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 line 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 Jo 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, but can only result in isolating certain sets 
of genes in a single pure line. 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.” 
