46 SIZE INHERITANCE IN RABBITS. 



independent units for certain morphological characters as for certain 

 color characters. From structural characters that can be absent and 

 thus make ratios recognizable, to those in which the " recessive" forms 

 can not be distinctly separated, is a short step. The phenomena shown 

 by these two types of characters are strikingly similar. Both show wide 

 variations in F 2 not found in the parental or first hybrid generations. 

 Both types show similar wide variations after crosses between certain 

 strains that appear to be alike. The appearance of Fahncntypen in 

 crosses between Rispentypen is an example of the type which gives 

 definite ratios; crosses between long beets that give very long forms 

 is an example of the type which does not afford ratios. Any theory 

 to account for the wide variations in F 2 of crosses involving characters 

 of this latter type must also account for this similarity with cases where 

 ratios are found and, again, with cases where simple Mendelian ratios 

 and complex ones are found for the same character-differences. 



The conception of many independent units presented by Nilsson-Ehle 

 can be applied to all the facts. It is the logical outcome of the pure- 

 line doctrine of de Vries and Johannsen; in its breadth of application 

 and its comprehensibility this theory, based on the purity of gametes 

 and unit factors, is very attractive. Through its use as a working 

 hypothesis important facts have been discovered and an important 

 system of inheritance has been developed. 



Castle (1912) has shown that this hypothesis is not the only one 

 that can be raised to satisfy the facts here presented. He says: 



"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. Variability would result whether the 

 growth-inducing substances were localized or not, provided only they were 

 not homogeneous in distribution throughout the cell. Crossing would increase 

 variability beyond the first generation of offspring, because it would increase 

 the heterogeneity of the zygote in special substances (though not in its total 

 content of such substances) and this heterogeneity of structure would lead to 

 greater quantitative variation in such materials among the gametes arising 

 from the heterozygote. Thus greater variability would appear in the second 

 hybrid generation. As a matter of fact we know that protoplasm is not 

 homogeneous, and that there are substances widely distributed in the cell, not 

 localized in chromosomes, which may well have an influence on size." 



To this East (1912) replied, making a very clear presentation of the 

 case of multiple factors. He claims qualitative characters can be 

 described in the same way and that one does not gain a system thereby. 



At present it seems that there is recorded more actual work favoring 

 the hypothesis of multiple factors than any alternative hypothesis. 



