=279. Concept.= The idea of polyphylesis, as advanced by Engler, contains two distinct concepts: (1) that a species may arise in two different places or at two different times from the same species, and (2) that a genus or higher group may arise at different places or times by the convergence of two or more lines of origin. It is here proposed to restrict polyphylesis, as its meaning would indicate, to the second concept, and to employ for the first the term polygenesis,[37] first suggested by Huxley in the sense of polyphylesis. The term polyphylesis is extended, however, to cover the origin of those species which arise at different places or times from the convergence of two or more different species, a logical extension of the idea underlying polyphyletic genera, though it may seem at first thought to be absurd. Polygenesis may be formally defined as the origin of one species from another species at two or more distinct places on the earth’s surface, at the same time or at different times, or its origin in the same place at different times. Polyphylesis, on the contrary, is the origin of one species from two or more different species at different places, at the same time or at different times. It is evident that what is true of species in this connection will hold equally well of genera and higher groups. Opposed to polygenesis is monogenesis, in which a species arises but once from another species; with polyphylesis is to be contrasted monophylesis, in which the species arises from a single other species. It will be noticed at once that these two concepts are closely related. The following diagrams will serve to make the above distinctions more evident: [Illustration: I. Polygenesis II. Polyphylesis III. Monogenesis (Monophylesis)] In I, a species A, becomes scattered over a large area in a series of places, _m_ ... _m^n_, with the same physical factors, in any or all of which may arise the new species _a_. In II, a species with xerophytic tendency, _A_, and one with mesophytic tendency, _B_, in the course of migration find themselves respectively in a more mesophytic habitat, _m_, and a more xerophytic one, _x_, in which either may give rise to the new form, _c_, which is more or less intermediate between _A_ and _B_. In III, the method of origin is of the simplest type, in which a species is modified directly into another one, or is split up into several.