486 ZOOLOGY 



Because the history is so much shortened, only some of the 

 more profound stages are recapitulated, and the advances of the 

 race in thousands of years are compressed into moments in the 

 individual history. Furthermore, each species has introduced 

 idiosyncrasies that belong in no way to the race. For these 

 reasons the general truth of recapitulation must not be applied 

 too literally or widely. 



489. The Principal Factors Entering into Evolution. As was 



indicated earlier, biologists no longer question the idea of the 

 evolution of the animal kingdom. They are now seeking to 

 find the principal elements that are bringing about this result 

 and how each contributes to the process. We doubtless have not 

 found all the factors. However, those mentioned below cer- 

 tainly enter into the explanation. 



Evolution could not take place without variation, nor with- 

 out some device to preserve (i.e., repeat) and to accumulate these 

 variations from generation to generation after they arise. Fur- 

 thermore, evolution seems to follow rather definite courses. 

 These courses are not haphazard. They have led toward adap- 

 tation of the organisms to their most pressing surroundings. 

 In other words there seems to have been selection and elimination 

 of variations, and thus a guidance of evolution. 



The following outline may aid the student in grasping some 

 of the important elements of the case. 



i . Variation makes evolution possible, but only this'. We have 

 learned (60) that body cells and germ cells lead a parallel 

 life in every organism. They are very different in their proc- 

 esses and their fate. Theoretically variations may arise in 

 either of two ways: 



A. The body of a given organism may change by the direct 

 effect of the environment upon the body or through the use 

 or disuse of its organs. 



B. Changes may occur in germ cells. 



a. Germ cells may be modified by their environment, 

 which is chiefly the body in which they are being housed. 



b. Germ cells may be modified by the mingling of the two 

 different germ cells, ovum and sperm, that united to 



