C. In physics it has a similar meaning. It was in this sense that the classical experiments of R. A. Millikan, by which he succeeded in isolating an ion, were performed.
D. In phylogeny (theories of descent and in ontogeny (developmental history), isolation conveys in general the idea of freedom from crossing with the parent, or other, stocks, in the broader meaning which has been given to the term since the days of Charles Darwin. In this sense isolation is synonymous with biolog ical and ecological separation; and also with habitudinal segregation. Isolation and all its synonyms, signify that the descendants of one ancestral form living amid a definite set of eco logical, or environmental conditions, begin to adapt themselves to different sets of conditions, as a result of which, they become ecologically separated or segregated. Isolation with three other factors may enter into any process of speciation,— the latter name being applied by O. F. Cook to the process, the operation of which brings about the differentiation of one species into several coexisting species by the prevention of free crossing between groups existing at the same, time. It is rabove all the setting of individuals in groups, the members of each group intergenerating, and so securing racial generalization, or fundamental unity os inheritance, within each group, while between the groups there is prevention from free cross ing. Reflexive selection, on the other hand, is applied to that kind of selection which may be described as depending on the relations of the members of a species to each other. Its most familiar forms are sexual selection and i social selection. Now isolation is in its very nature the suspension not only of one form, but of all forms of reflexive selection between the separated portion of a species. Isolatiop as one of the four main factors of evolution, im plies that the process of transformation is a very complex affair. We, on accepting isola tion, recognize, .by implication, in evolution the ultimate outcome of a number of factors, each of which has its own special efficacy, and each of which may also at times operate in a manner antagonistic to all of the others. This view, eclectic in its nature, is perhaps to be eegarded as the most satisfactory explanation of the or ganic world and its upbuilding that has yet been put forward. But it must be repeated that it recognizes of the various factors thus engaged, the following four as the most potent and the most essential, and also as co-ordinate: The two Laniarckian factors, variation and inherit: ance, the Darwinian factor, natural selection, and the factor first postulated by Moritz Wag ner, or isolation. Even Lamarck recognized that distinct organic types could not be main tained without some form of isolation, and such Neo-Lamarckians of to-day as Professor Pack ard have been even more emphatic in insisting on this principle as being among the essential conditions for any genuine divergent evolution. Charles Darwin used isolation as equivalent to geographical separation while later writers have come to use it as equivalent to independent gea eration. Isolation differs from selection in that the latter denotes the exclusion of certain kinds from opportunity to propagate, while the former denotes the division of those that propa-, gate into classes that are prevented from inter generating. Isolation, or the prevention of in tergeneration, whether it be through separation or segregation is also called independent gener ation. Charles Darwin endeavored to explain the origin of species by the agency of natural selection plus the Lamarckian factors, and inheritance, and found in Wallace. Huxley and Haeckel, along with many other ingenious supporters. On other sides, however, the specifically Darwinian explanation encoun tered vehement opposition. Moritz Wagner,
for example, regarded free intercrossing, as an insurmountable obstacle to the establishment of new modifications and contended very ably that the isolation of a few individuals, a condition which would occur most frequently during mi grations, was a postulate in account ing for the origin of each new variety or spe cies. In these contentions Wagner showed the great value of migration and the intervention of geographical harriers in accounting for the process of speciation. The absence of in terbreeding, as that resulting from geographical isolation,— what August Weisman calls amixia, — is substantially the prevention of free inter crossing, and thus a form of isolation. And Hugo de Vries, while his classical investigations resulted in the demonstration that it is possible to produce species, or true breeding forms, out of mutations by pedigree culture, still he failed to see that the essential factor is not the qual ity of the material he worked with (the muta tions), bat that it is the pedigree — culture, and that this corresponds to the well-known factors, selection and isolation. The importance of iso lation as a co-ordinate factor with selection in the evolution of species is now gaining wide recognition. George J, Romanes insisted that isolation, or the prevention of free in tercrossing, organic evolution is in no case pos sible.° And he even went so far as to urge that isolation °has been the exclusive means of Modification, or more correctly, the universal condition of its But here it is to be remembered that Romanes failed to discriminate clearly be tween selection and isolation. It was the cus tom when he wrote to describe any influence that tends to transform species as a form of selection. For instance: Romanes says of in fertility between varieties of the same species: °I will call this principle physiological selection or segregation of the fit.° Since 1886 when Romanes wrote, however, isolation has by gen eral consent come to mean the prevention of between groups existing at the same time. (Rev. John T. Gulick). And in accordance with this usage even Romanes later substituted physiological isolation for physiolog ical selection, which Seebohm suggests, as does Gulick also, is a better term. But when Ro manes defined isolation itself he extended its meaning so as to include the prevention of crossing between those members of the group who succeed in living and propagating and those who die without propagating; a definition of, isolation• which makes it include natural selection as one of its many forms. Modern nomenclature would however restrict the term selection to the influences that determine the survival, or continued propagation, of the fit innate variations of anygiven group, and the elimination, that is, the disappearance of the unfit, thus preventing the crossing of fit with unfit. And naturalists would likewise restrict isolation to the prevention of free-crossing be tween groups existing at the same time. And present day naturalists are also gradually com ing to the conclusion that if natural selection works without isolation, only monotypic evolu tion can result. It is even evident that varia tion, inheritance, plus natural selection, working together, do not suffice for the interpretation of the whole process of evolution. They fail to account for the fact that often two or more dif ferent forms have originated from a single an cestral type. They are capable at best of ex plaining the transformation of one existing form into one other form. So it came about that the principle of isolation has found recent and able supporters in Baur, Ortman, Gulick and many others.