General Survey 



almost a century the answers have been sought, through 

 speculation and reasoning, through observation and experi- 

 ment. After much conflict of opinion and many changes 



Figure 5. Diagram of the descent of generations in Protozoa or 

 other organisms reproducing from a single parent; as compared with 

 that in organisms reproducing from two parents. The lines represent 

 the lives of individuals or germ cells, beginning at the left and passing 

 to the right. A, uniparental reproduction by fission. The line of 

 ancestry traced back from any individual at the right is always single; 

 and there is no corpse found at any point, the present body transform- 

 ing directly into the bodies of the next generation. B, biparental repro- 

 duction by union of germ cells, as in the higher organisms. The tri- 

 angular structures are the bodies, the lines the germ cells. The line of 

 ancestry from any individual traced back from the right forks at each 

 generation, becoming in a few generations multiplex. The bodies of 

 any generation are not continuous with the bodies of the previous gen- 

 eration the latter dying, while the new bodies are produced by germ 

 cells from two diverse lines. 



