Origin of Species Variorum

such rules;— all naturally follow if we admit the common parentage of allied forms, ... together with their modification through natural selection, with its contingencies of extinction and divergence of character. In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however different they may be in structure. If we extend the use of this element of descent,— the one certainly known cause of similarity in organic beings,— we shall understand what is meant by the Natural System: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms, varieties, species, genera, families, orders, and classes.

On this same view of descent with modification, all the great facts in Morphology become intelligible,— whether we look to the same pattern displayed by the different species of the same class in their homologous organs, to whatever purpose applied; or to the homologous parts in each individual animal and plant.

On the principle of successive slight variations, not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can understand the great leading facts in Embryology; namely, the close resemblance in the individual embryo of the parts which are homologous, and which when matured .. become widely different ... in structure and function; and the resemblance in allied though very distinct species of their homologous parts or organs, though fitted in the adult state for purposes as different as is possible. Larvæ are active embryos, which have been specially modified in a greater or less degree in relation to their habits of life, with their modifications inherited at a corresponding age. On