Page - 497 - in The Origin of Species

SUMMARY 497 by naturalists in their classifications,—the value set upon characters, if constant and prevalent, whether of high or of the most trifling importance, or, as with rudimentary organs, of no importance,—the wide opposition in value be- tween analogical or adaptive characters, and characters of true affinity; and other such rules;—all naturally follow if we admit the common parentage of allied forms, together with their modification through variation and natural selec- tion, with the 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, dimor- phic forms, and acknowledged varieties of the same species, however much they may differ from each other 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, most of 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 serial and lateral homologies in each individual animal and plant. On the principle of successive slight variations, not neces- sarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can under- stand the leading facts in Embryology ; namely, the close resemblance in the individual embryo of the parts which are homologous, and which when matured become widely dif- ferent in structure and function ; and the resemblance of the homologous parts or organs in allied though distinct species, though fitted in the adult state for habits as different as is possible. Larvc-e are active embryos, which have been spe- cially modified in a greater or less degree in relation to their habits of life, with their modifications inherited at a corre- sponding early age. On these same principles,—and bearing in mind that when organs are reduced in size, either from