likewise homologous parts, yet of another kind; that is they are the same parts existing in animals of different species.

In contrast to this, 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 of the flying-dragon, for the latter is formed of certain ribs, and not of limb bones.

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

TARSAL BONES OF DIFFERENT LEMUROIDS.

ig ht-hand figure, Tarsus of Galago; left-hand figure, Tarsus of Cheirogaleus.)

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;"1 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

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

and the dog before spoken of.1 For this relationship Mr. Ray Lankester has proposed the term "homoplasy."

"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 a horse, or the limbs of a centipede. The latter animal is a striking example 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 a six-jointed abdomen (the so-called tail), in front of which is a large solid mass (the cephalo-thorax), terminated anteriorly by a median process (the rostrum). On the under-surface of the body we find a quantity of moveable appendages. Such are, eg., feelers (Fig. 9), jaws (Figs. 6, 7, and 8), foot-jaws (Fig. 5), claws and legs (Figs. 3 and 4), beneath the cephalo-thorax; and flat processes (Fig. 2), called "swimmerets," beneath the so-called tail or abdomen.

Now, these various appendages are distinct and different enough as we see them in the adult, but they all appear in the embryo as buds of similar form and size, and the thoracic limbs at first consist each of two members, as the swimmerets always do.

This shows what great differences may exist in size, in form, and in function, between parts which are develop

1 See antè, p. 76.

mentally the same, for all these appendages are modifications of one common kind of structure, which becomes differently modified in different situations; in other words, they are serial homologues.

The segments of the body, as they follow one behind the other, are also serially alike, as is plainly seen in the abdomen or tail. In the cephalo-thorax of the lobster, however, this is disguised. It is therefore very interesting