DEGENERATION. 391 



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 inter- 

 est 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 look- 

 ing 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 num- 

 bers the squids and cuttles of to-day present, in the extreme devel- 

 opment 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 foot- 

 ing, 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 devel- 

 oped, as a rule, the companion organ being rudimentary and degener- 

 ate. 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 j^ossess little effect in inducing such an unsymmetrical re- 

 trogression 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 considera- 

 tion 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. 



