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 conditions.

Such parts, whether more or less modified, unless their common origin became wholly obscured, would be serially 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 homologous 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 indefinite 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 proposes to call the structures which resemble each other in distinct animals, owing to their descent from a common progenitor with subsequent modification, "homogenous"; and the resemblances which cannot thus be accounted for, he proposes to call "homoplastic".

For instance, he believes that the hearts of birds and mammals are as a whole homogenous--that is, have been derived from a common progenitor; but that the four cavities of the heart in the two classes are homoplastic--that is, have been independently developed.