CHAPTER XVII 

 PARASITISM AND DEGENERATION 



Les causes de revolution regressive peuvent se ramener a une 

 seule, la limitation des moyens de subsistence, de la, la lutte pour 

 1'existence entre les organismes ou les societes. et entre leurs parties 

 composantes. DEMOOR, MASSART, and VANDERVELDE. 



A SPECIAL kind of adaptation is that shown by parasitic 

 animals. The relations of parasitic animals to their hosts 

 appear in many familiar examples, 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 parasites, 

 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 prey or 

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

 then we call them parasites. And the difference in habit 

 between a lion and an intestinal worm is large enough and 

 marked enough to make verv clear to us what is meant w r hen we 



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speak of one as predatory and the other as a parasite. But 

 how shall we class the lamprey, that swims about until it finds 

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

 lives mostly free, hunting its prey, clinging to it for a while, 

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

 are the hag fishes CMyxine) marine eellike fishes that attach 

 themselves by a suckerlike mouth to living fishes and gradually 

 scrape and eat their way into the abdominal cavity of the host. 

 These " hags " or " borers ' ;l approach more nearly to the con- 



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