152 EVOLUTION AND GENETICS 



at present we cannot give a satisfactory explanation 

 of this relation if we assume that evolution has come 

 about by the same kind of processes that we observe 

 today when new mutants arise. There are, however, 

 certain considerations that put the situation in a 

 somewhat different light. 



In the first place there is no such sharp contrast 

 as imjDlied in the statement just made between domi- 

 nant and recessive genes. Many genes classified as 

 recessive produce some effect in hybrid combination 

 on the character most affected. 



In the second place if recessive mutant genes may 

 sometimes revert to the original type (for which 

 there is some evidence at present but not enough 

 perhaps to be entirely convincing) it follows that 

 there may be no essential difference between the 

 kinds of genes in question. 



In the third place it is possible that some or even 

 many of the commonly observed mutant genes rep- 

 resent degradation products of the old genes (that 

 is, simjiler chemical bodies) that are more frequently 

 produced than more com^ilex bodies. Even if this is 

 true it does not follow that more complex genes may 

 not also arise by mutation and some of these might 

 be dominants to the old gene. At present, however, 

 this is purely speculative. 



In the fourth place it is known that new domi- 

 nant genes do arise. There need be no necessary re- 

 lation between the dominance of a gene and an 



