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.
[Illustration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.] The term "homologous" may be applied to parts in two individual animals of different kinds, or to different parts of the same individual.

Thus "the right and left hands," or "joints of the backbone," or "the teeth of the two jaws," are homologous parts of the same individual.

But the arm of a man, the fore-leg of the horse, the paddle of the whale, and the wing of the bat and the bird are all also homologous parts, yet of another kind, _i.e._ they are the same parts existing in animals of different species.
On the other hand, the wing of the humming-bird and the wing of the humming-bird moth are not homologous at all, or in any sense; for the resemblance between them consists solely in the use to which they are put, and is therefore only a relation of _analogy_.

There is no relation of _homology_ between them, because they have no common resemblance as to their relations to surrounding parts, or as to their mode of origin.
Similarly, there is no homology between the wing of the bat and that {158} of the flying-dragon, for the latter is formed of certain ribs, and not of limb bones.
[Illustration: SKELETON OF THE FLYING DRAGON.
(Showing the elongated ribs which support the flitting organ.)] Homology may be further distinguished into (1) a relationship which, on evolutionary principles, would be due to descent from a common ancestor, as the homological relation between the arm-bone of the horse and that of the ox, or between the singular ankle bones of the two lemurine genera, cheirogaleus and galago, and which relation has been termed by Mr.Ray Lankester "homogeny;"[162] and (2) a relationship induced, not derived--such as exists between parts closely similar in relative position, but with no genetic affinity, or only a remote one, as the homological relation between the chambers of the heart of a bat and those of a {159} bird, or the similar teeth of the thylacine and the dog before spoken of.
For this relationship Mr.Bay Lankester has proposed the term "homoplasy." [Illustration: TARSAL BONES OF DIFFERENT LEMUROIDS.
(Right tarsus of Galago; left tarsus of Cheirogaleus.)] [Illustration: A CENTIPEDE.] "Serial homology" is a relation of resemblance existing between two or more parts placed in series one behind the other in the same individual.
Examples of such homologues are the ribs, or joints of the backbone of{160} a horse, or the limbs of a centipede.