INTRODUCTION. XIX 



feed at the expense of a neighbour, either establishing 

 themselves voluntarily in his organs, or quitting him 

 after each meal, like the leech or the flea. 



But when the larva of an ichneumon devours, organ 

 after organ, the caterpillar which serves him as a nurse, 

 and at last eats her entirely, can we call him a parasite? 

 According to Lepelletier de Saint-Fargeau, w^ho has so 

 successfully treated these questions, the parasite is he 

 who lives at the expense of another, eating that which 

 belongs to him, but not devoming his nm'se herself. Nor 

 is the ichneumon a carnivorous animal, for the true 

 beast of prey cares nothing at any period of his exist- 

 ence for the life of his victim. 



True parasites are very commonly found in nature, 

 and we should be wrong were we to consider that they 

 all live a sad and monotonous life. Some among them 

 are so active and vigilant that they sustain themselves 

 dm'ing the greater part of their life, and only seek for 

 assistance at certain determinate periods. They are 

 not, as has been supposed, exceptional and strange 

 beings, without any other organs than those of self- 

 preservation. There is not, as was formerly supposed, 

 a class of parasites, but all the classes of the animal 

 kingdom include some among their inferior ranks. 



We may divide them into different categories. 



In the first of these we will place together all those 

 which are free at the commencement of their life, which 

 swim and take their sport without seeldng assistance 

 from others, until the infirmities of age compel them to 

 retire into a place of refuge. They live at first like true 

 Bohemians, and are certain of getting invalided at last 

 in some well-arranged asylum. Sometimes both the 



