298 THE THEORY OF THE GENE 



A later experiment, devised by Wright and carried out 

 by Castle, showed, in fact, that the results had been due 

 to the isolation of modifying genes for width of stripe. 

 The test was as follows : Each of the highly selected races 

 was back-crossed to wild rats, that is, rats with uniform 

 coat color, and a second (extracted) generation of 

 spotted rats obtained. The process was repeated uith 

 the (Fo) spotted rats obtained from the first back-cross. 

 It was found after back-crossing for two or three genera- 

 tions that the selected stock began to change back, so to 

 speak, to its original state. The selected race with a nar- 

 rower stripe changed toward a broader stripe and the 

 selected race with a broader stripe changed toward a 

 narrower stripe. In other w^ords, the two selected races 

 became more and more like each other, and more like the 

 original race from which they had started. 



This result is completely in accord with the view that 

 modifying factors are present in the wild rats that affect 

 the width of the stripe in animals that are already 

 striped. In other words, the original selection had 

 changed the character of the stripe by sorting out those 

 genes that made it broader or narrower. 



At one time Castle went so far as to claim that the 

 results of the experiments with hooded rats reestablished 

 a view that he ascribed to Darwin, namely, that selection 

 itself brings about a change in the hereditary materials 

 in the direction in which the selection takes place. If this 

 were really Darwin's meaning, such an interpretation of 

 variability might seem greatly to strengthen the theory 

 of natural selection as the method by which evolution has 

 taken place. Castle said in 1915: ''All the evidence we 

 have thus far obtained indicates that outside modifiers 

 will not account for the changes observed in the hooded 

 pattern, itself a clear Mendelian unit. We are forced to 

 conclude that this unit itself changes under repeated 



