These tendencies to lateral and serial repetition show themselves in ways which cannot be accounted for by inheritance from ancestral forms, but loudly proclaim the presence and action of some internal force tending to produce such homologous repetitions in organisms in different animals. Thus even in ourselves, when we compare our leg and foot with our arm and hand, we find that they have homologous features which cannot be accounted for as being inheritances from supposed ancestral animals. Our extremities resemble each other in the texture of the skin, the shape of the nails, and other points, and these resemblances are not due to external conditions, but exist in spite of them; and comparative anatomy reveals to us countless similar examples in the animal kingdom. Limbs can hardly be more unlike in form and position than are the arms and legs of birds, and yet we meet with breeds of fowls and pigeons the feet of which are furnished with what are called “boots,” that is, with long feathers which grow on the side of the foot, serially corresponding with that of the hand, which grow the feathers of the wing. Again, in disease, and in cases of monstrosity or congenital malformation, nothing is more common than to find precisely similarly diseased conditions, or similar abnormalities of structure, affecting serially or laterally homologous parts, such as corresponding parts of the two arms or two legs, or of the right (or left) arm and hand and leg and foot respectively. Altogether it seems then to be undeniable that the characters and the variations of species[33] are due to the combined action of internal and external agencies acting in a direct, positive, and constructive manner. [33] The existence of internal force must be allowed. We cannot conceive of a universe consisting of atoms acted on indeed by external forces, but having no internal power of response to such actions. Even in such conceptions as those of “physiological units” and “gemmules” we have (as the late Mr. G. H. Lewes remarked) given as an explanation that very power the existence of which in larger organisms had itself to be explained. It is obvious, however, that no character very prejudicial to a species could ever be established, owing to the perpetual action of all the destructive forces of nature which destructive forces, considered as one whole, have been personified under the name “natural selection.”