CHAPTER X\n 

 PARASITISM AND DEGENERATION 



Les causes dc IV'volutioii re<!;rossiv" j^'uvcnt so niniciicr :\ uiio 

 seule, la limitation des moyeiis de suhsistaiico, dc Im, la lutto jtour 

 Texistence entre les or<!;anisnies ou Ics sociotrs. ct entre leurs parties 

 composantes. — Demooh, Massakt, and Vandekvelde. 



A SPECIAL kind of adaptation is that shown by parasitic 

 animals. The relations of parasitic animals to their hosts 

 appear in many familiar exam])les, and the results of this para- 

 sitic life, or at least the conditions that seem always to attend 

 it, namely the degeneration, slight or extreme, of the ])arasites, 

 is also familiar to all observers of animal life. The term para- 

 sitism, as well as the term degeneration, cannot be very 

 rigidly defined. To prey upon the bodies of other animals is 

 the common habit of many creatures. If the animals which 

 live in this way are free, chasing or lying in wait for or snaring 

 their prey, we speak of them in general as predatory animals. 

 But if they attach themselves to the body of their jn-cy or 

 burrow into it, and are carried about by it, live on or in it, 

 then we cjdl them ])arasites. Ai;d the difTerencc in habit 

 between a lion and an intestinal worm is large enough and 

 marked enough to make very clear to us what is meant wIkmi wo 

 speak of one as predatory and the other as a ])arasite. Hut 

 how shall we class the lam})rey, that swims about imtil it finds 

 a fish to which it clings, while sucking away its blood? It 

 lives mostly free, hunting its j^rey, clinging to it for a while, 

 and is carried about by it. Closely related to the lampreys 

 are the hag fishes (}fi/xine) marine eellike fishes that attach 

 themselves by a suckerlike mouth to living fishes and gradually 

 scrape and eat their way into the abdominal c'avity of the liost. 

 These "hags" or "borers" aj)proach more nearly to the cou- 



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