LETHAL FACTORS IN DROSOPHILA 



545 



Since no lethal has been found in the hundreds of pairs of 

 fresh wild stock examined and since lethals have occurred in 

 many of the inbred stocks, it may appear that inbreeding (with 

 its constraints of confinement and homogeneous feeding) tends 

 to cause the mutation to appear or else the removal of the flies 

 from the competition that takes place under wild conditions 

 makes possible the preservation and continuance of any lethal 

 factors that may appear. 



A discussion of these alternatives would involve a fuller knowl- 

 edge than we have at present of the causes of mutations and the 



