CHAPTER n 



EVOLUTION 



When Charles Darwin published his epoch-making 

 work, On the Origin of Species by Means of Natural Se- 

 lection, one hundred years ago, he had grasped the two 

 essential processes underlying organic evolution: biolog- 

 ical variation and natural selection. Our knowledge of 

 both processes, however, has been greatly enhanced 

 since Darwin's time. It was not until the early decades 

 of the present century that biologists clearly distin- 

 guished between acquired characters, or those that are 

 acquired during the life of the individual, and genetic 

 characters, i.e., those that are inheritable generation after 

 g^ieration. There is no evidence that acquired characters 

 are ever inherited, and there is much positive evidence 

 to refute the view. Consequently the raw materials of 

 evolution must be sought in changes in the genetic 

 characters. 



Early in this century it was shovm that genetic char- 

 acters are determined by the chromosomes of the ceU, 

 and it has since been established that each chromosome 

 consists of many units called genes (from gignesthai = to 

 be bom) arranged in a definite sequence along the 

 chromosome. Each gene has a very profoimd influence 

 on one or more body characters, and a *genic map' of 

 the chromosomes can be made by observing the behavior 

 of these characters in heredity. In g^ieral, genes are 

 quite stable and are passed on from one generation to 



