ration: 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 centiped