The three forms of endonomic isolation are (1) industrial isolation, arising from the activities by which an organism protects itself against' adverse influences in an environment, or by which' it finds and then appropriates special resources in the environment; and hav ing three forms: (a) Sustentational isolation, arising from the use of different methods of obtaining sustentation by members of the same species; (b) protectional isolation, or isola tion from the use of different methods of protection against adverse influence in the environment; (c) nidificational isolation, the prevention of free interbreeding between the different sections of a species by diversity of industrial habits,— the separation of the in dividual from the mass of the same species by an industrial habit; (2) chronal isolation, isola tion arising from the relations i.i which an organism stands to times and seasons; it is of two kinds: (a) Cyclical isolation, arising from the fact that the life-cycles of the different sections of a species dp not mature simul taneously; (b) Seasonal isolation, produced whenever the season for reproduction in any section of the species is such that it can not interbreed with other sections of the species; (3) migrational isolation, caused by the powers of locomotion in an organism.
The four forms of heteronomic isolation are; (1) transportational isolation, caused by the activities in the environment that distribute the organisms in different districts; (2) geo logical isolation, caused by geological changes which divide the territory occupied by a species into sections; (3) fertilizational isolation, a segregative rather than a separative form, in that it perpetuates a segregation previously produced, depending on divergence of char acter already clearly established, and therefore on some form of isolation that has preceded. The forms of isolation that precede fertiliza tional isolation are in the majority of cases local, but may be chronal or impregnational; (4) artificial isolation arising from the rela tions in which the organism stands after an attempt has been made to rationalize its environment.
Attention having been given to other fea tures other groupings of these various sorts of isolation is possible. Thus potential isola tion, segregate vigor and segregate fecundity might be brought together as forms of physiological isolation, in the sense in which Romanes uses the term in his work 'Darwin and after Darwin.' And thus, too, all forms of environal and regressive isolation, including as environal isolation does, all forms of en donomic and heteronomic isolation might not unreasonably be brought under a single caption as forms of coincident isolation. All
forms of isolation are talked about by natural ists who saw a necessity to specify the modes of operation manifest in the behavior of bionomic isolation, which, with three other principles, already spoken' of, serves to ex plain the process of organic evolution.
Through such isolation the swamping or leveling effects of free intercrossing, or mix ing with allied varieties or incipient species,, are prevented. As a consequence, variations or nascent species become fixed or localized, be ing prevented from spreading by some geo graphic or topographic barrier, with the result that there are many thousands of local races, varieties and species; indeed, probably over half of the number of known species are such forms. Not only species, but genera and higher groups are thus isolated. Thus the marsupials of Australia are, with one or two exceptions, confined to that continent, the connection once existing with Asia having been cut off.
Examples of These are found among cave animals (q.v.) where animals, con fined to the nether world, living in total dark ness, are prevented from breeding with their ancestors of the upper world. The deep-sea fauna is another such assemblage, living in gloom and in water at the freezing point, al though at the surface the winter temperature of the sea may be 80-85° F. Other examples of the result of isolation are the assemblage of animals peculiar to certain islands, to basins walled in by mountain chains, valleys, deserts and Alpine summits. Interesting cases of isolation on islands are the gigantic moa birds of New Zealand; the local species of birds confined to the different islands of the Gala pagos archipelago, also the land shells living in the different valleys of Oahu, one of the Hawaiian Islands.
Consult Dewar and Finn, 'The Making of a Species' (London 1909); Gulick, J. T., 'On Diversity of Evolution under One Set of Ex ternal Conditions' (in Linnean Society of Lon don, Journal of zoology, London 1872) ; id., 'Divergent Evolution and the Darwinian Theory' (in American Journal of Science, New Haven, January 1890) ; Hutton, F. W., Racial and HabitudinaP (Wash ington 1905) ; Jordan, (Isolation as a Factor in Organic Evolution) (in 'Fifty Years of Dar winism,' New York 1909).