retrogressive while the other sex remains structurally fully developed. Such a case is illustrated by the males of those remarkable organisms, the Rotifera, or "wheel-animalcules" (Fig. 19). These minute creatures, inhabiting our fresh waters, may be desiccated and dried, and revived, on the application of moisture, many times in succession. But in their ordinary existence, and in the details of their structure, the "wheel-animalcules" present details equally interesting with their exhibition of "potential vitality." The female animalcules possess a complete digestive system, a set of water vessels, a nervous ganglion, and other belongings; but their partners are decidedly inferior creatures, since their digestive system becomes totally abortive, while in size the males are likewise far excelled by the lady rotifers. How this degeneration and disappearance of digestive apparatus and the inferiority of size have been produced in the male rotifers may be a matter regarding which difference of opinion will certainly exist in biological minds. The fact that retrogression is here illustrated, however, can not be questioned. It may also be added that, in all probability, the extreme development of the function of perpetuating the species and the extraordinary fertility of production witnessed in these animalcules, may satisfactorily account for the abrogation of digestion in favor of reproduction. Thus, to the other causes of degeneration in animal life and structure, we may append that which takes origin from the extreme or excessive development of one function over another. Physiological development in one direction, overstepping the natural and ordinary limits, runs concurrently with destruction of life's equilibrium, and naturally tends to produce degeneration and simplification of other organs and other duties of life.
How far the theory of degeneration we have thus briefly discussed may be applied in explanation of the peculiarities of animal structure, remains as a task for the future of biology to satisfactorily determine. Possibly the corrections which the future of every hypothesis carries with it may be many and sweeping. The deductions and inferences we extract from a study of degeneration to-day may perchance be falsified by the higher and newer views of the to-morrow of biological science. But enough has been said to show that, even in a cursory review of the doctrine of degeneration and retrogression, many phases of living histories become theoretically plain; and it argues hopefully for the correctness and value of the doctrine before us that it has, so far as it has been logically pursued, fitted compactly and harmoniously enough with ascertained facts and with received views of the origin of animals and plants. That higher forms of life than the sea-squirt and insect race are by no means exempt from the influence of retrogressive change is an observation worth noting at the close of our researches. We know, for instance, of lowly structures in shell-fish life appearing in the midst of highly organized frames. A mussel, a cockle, or an oyster, whose early development runs in parallel lines
