to that of the snail and whelk class, is nevertheless esteemed less highly organized than the latter. The mussel or oyster tribe possess no head; the snails and their allies, as every one knows, not merely exhibit a well-developed head, but have that extremity provided with eyes, tentacles or feelers, and other addenda of the front region of the animal body. Hence it is more than probable that the mussel, headless and inclosed in its shell and possessing relatively little interest in the affairs of the outer world, is an example of a degenerated type of mollusks. The mussels and their relations stand, in fact, at the opposite extreme of development in this respect from those well-known mollusks, the cuttle-fishes. In these creatures, the tendency to head-development—or what Professor Dana calls "cephalization"—reaches its maximum, as any one may readily enough suppose on looking at an octopus or squid, with its great head, its enormous eyes, and its nerves massed together to form a brain inclosed in a kind of skull. Even as compared with the earlier cuttle-fishes—whose shells, under the name of ammonites and the like, we find fossilized in large numbers—the squids and cuttles of to-day present, in the extreme development of head, a noteworthy advance. Thus, while the one molluscan tribe of mussels and their neighbors has degenerated and gone to its own lowly place in the series, other groups, starting on an equal footing, have advanced, and, through progressive evolution, have produced those higher manifestations of molluscan life that teem in the seas of to-day. Even among the vertebrate animals we meet with examples of degenerative tendency which are not so easily explicable as the foregoing illustrations. In most snakes only one lung is fully developed, as a rule, the companion organ being rudimentary and degenerate. In birds, the egg-producing organs are similarly developed on one side only. How degeneration should be thus partial and affect one half of an animal's frame, so to speak, is very hard to discover. External conditions of life and the influences of surroundings could apparently possess little effect in inducing such an unsymmetrical retrogression of parts. Most probably we shall find the solution of such conditions to exist within the operation of some deep-seated law of the living constitution, and in the effects of that law in molding, or even contorting, the animal frame.
It constitutes one of the chief glories of biological science, as pursued among us to-day, that its studies are of far-reaching order, and lead, as the results of their natural extension, to the consideration of fields of thought often widely removed from the original topic which interests the reader. The present subject of degenerative changes, regarded as part and parcel of the living constitution, can readily be shown to possess applications far removed from zoology and botany, and extending into the most intimate spheres and phases of human history itself. Degenerative change in human tissues is medically symptomatic of very many of the ills to which flesh is heir.
