202 MULTIPLE FACTORS 



direction. In other words it was intended to discover 

 whether a new genetic type, with a new mode, could 

 be estabUshed as a result of selection, so that the 

 original bounds of variabihty would be transgressed. 

 Castle has interpreted his results to mean that 

 through selection or after selection, a unit character 

 can be changed. He has used at times a word fa- 

 mihar to readers of Darwin, namely ''potency." The 

 potency of a factor as well as of a character is sup- 

 posed to be a somewhat variable element. 



It is obvious that it would be exceedingly difficult 

 to establish such an interpretation, because in order 

 to prove that selection can alter a factor it would first 

 be necessary to prove that recombinations of multiple 

 factors were not responsible for the variations of the 

 ''unit" character. The results with rats are in har- 

 mony with the theory of multiple factors, and hence 

 in harmony with the whole body of Mendelism. 

 There are no a priori grounds for regarding quantita- 

 tive factors as differing from other Mendelian factors, 

 and many cases are known in which quantitative 

 factors conform in every respect to Mendel's 

 principles. 



In support of the view that the particular character 

 of the hooded rat differs from the wild rat by a single 

 factor Castle has pointed out that this is estabhshed 

 by the Mendehan ratio, 3:1, that obtains when these 

 types are crossed. But the 3 :1 ratio does not estab- 

 hsh this view. The ratio only shows that a recessive 

 factor for hoodedness must be present in order that 

 the rats may be hooded at all. Other factors that 



