Now, it is here contended that the relationships borne one to another, by various component parts, imply the exist ence 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 backward, by the intercalation of another step between the action of the internal condition or power and its external result. In the mean time, 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 homol

ogy,' but it is only necessary to consider the three kinds above enumerated.

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

DUE

000

WING-BONES OF PTERODACTYL, BAT, AND BIRD.

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

1 For an enumeration of the more obvious homological relationships see Ann. and Mag. of Nat. Hist. for August, 1870, p. 118.

between the wing of the bat and that of the flying-dragon, for the latter is formed of certain ribs, and not of limbbones.

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

SKELETON OF THE FLYING-DRAGON.

(Showing the elongated ribs which support the flitting organ.)

2

genera, cheirogaleus and galago, and which relation has been termed by Mr. Ray Lankester "homogeny; 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 bird, or the similar teeth of the thylacine and

2 See Ann. and Mag. of Nat. Hist., July, 1870.

the dog before spoken of. For this relationship Mr. Ray Lankester has proposed the term "homoplasy."

TARSAL BONES OF DIFFERENT LEMUROIDS.

(Right tarsus of Galago; left tarsus of Cheirogaleus.)

"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

A CENTIPEDE.

are the ribs, or joints of the backbone of a horse, or the limbs of a centipede. The latter animal is a striking ex

ample of serial homology. The body (except at its two ends) consists of a longitudinal series of similar segments Each segment supports a pair of limbs, and the appendages of all the segments (except as before) are completely alike A less complete case of serial homology is presented by Crustacea (animals of the crab class), notably by the squilla and by the common lobster. In the latter animal we have

SQUILLA.

a six-jointed abdomen (the so-called tail), in front of which is a large solid mass (the cephalo-thorax), terminated anteriorly by a jointed process (the rostrum). On the under