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.
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