322 



EVOLUTION AND GENETICS 



at a rate up to 150 times the normal. Among the mutations 

 produced from treated flies there were many lethal characters 

 and recessives, but such familiar things as multiple allelomorphism 

 were also produced. 



The experiments are of great interest in evolution because they 

 show definitely that genes may respond directly to conditions of 



r\ 



o 



\J \J 



Gametes 



r-\ 



(O 



KJ 



Fig. 186. ^Diagram to show the inheritance of a lethal factor in DrosopMla. 

 1 indicates the lethal factor and L its normal allelomoi-ph. The male at the 

 right in the Fi, generation fails to develop so that the ratio of sexes in such 

 a generation is two females to one male. 



the external environment. The appearance of reverse mutations, 

 moreover, establishes the reversibility of slight evolutionary 

 changes. 



Genes in the Development of the Individual. Beyond these 

 facts of inheritance, showing close association of characters in 

 the individual and genes in its chromosomes, we may well inquire 

 how the genes act during ontogeny to condition the differentiation 



