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been termed by Mr. Ray Lankester "homogeny; (2) a relationship induced, not derived—such as exists between parts closely similar in relative position, but with
PART OF THE SKELETON OF THE LOBSTER.
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. 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 abdovien or tail. In the cephalo-thorax of the lobster, however, this is disguised. It is therefore very interesting
to find that in the other crustacean before mentioned, the Squilla, the segmentation of the body is more completely preserved, and even the first three segments, which go to compose the head, remain permanently distinct.
Such an obvious and unmistakeable serial repetition of parts does not obtain in the highest, or backboned animals, the Vertebrata. Thus in man, and other mammals, nothing of the kind is externally visible, and we have to penetrate to his skeleton to find such a series of homologous parts.
There, indeed, we discover a number of pairs of bones, each pair so obviously resembling the others that they all receive a common name—the ribs. There also (i.e. in the skeleton) we find a still more remarkable series of similar parts, the joints of the spine or backbone (vertebræ), which are admitted by all to possess a certain community of structure.
It is in their limbs, however, that the Vertebrata present their most conspicuous example of serial homology—almost the only serial homology noticeable externally.
The facts of serial homology seem hardly to have excited the amount of interest they certainly merit.
Very many writers, indeed, have occupied themselves with investigations and speculations as to what portions of the leg and foot answer to what parts of the arm and hand, a question which
SPINE OF GALAGO
has only recently received a more or less satisfactory solution through the successive concordant efforts of Professor Humphry, Professor Huxley, the author of this work, and Professor Flower. Very few writers, họwever, have devoted much time or thought to the question of serial homology in general. Mr. Herbert Spencer, indeed, in his very interesting “First Principles of Biology,” has given forth ideas on this subject, which are well worthy of careful perusal and consideration, and some of which apply also to the other kinds of homology mentioned above. He would explain the serial homologies 'of such creatures as the lobster and centipede thus : Animals of a very low grade propagate themselves by spontaneous fission. If certain creatures found benefit from this process of division remaining incomplete, they would (on the theory of “Natural Selection”) transmit their selected tendency to such incomplete division to their posterity. In this way, it is conceivable that animals might arise in the form of long chains of similar segments, each of which chains would consist of a number of imperfectly separated individuals, and be equivalent to a series of separate individuals belonging to kinds in which the fission was complete. In other words, Mr. Spencer would explain it as the coalescence of organisms of a lower degree of aggregation in one longitudinal series, through survival of the fittest aggregations. This may be so. It is certainly an ingenious speculation, but facts have not yet been brought forward which demonstrate it; otherwise, this kind of serial homology might be termed “homogenetic."
1 Treatise on the Human Skeleton, 1858.