226 



THE POPULAR SCIENCE MONTHLY. 



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 pos- 

 sibilities 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 elabo- 

 rate 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, per- 

 chance, 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 an 



animal or plant attaches itself partly or 

 wholly to another living being, and be- 

 comes more or less dependent upon the 

 latter for support and nourishment, it ex- 

 hibits, as a rule, retrogression and degen- 

 eration. 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 

 S lower life as analogous instances of men- 

 Fio. lo.-CoMMox Tapewohm ( 7Vemz tal dependence on others act in our- 



*sohi/m}. 1. TliohoafI extremity, mas:- . ^ . . 



nifled, pho\vin<; hooks (a), and suck- selves. The destruction of characteristic 



erp (6, c); d, neck, with immature ,..-,,. , , . . j, 



joints. 2. A jointjartrelymairnifled, individuality and the extmctlOll 01 pcr- 

 showinK the branchint,' ' ovary," in ,. , i- j? .i - j* 



. which the numerous eggs of each sonality are natural results 01 tnat lorm 



joint are matured. <. . , . , . /. -, 



01 association wherein one lorm becomes 

 absolutely dependent on another for all the 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 



