CHAPTEK I. 

 NATUBE AND OBGANIZATION OF PAEASITES. 



THE term " Parasite/' in its widest sense, includes all those creatures 

 which inhabit a living organism, and obtain nourishment from its 

 body. 



This definition includes not only vegetable and animal parasites 

 (phytoparasites and zooparasites), but also parasites on plants and 

 on animals. The larva that inhabits the wood of a tree or the pulp 

 of a fruit is to be regarded as a parasite in no less degree than the 

 thread-worm of the human intestine ; and the beetle that defoliates 

 our forests is quite as much a parasite as the louse upon the feathers 

 of the swallow. Parasitic life, then, as thus understood, is an ex- 

 ceedingly widespread phenomenon. 



So long as the term " parasite " was confined to certain special 

 forms, as was the case formerly, it followed as a necessary con- 

 sequence that parasitism was an isolated phenomenon, and bore no 

 relation to any other mode of existence. Now, however, this view is 

 known to be false, a matter of great importance when we come to 

 study the subject from a historic point of view. It is not merely 

 the intestinal worms and allied forms that are to be included 

 among parasites, but also numerous creatures that sometimes resemble 

 so completely certain free-living animals, except in the nature of their 

 food, that an independent mode of existence has been actually as- 

 cribed to them. Does it correspond with the common view of the 

 peculiar nature of parasitism, that a creature which, according to the 

 definition just given, ought to be regarded as a parasite, should be 

 sharply distinguished from another free-living animal simply because 

 it feeds upon a living branch instead of dead wood, or on green 

 foliage instead of withered leaves ? Do not the value and meaning 

 of these differences appear less than those which obtain between car- 

 nivorous animals on the one hand and herbivorous on the other ? 



The question raised here remains the same, when we limit more 

 narrowly the conception of parasitism, which on practical grounds 

 is advisable for the purpose of this work, and confine it entirely to 

 animals living as parasites upon other animals. 



