Inheritance of Qiianiilatlve Characters 91 



slight variations he regarded as mere lluctuations. I-'urthermore, 

 he found that the character of his pure Hne behaved in crossing as 

 a simple unit character and that no complex factors were involved. 

 With this evidence he should not have been able to effect any 

 permanent changes by selection, but this is exactly what he did. 

 Selecting in opposite directions, he developed two new strains, 

 the boundaries of the new strains being distinct from one another 

 and distinct from the boundaries of the original strain, that is, 

 the non-selected type that he started with. 



Castle's next step was significant. He crossed each of his 

 new strains with the same wild race, the result being that each of 

 his new strains behaved as a simple and distinct recessive unit. 

 The high pigmentation strain ''came out of the cross" with the 

 characteristic high pigmentation; the low pigmentation strain 

 came out with the characteristic low pigmentation. 



The conclusion from this series of experiments may be given 

 in Castle's words, as follows: ''The conclusion seems to me 

 unavoidable that in this case selection has modified steadily and 

 permanently a character unmistakably behaving as a simple Men- 

 delian unit." The importance of this conclusion is evident. 

 Mendelism had been based upon the conception that unit char- 

 acters could not be modified. Mendelians of the "mutationist" 

 school had granted only two possible methods for the origin of 

 new races: (i) by recombinations of existing characters through 

 hybridizing; (2) by the sudden and complete dropping out of an 

 existing unit or the equally sudden addition of a new unit, both 

 of which possibilities might arise from mutation. No "mutation- 

 ist" would grant, however, the possibility of modifying an existing 

 unit character, the thing which Castle claimed to have done, 

 basing his claim upon well-controlled experimental breeding. If 

 Castle's contention were true, it would result in the fundamental 

 modification of Mendel's law. The whole mechanism would have 

 to be modified to take into account new fields of variation that had 

 not been thought to exist. 



The statements of the "mutationists" in reference lo these 

 experiments should be considered. They attempted to explain 

 Castle's results through the cumulative factor mechanism. 

 The claim was made that Castle had started with a character 



