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of one species with those of other and 1860 1861 1866
of one species with those of another and 1859
in 1869 1872

we can indicate but 1859 1860 1861 1866
are homologous, but 1869
can be shown to be homologous, only a 1872

that 1859 1860 1861 1866
can be indicated; that 1869
such as the valves of Chitons, can be indicated; that 1872

6 blocks not present in 1859 1860 1861 1866 1869; present in 1872
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 to distinct parts of the same organism, having varied in an analogous manner; and in part to similar modifications, having been preserved for the same general purpose or function,— of which many instances have been given.

but 1859 1860 1861
in their present state, but 1866
as they now exist, but 1869 1872

element. 1859 1860 1861
and simpler element. 1866 1869 1872

a modification of this nature having 1859 1860 1861
such modifications having actually 1866
this having 1869 1872

On my view these terms 1859 1860 1861 1866
According to the views here maintained, such language 1869 1872

during a long course of descent 1859 1860 1861 1866
OMIT 1869 1872

legs, or from some simple appendage, is 1859 1860 1861
though simple legs, is 1866
though extremely simple legs, is 1869
though extremely simple legs, is in part 1872

←Subtitle not present 1859 1860 1861 Embryology and Development . 1866 1869 1872
1 blocks not present in 1859 1860 1861; present in 1866 1869 1872
This is one of the most important departments of natural history.

casually remarked that certain organs 1859 1860 1861
remarked that various parts and organs of the same individual animal are during an early embryonic period exactly like each other, but become 1866
stated that various parts and organs 1869
stated that various parts 1872

individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. 1859 1860 1861
adult state widely different and serve for widely different purposes. 1866
same individual are exactly like each other during an early embryonic period, but in the adult state become widely different and serve for widely different purposes. 1869
same individual which are exactly alike during an early embryonic period, become widely different and serve for widely different purposes in the adult state. 1872

the parts of one species with those of other and distinct
species
species,
we can indicate but few serial
homologies
homologies,
homologies;
that is, we are seldom enabled to say that one part
or organ
or organ
is homologous with another
part in
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
part,
as we find in the other great classes of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ;
vertebræ:
the jaws of crabs as metamorphosed legs; the stamens and pistils
in
of
flowers as metamorphosed leaves; but it would in
most
these
cases
probably
probably
be more correct, as Professor Huxley has remarked, to speak of both skull and vertebræ,
both
both
jaws and legs,
&c.,
&c.,—
as having been metamorphosed, not one from the
other
other,
but from some common element.
Most naturalists,
Naturalists,
however, use such language only in a metaphorical
sense;
sense:
they are far from meaning that during a long course of descent, primordial organs of any kind— vertebræ in the one case and legs in the other— have actually been
converted
modified
into skulls or jaws. Yet so strong is the appearance of a modification of this nature having occurred, that naturalists can hardly avoid employing language having this plain signification. On my view these terms may be used literally; and the wonderful fact of the jaws, for instance, of a crab retaining numerous characters, which they
probably would
would probably
have retained through inheritance, if they had really been metamorphosed during a long course of descent from true legs, or from some simple appendage, is explained.
Embryology .—
It has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike.