472 MISSES K. FOOT AND B, C. STEOBELL : 



our results on Mendelism, or on the many ingenious hypotheses designed to- 

 adapt Mendel's law fo some later experimental results. As, however, we are 

 in entire sympathy with Castle's interpretation o£ size-characters, and believe 

 his thorough knowledge of the subject entitles him to speak^with authority, 

 we would quote some of his recent conclusions that appear to us as sustained 

 by the facts of our experimental work on both the intromittent organ and 

 the genital spot of Eiiscliistns. 



Castle says : — " It is evident that size is not a simple unit character, 

 for there is no dominance and no evidence of segregation other than the 

 increased variability of the second hybrid generation 



" Dominance is clearly absent and the only fact suggesting segregation 

 is the increased variability of the second as compared with the first hybrid 

 generation. This fact, however, maybe accounted for on other grounds than 

 the existence of multiple units of varying power. 



" If size-differences are due to quantitative variations in special materials 

 within the cell, it is not necessary to suppose that these materials are localized 

 in chunks of uniform and unvarying size, or that they occur in any particular 

 number of chunks, yet the genotype hypothesis involves one or both of these 

 assumptions. Both are unnecessary." (Castle^ '12 «.) 



" The results of all observers, as regards the inheritance of ordinary 

 differences in size, are closely in accord. When two races differing in size 

 are crossed the immediate offspring are intermediate in size. The next 

 generation of offspring is likewise intermediate, but more variable as a rule^ 

 and it has been found possible in some cases to select from them forms as 

 extreme in size as the original parents. To interpret such cases as Mendelian, 

 requires the assumption that no single unit or factor is concerned in the size- 

 difference, but many wholly independent units. For a single Mendelizing 

 unit would produce a wholly different result. But suppose we allow the 

 assumption that many independent Mendelizing units or factors are concerned 

 in the inheritance of size. The pure line hypothesis is not benefited by this 

 assumption unless we suppose further that these hypothetical factors do not 

 vary. But this is an assumption wholly without warrant." (Castle, '14 /^) 



" The increased variabilit// of the Fg generation is the only evidence of 

 Mendelism in size crosses." .... 



" On any hypothesis size-differences must depend on many mutually 

 independent factors or causes ... It would be rash to assume that all the 

 factors concerned are Mendelizing factors, in the total absence of the two 

 usual accompaniments and criteria of Mendelism, dominance and segregation 

 in recognisable Mendelian ratios." (Page 2.) 



He says the facts observed for body-size in rabbits and other quantitati-v e 

 characters in animals and plants are, Fi intermediate and Fg also intermediate,, 

 but more variable than Fi, and he adds, '^ If we call this Mendelism, we shall 

 need to explain that it is not the Mendelism of Mendel himself, but original. 



