2), called "swimmerets," beneath the so-called tail or abdomen.
[Illustration: PART OF THE SKELETON OF THE LOBSTER.] Now, these various appendages are distinct and different enough as we {162} 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 developmentally 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 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.
[Illustration: SPINE OF GALAGO ALLENII.] 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 (vertebrae), which are admitted by all to possess a certain community of structure.{163} It is in their limbs, however, that the Vertebrata present the most obvious and striking 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 has only recently received a more or less satisfactory solution through the successive concordant efforts of Professor Humphry,[163] Professor Huxley,[164] the Author of this work,[165] and Professor Flower.[166] Very few writers, however, 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 careful perusal and consideration, and some of which apply also to the other kinds of homology mentioned above.