Page:Popular Science Monthly Volume 19.djvu/238

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incentive is there for the progress and evolution of the races which dwell in their midst.

This somewhat lengthy introduction to the subject of degeneration and its results is in its way necessary for the full appreciation of the fashion in which degeneration relates itself to the other conditions of life. From the preceding reflections it becomes clear that three possibilities of life await each living being. Either it remains primitive and unchanged, or it progresses toward a higher type, or, last of all, it backslides and retrogresses. As the first condition, that of stability, is, as already noted, perfectly consistent with the doctrine of descent, so are the two latter conditions part and parcel of that theory. The stable state forces the animal to remain as it now is, or as it has been in all times past; the progressive tendency will make it a more elaborate animal: and the progress of degeneration will, on the other hand, tend to simplify its structure. It requires no thought to perceive that progress is a great fact of nature. The development of every animal and plant shows the possibilities of nature in this direction. But the bearings of degeneration and physiological backsliding are not, perchance, so clearly seen; hence, to this latter aspect of biology we may now specially direct our attention.

That certain animals degenerate or retrogress in their development before our eyes to-day, is a statement susceptible of ready and familiar illustration. No better illustrations of this statement can be found than those derived from the domain of parasitic existence. When anPSM V19 D238 Common tapeworm.jpgFig. 10.-Common Tapeworm (T├Žna `solium). 1. The head extremity, magnified showing hooks (a) and suckers (b, c); d, neck, with immature joints. 2. A joint largely magnified showing branching "ovary" in which the numerous eggs of each joint are matured. animal or plant attaches itself partly or wholly to another living being, and becomes more or less dependent upon the latter for support and nourishment, it exhibits, as a rule, retrogression and degeneration. The parasitic "guest" dependent on its "host" for lodging alone, or it may be for both board and lodging, is in a fair way to become degraded in structure, and, as a rule, exhibits degradation of a marked kind, where the association has persisted sufficiently long. Parasitism and servile dependence act very much in structural lower life as analogous instances of mental dependence on others act in ourselves. The destruction of characteristic individuality and the extinction of personality are the natural results of that form of association wherein one form becomes absolutely dependent on another for all conditions of life. A life of attachment exhibits similar results, and organs of movement disappear by the law of disuse. A digestive system is a superfluity