56 GENETIC VARIATIONS 



tions, or orthogenetic series reported by JoUos. As matters 

 stand, the more radical conclusions of Jollos cannot be con- 

 sidered established. 



The discovery of agents producing mutation brings us 

 squarely to the question in which these lectures are primarily 

 interested. Both the known agents are powerfully injurious. 

 Both kill a large proportion of the individuals exposed to 

 them ; both leave many of the others abnormal or imperfect. 

 Both damage the genetic system, producing changes that 

 make it impossible for the individuals to develop. And both 

 produce inherited changes that are manifested mainly as 

 abnormalities, weaknesses and imperfections. 



Are these the type of changes on which evolution is based ? 

 Amid the carnage engendered by blighting radiation and 

 unbearable heat, do we find produced the changes that mean 

 progressive evolution? The gene mutations produced by 

 radiation and by heat are of the same types as occur sponta- 

 neously. It is commonly assumed and urged that the muta- 

 tions observed in experimental genetics are the raw material 

 of evolutionary change; are visible steps in evolution. Is this 

 to be considered established ? This question we take up in our 

 next chapter. 



