation are (1) sexual isolation which arises between groups of the same species that have been separated' by geographical barriers for several generations, and have in the meantime attained clfvergent forms of inherited characters by which they recognize each other, and also dif ferent methods of calling and winning each other; (2) social isolation which arises when ever two groups of a species have been separated by geography for many generations and in the meantime gain divergent social habits and instincts, rendering them unfit for being associated in one intergenerating group when brought together in one district. It is important to notice here that geographical isolation has ceased but the groups continue as separate groups through the influence of social isolation.
The eight forms of impregnational isolation are: (1) dimensional isolation, of which there is an example when local varieties of birds or of mammals that have become very divergent in size are brought together in the same dis trict; (2) structural isolation, which arises when local varieties that have become so far divergent in structure as to be incompatible are brought together into the same district; (3) potential isolation, of which there are two forms: (a) Complete potential isolation which exists between types when their sexual elements are incapable of union in fertilized germs under any conditions; (b) Prepotential isola tion which exists when cross-fertilization is possible if the alien fertilizing element has been applied sometime in advance; but which if the fertilizing element of the same species is applied at the same time, or in some cases, at any time during the several hours that follow, mixed fertilization is prevented by the pre potence of the pure fertilizing element; (4) segregate fecundity, the relation in which the species or the varieties stand to each other when intergeneration of members of the same species or varieties results in higher fertility than the crossing of different species or varie ties; (5) segregate vigor, the relation in which species or varieties stand to each other when the intergeneration of members of the same species or variety produces offspring of more vigor than those produced by crossing with other species or varieties; (6) segregate adapta tion, the relation in which species or varieties stand to each other when the intergeneration of individuals of the same species or variety produces offspring better adapted than the off spring produced by crossing with other species or varieties; (7) segregate freedom from com petition, the segregate access to unused re sources which results when the pure offspring have freer access to unused resources than do the cross-breeds or the original stock; (8) segregate escape from enemies arises when ever the pure offspring of a divergent variety are able to occupy a position freer from enemies than that occupied by the original stock.