n as
real parts, or conditions of parts, of real wholes; they are, therefore,
true relations, such, _e.g._, as those between the right and left hand,
between the hand and the foot, &c.

The component parts of each concrete whole have also a relation of
resemblance to the parts of other concrete wholes, whether of the same{156}
or of different kinds, as the resemblance between the hands of two men, or
that between the hand of a man and the fore-paw of a cat.

Now, it is here contended that the relationships borne one to another by
various component parts, imply the existence of some innate, internal
condition, conveniently spoken of as a power or tendency, which is quite as
mysterious as is any innate condition, power, or tendency, resulting in the
orderly evolution of successive specific manifestations. These
relationships, as also this developmental power, will doubtless, in a
certain sense, be somewhat further explained as science advances. But the
result will be merely a shifting of the inexplicability a point backwards,
by the intercalation of another step between the action of the internal
condition or power and its external result. In the meantime, even if by
"Natural Selection" we could eliminate the puzzles of the "origin of
species," yet other phenomena, not less remarkable (namely, those noticed
in this chapter), would still remain unexplained and as yet inexplicable.
It is not improbable that, could we arrive at the causes conditioning all
the complex inter-relations between the several parts of one animal, we
should at the same time obtain the key to unlock the secrets of specific
origination.

It is desirable, then, to see what facts there are in animal organization
which point to innate conditions (powers and tendencies), as yet
unexplained, and upon which the theory of "Natural Selection" is unable to
throw any explanatory light.

The facts to be considered are the phenomena of "homology," and especially
of serial, bilateral, and vertical homology.

The word "homology" indicates such a relation between two parts that they
may be said in some sense to be "the same," or at least "of similar
nature." This similarity, however, does not relate to the _use_ to which
parts are put, but only to their relative position with regard to other
parts, or to their mode of origin. There are many kinds of {157}
homology,[161] but it is only necessary to consider the three kinds above
enumerated.

[Illust