Page - 476 - in The Origin of Species

476 ORIGIN OF SPECIES developed from such cells. It must suffice for our purpose to bear in mind that an indefinite repetition of the same part or organ is the common characteristic, as Owen has re- marked, of all low or little specialised forms; therefore the unknown progenitor of the Vertebrata probably possessed many vertebrae ; the unknown progenitor of the Articulata, many segments; and the unknown progenitor of flowering plants, many leaves arranged in one or more spires. We have also formerly seen that parts many times repeated are eminently liable to vary, not only in number, but in form. Consequently such parts, being already present in consider- able numbers, and being highly variable, would naturally afford the materials for adaptation to the most different pur- poses ; yet they would generally retain, through the force of inheritance, plain traces of their original or fundamental resemblance. They would retain this resemblance all the more, as the variations, which afforded the basis for their subsequent modification through natural selection, would tend from the first to be similar; the parts being at an early stage of growth alike, and being subjected to nearly the same con- ditions. Such parts, whether more or less modified, unless their common origin became wholly obscure, would be se- rially homologous. In the great class of molluscs, though the parts in distinct species can be shown to be homologous, only a few serial homologies, such as the valves of Chitons, can be indicated; that is, we are seldom enabled to say that one part is homol- ogous with another part in the same individual. And we can understand this fact for in molluscs, even in the lowest members of the class, we do not find nearly so much indefi- nite repetition of any one part as we find in the other great classes of the animal and vegetable kingdoms. But morphology is a much more complex subject than it at first appears, as has lately been well shown in a remarkable paper by Mr. E. Ray Lankester, who has drawn an important distinction between certain classes of cases which have all been equally ranked by naturalists as homologous. He pro- poses to call the structures which resemble each other in distinct animals, owing to their descent from a common pro- genitor with subsequent modification, homogenous; and the