110 EVOLUTION AND GENETICS 



that it might soon appear in all as the result of what- 

 ever change induced its appearance in so many. 



Now this dilemma is to some extent at least met 

 by the modern theory of the stability of the gene. A 

 gene is not lost by residing in the same cell with a 

 gene of another kind. It may, if mere chance favors 

 its perj^etuation, spread, and once inoculated in the 

 race it may produce in time enough individuals to 

 start a new tyj^e. Our modern knowledge of the be- 

 havior of the gene meets to some extent the difficulty 

 raised by Fleming Jenkin and reestablishes the 

 strength of Darwin's theory, but, on the other hand, 

 it should be clearlv understood that the chance of a 

 recessive gene becoming widely disseminated is ex- 

 tremely small even though the character it represents 

 may be a beneficial one. 



The stability of the gene also enables us to under- 

 stand how a gene, if it is recessive, representing a 

 character that is even injurious to the race, may be- 

 come spread, localty at least, in a group without 

 detriment to the individuals carrviuia: one oene. This 

 explains the frequent occurrence, in certain re- 

 stricted populations, of the appearance at times of 

 certain abnormal types, as seen, for instance, in 

 night blindness, and "bleeding" in man. 



There is another result, clearly established by the 

 genetic work on Drosophila, that is favorable to the 

 final establishment of a new type or character if it 

 is beneficial. JMost, perhaps all, of the mutations ap- 



