202 MULTIPLE FACTORS 



direction. In other words it was intended to discover 

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

 be established as a result of selection, so that the 

 original bounds of variability 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- 

 miliar 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 know T n 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 established 

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

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

 lish 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 



