74 



THE AmERICAN NATURALIST 



[Vol. LI 



new form being not merely fully differentiated at its first 

 appearance, but also fully able to survive. 



Natural selection is, from the point of view of modern 

 genetics, a somatic tlieorj^ It begins and ends in somatic 

 differences. Except under the most unusual and rare of 

 conditions natural selection can not possibly operate 

 directly upon germ-cells. It must, from the very nature 

 of the case, work only indirectly upon the germ through 

 the soma. Now it is a historical fact that just so long as 

 the study of heredity confined itself solely to the somatic 

 results of the process, substantially no advance in knowl- 

 edge was made. Only when it was clearly perceived that 

 heredity is primarily and fundamentally a problem in the 

 physiology of germ cells, and that the soma is, as some 

 one has said, only the mechanism which the fertilized egg 

 uses to produce another fertilized egg like itself, did we 

 begin to make progress. Can we suppose that these con- 

 siderations have no meaning or application in our at- 

 tempts to solve the larger problem of organic evolution? 



If natural selection only could act directly upon germ 

 cells we should have a different story to tell. The writer 

 has lately been experimenting^^ with an agent which acts 

 with extraordinary precision and definiteness in a selec- 

 tive manner upon the germ cells, killing or inactivating 

 the weak, and leaving only the strong and resistant to 

 produce zygotes, and somata. Tliis substance is alcohol. 

 In the case at least of tlip domestic fowl the evolutionary 

 effects produced l\v tliis siil)stnnce are remarkable in their 

 magnitude and dofiriiteiK'ss. It is not possible here to go 

 into details, but hrloflv it iiiav said that the first general 

 result of tlie conlliiiuMl ndnmrKtratimi of ethyl or methyl 



