INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 125 



pends on the host for the performance of these functions. The 

 explanation of this degeneration of useless or unused organs 

 is not quite certain. It is known that disuse causes structures 

 to deteriorate in the life of the individual, and some naturalists 

 claim that part of this loss is transmitted to the next genera- 

 tion. The claim is denied by many, who are disposed to con- 

 sider that it is merely a case of natural selection working for 

 simplification of organs and the economizing of materials. 

 The reproductive organs on the contrary become much more 

 complicated and the reproductive elements are produced in great 

 abundance. This is an adaptation to the difficulties involved 

 in finding the special host in which development may proceed. 

 This is more striking because many parasites require two dif- 

 ferent hosts in order to complete the life cycle, and great 

 mortality accompanies the passage from one host to another. 

 A good illustration of such parasites is the tape-worm which 

 infests the trout in Yellowstone Lake. The larvae enter the 

 tissues of the trout and by their ravages weaken and kill the 

 host. The dead fish are eaten by pelicans. The worms de- 

 velop to the adult, sexual condition in the digestive canal of 

 this second host and the eggs or young embryos escape into 

 the water with the excreta and from there are taken up by 

 other trout whose destruction is again wrought by the tissue- 

 infesting larvae. This passage from one host to another prob- 

 ably arose and is helped by the carnivorous habit among ani- 

 mals. 



The parasites are almost exclusively invertebrates. The 

 worms and arthropods furnish the most numerous representa- 

 tives. The gregarines, among the Protozoa, are internal para- 

 sites, sometimes being parasitic within the cells. There are only 

 a few parasitic vertebrates, and these are transient. They be- 

 long to the lower fishes (lamprey, Fig. 62). 



Parasitism is a very successful adaptation to a much limited 

 environment in which the organism has bartered its original 

 powers for a life of comparative ease. The only necessity 

 still resting upon it is in the matter of reproduction, and the 



