he materials for adaptation to the most different purposes; 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 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. Mr. Lankester
also adduces the close resemblance of the parts on the right and left
sides of the body, and in the successive segments of the same individual
animal; and here we have parts commonly called homologous which bear no
relation to the descent of distinct species from a common progenitor.
Homoplastic structures are the same with those which I have classed,
though in a very imperfect manner, as analogous modifications or
resemblances. Their formation may be attributed in part to distinct
organisms, or t