Tissues and organs degenerate in individual animals, as animal frames retrogress in their entirety. Cells retrograde and fibers degenerate in our bodies, just as the sea-squirt's frame exhibits, as a whole a universal, physiological backsliding. Nor may many of our diseases alone be esteemed mere examples of degeneration affecting our tissues. The termination and decline of life itself and the age that really "melts in unperceived decay" are, in reality, examples of natural degeneration also. The decline of existence is largely a retrogression of structure. There can be no such thing as a really "green old age," any more than we can speak of "the sere and yellow" of the autumnal leaf as imitating the verdant nature of the spring blossom. Nay, stranger still is it to discern that the full flush of life's vigor is accompanied by degenerative changes as typical as those which mark life's decline. For every tissue wastes as it works; and cells degenerate, die, and are cast off from every surface and tissue of our frames as the natural result of living and being. "Generally speaking," says a writer, in discussing the degeneration of human tissues, "those parts which live most slowly are those of which the duration is the greatest, and in which there is consequently the least frequent change. Of the exuviation of epidermic structures en masse—a process altogether comparable to the fall of the leaf—we have striking examples in the entire desquamation of serpents, the molting of the plumage in birds, and the shedding of the hair in mammalia; and, in the shedding of the antlers of the stag, we have an example of the exuviation of a highly organized and vascular part, which periodically dies, and which, being external, is cast off entire. 'What means all this,' says Sir James Paget, 'but that these organs have their severally appointed tissues, degenerate, die, are cast away, and in due time are replaced by others, which in their turn are to be developed to perfection, to live their life in the mature state, and to be cast off?'" And, again, the same high authority remarks that "it is, further, probable that no part of the body is exempt from the second source of impairment; that, namely, which consists in the natural death or deterioration of the parts (independent of the death and decay of the whole body) after a certain period of their life. It may be proved, partly by demonstration and partly by analogy, that each integral or elemental part of the body is formed for a certain natural period of existence in the ordinary conditions of active life, at the end of which period, if not previously destroyed by outward force or exercise, it degenerates and is absorbed, or dies and is cast out; needing, in either case, to be replaced for the maintenance of health." To these weighty words we may lastly add the opinion of Dr. Carpenter, who remarks that, "when the adult type has once been completely attained, every subsequent change is one rather of degeneration than of development, of retrogression rather than of advance."
Degeneration is thus an invariable concomitant of life. So far