DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 



133 



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 hfe 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 upon 

 mutations which appear in various ways, all tending toward 

 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 



Fig. 64. 



Fig. 64. Lake Lamprey {Petromyzon marinu^) clinging to Sucker. (From Bull. U. S. Fish Com- 

 mission, b> Surface.) 



Questions on the figure. — Does it seem that this is an instance of parasitism 

 or simple preying? What special organs has the lamprey adapting it to this 

 habit? What references can you find to the breeding habits of the lamprey? 



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 mor- 

 tality 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 larvee enter the 

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

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

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



