Evolution, in the sense of transformism, is opposed to fixism, the older theory of Linné, according to whom no _specific_ change is possible in plants and animals, all organisms being assumed to have persisted in essential sameness of type from the dawn of organic life down to the present day. The latter theory admits the possibility of environmentally-induced modifications, which are non-germinal and therefore non-inheritable. It also admits the possibility of germinal changes of the varietal, as opposed to the specific, order, but it maintains that all such changes are confined within the limits of the species, and that the boundaries of an organic species are impassable. Transformism, on the contrary, affirms the possibility of specific change, and assumes that the boundaries of organic species have actually been traversed. What, then, is an organic species? It may be defined as a group of organisms endowed with the _hardihood_ necessary to survive and propagate themselves under natural conditions (_i.e._ in the wild state), exhibiting a common inheritable type, differing from one another by no major germinal difference, perfectly interfertile with one another, but _sexually incompatible_ with members of an alien specific group, in such wise that they produce hybrids wholly, or partially, sterile, when crossed with organisms outside their own specific group. David Starr Jordan has wisely called attention to the requisite of viability and survival under natural conditions. “A species,” he says, “is not merely a form or group of individuals distinguished from other groups by definable features. A complete definition involves longevity. A species is a kind of animal or plant which has run the gauntlet of the ages and persisted.... A form is not a species until it has ‘stood.’” (_Science_, Oct. 20, 1922, p. 448.) Sexual (gametic) incompatibility as a criterion of specific distinction, presupposes the bisexual or biparental mode of reproduction, namely, syngamy, and is therefore chiefly applicable to the metista, although, if the view tentatively proposed by the protozoölogist, E. A. Minchin, be correct, it would also be applicable to the protista. According to this view, no protist type is a true species, unless it is maintained by syngamy (_i.e._ bisexual reproduction)—“Not until syngamy was acquired,” says Minchin, “could true species exist among the Protista.” (“An Introduction to the Study of the Protozoa,” p. 141.)