Other Explanations 357 



special body parts. Is a large rabbit larger because he is larger 

 throughout his body or because certain parts only of his body 

 are larger? Castle and Gregory, as the result of an embryo- 

 logical study, showed that generally rabbits of a large race grow 

 more rapidly throughout development, are larger at birth, and 

 continue to grow more rapidly and for a longer time after birth. 

 In such rabbits, therefore, greater growth is a general phenome- 

 non throughout the body. On the other hand, there are also 

 special genes which increase the growth of special body parts, 

 such as length of hair, ear, and tail. Summing up, Wright con- 

 cludes that the inheritance of body size is chiefly a general 

 phenomenon throughout the body but that groups of organs 

 (as legs of mammals or legs and wings of birds) and even indi- 

 vidual organs themselves may vary in size independently of 

 general body size. 



Castle's genes for body size are very different from the dupli- 

 cate, cumulative, nondominant genes that have been described 

 for corolla length in tobacco and for other quantitative char- 

 acters. They are not duplicates, they do not act cumulatively, 

 and they are not nondominant. Some act in a positive direction 

 and some in a negative direction, and they all differ in the 

 amount they increase or decrease the various elements of body 

 size. Some quantitative characters such as body size in mice 

 are clearly not the result of polymeric genes. Some quantita- 

 tive characters such as Nilsson-Ehle's wheat appear to result 

 from polymeric genes. Most quantitative characters, however, 

 are not conclusively proved to result either from polymeric genes 

 or from some other type of plural determiners. For those 

 characters, the theory of multiple factors is very valuable, pro- 

 vided that it is recognized as a working hypothesis only and 

 that without further evidence it is not understood to be a true 

 representation of facts. The problem of quantitative characters 

 is a very important one, for many physical traits show continu- 

 ous variation and almost all psychological traits are of that 

 nature. 



Mather has concluded that quantitative characters are the 

 basis of differences between species. He divides characters into 

 oligogenic and polygenic. Oligogenic characters are controlled 

 by only a few genes each of which has a large effect when com- 

 pared with nonheritable fluctuation. Almost all the characters 



