Origin of Species Variorum

extend the use of this element of descent,— the only 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, and the grades of acquired difference are 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 in the homologous organs, to whatever purpose applied, of the different species in the same class; or to the homologous parts constructed on the same pattern 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 resemblance in an individual embryo of the homologous parts, which when matured .. become widely different from each other in structure and function; and the resemblance in different species of the same class of the homologous parts or organs, though fitted in the adult members for purposes as different as is possible. Larvæ are active embryos, which have become specially modified in relation to their habits of life, through the principle of modifications being inherited at corresponding ages. On this same principle— and bearing in mind, that when organs are reduced in size, either from disuse or selection, it will generally be at that period of life when the being has to provide for its own wants, and bearing in mind how strong is the principle of inheritance— the occurrence of rudimentary organs and their final abortion, present to us no inexplicable difficulties; on the contrary, their pre- sence might