SIZE. 55 



case under discussion by an unequal persistence, among the F 2 zygotes, 

 of the increased growth stimulus observed in F x and due evidently to 

 the act of crossing, not to inheritance. (2) Increased variability in F 2 

 would also result if a blending occurs in FI, which is imperfect, so that 

 the gametes formed by the F! individuals are not all the exact mean of 

 the parental gametes, but fluctuate around that mean. 



What may we imagine the germinal basis of a blending character to 

 be? Perhaps some substance or ferment which varies in amount, larger 

 amounts producing larger results. If a 5 per cent solution of cane sugar 

 were poured into the same dish with a 10 per cent solution and then sam- 

 ples were dipped from this before the two solutions had been thoroughly 

 stirred together, it might very well happen that the samples would not be 

 of uniform strength. Any other result would be surprising. A char- 

 acter genuinely blending in heredity might be expected to behave in 

 this same way, the quantitatively different conditions found in parent 

 races not blending perfectly in a single generation of association 

 together in an FI zygote, which therefore would produce gametes less 

 uniform in character than those of the respective inbred parent races. 



The multiple factor interpretation of size inheritance, besides being 

 superfluous, meets with this serious logical difficulty: If we suppose 

 the difference between two races to depend upon a certain number of 

 independent factors whose action is cumulative, then a less difference 

 must be due to fewer factors, and the fewer factors concerned in a cross, 

 the more obvious is the segregation. But we do not find it easy to 

 detect segregation when races are crossed which differ little in size; 

 the general result is the same as when races are crossed which differ 

 widely from each other. It is difficult to detect any evidences of 

 segregation unless the parent races differ widely from each other, under 

 which condition, if multiple factors are involved, complete segregation 

 should occur least often. 



On the whole, the hypothesis of quantitative variations in a blending 

 character presents fewer difficulties as an explanation of size inheritance 

 than the hypothesis of multiple unvarying segregating factors. It is 

 to be preferred on the ground of simplicity alone, but it also accords 

 better with the results obtained in other fields. Jennings now finds, 

 contrary to his earlier observations on paramecium, which Calkins and 

 Gregory were unable to confirm, that size is a character varying even 

 in asexual reproduction, within what would be a "pure line" if the 

 theory of factorial constancy were true. My own observations of 

 rats and other rodents (Castle, 1915) may be cited to show that even 

 single Mendelian unit characters are quantitatively variable. If this 

 is so, the hypothesis of multiple factors as a general explanation of 

 variability is quite unnecessary and so should be discarded. 



