HEREDITY IN PURE LINES 379 



says that " the changes in question had not occurred in the gene for the 

 hooded pattern, but in the residual heredity." The situation appears 

 to be this: the hooded pattern is a black marking covering the head 

 and shoulders of the otherwise white body. It varies in the extent to 

 which it covers the body, and, by selecting the plus or minus indi- 

 viduals, a nearly complete black and a nearly complete white race was 

 produced. These were maintained in a pure hne for three generations 

 and then were bred with a pure white strain. After six generations 

 " the whitest individuals extracted from the dark hooded race were no 

 darker than the darkest individuals extracted from the white hooded 

 race. In other words repeated crossing with the non-hooded (wild) 

 race had caused the changes in the hooded character, which had been 



secured by selection, altogether to disappear Accordingly we 



are led to conclude that unit-characters or genes are remarkably con- 

 stant and that when they seem to change as the result of hybridization 

 or of selection unattended by hybridization, the changes are rather in 

 the total complex of factors concerned in heredity than in single genes." 



In a mutating race, however, such as Drosophila, changes in genes 

 surely do occur, as has been proved by the work of Morgan and his 

 collaborators. We do not know what causes changes in genes, but 

 we can demonstrate that they do occur. Selection cannot bring about 

 any change in single genes, ' ^ut can only result in isolating certain sets 

 of genes in a single pure Hne. This once done, selection ceases to be 

 effective in altering the character of the stock under selection. 



"The substance of our present knowledge as to changes in genes," 

 concludes Castle, "may be summed up in the statement that such 

 changes come or go suddenly in their entirety, and cannot, so far as 

 we know, be influenced by selection or any other controllable process. 

 Hence we may call changes in genes mutations." 



