ORGANIC RESPONSE 287 



have accordingly disappeared through " degenera- 

 tion." The fact of degeneration is a familiar accom- 

 paniment of the habit of parasitism, though the 

 method by which it comes about is little understood. 

 As in the malarial parasite, there is an alternation 

 of hosts in the life cycle of the tapeworm. In 

 most cases the completion of this cycle requires 

 that one host shall be eaten by the other. It follows 

 therefore that such parasites are most numerous in 

 carnivorous animals. 



Parasitism in Insects. Another group in which 

 parasitism nourishes is the one that has become 



FIG. 103. Caterpillar of Sphinx moth with cocoons of a parasitic wasp 

 attached. (Sanderson and Jackson.) 



adapted to all possible modes of existence except 

 that under the sea, the insects. A few insects 

 are parasitic on other animals (e.g. the bot-fly), but 

 the majority parasitize other insects. A very large 

 group of the Hymenoptera (ants, bees, and wasps) 

 are parasitic, and many of them specialize on the 



