COAT-COLOR IX HYHRIDS OF PEROMYSCTS EREMICUS. 55 



earlier disposed to concede. It is important, however, that he 

 should make clear just how much is involved in this admission. 



The evidence renders probable the segregation, in the second 

 hybrid gem-ration, of unlike hereditary materials derived from 

 the two i Tin-lit races. It likewise renders probable the independent 

 assortment of materials affecting the development of various single 

 character differences. Furthermore, it seems to show that each 

 one of these character differences, so far as it is hereditary at 

 all. is influenced by a considerable number of units which segregate 

 independently of one another. 



1 Mi :hr other hand, there is nothing in the present evidence to 

 support the view that the entire heritage of an organism is con- 

 1 by these segregating " factors." Nor would even a de- 

 lemonstration of the segregation of hereditary units consti- 

 tute a proof that the segregation was complete or that the units 

 were unchangeable or incapable of influencing one another. One's 

 belief- in regard to these matters must depend upon the totality of 

 'encc from afl sources, and few would claim that at present 

 tliis evidence is decisive in favor of any particular hypothesis. 



In the present state of our knowledge, it is still permissible 

 i-pect that the mendelian " genes " are not the only substances 

 on>iMe for the hereditary differences between one organism 

 and another. It has frequently been suggested that the genes 

 pl.tv the i''le of enzymes in the process of development. 

 I '.in an cn/\me must have a " substrate " to act upon, and the end 

 result depends upon the properties of both. Such viewpoint is in 

 harmony with the contention of Loeb " that the Mendelian factors 

 of heredity must have the embryo to work on and that the organ- 

 ism is not to be considered a mere mosaic of Mendelian factors " 

 i " The < irganism as a Whole," p. 247). 



results of the present studies are not offered as evidence 

 for such a modified or restricted mendelian viewpoint, though they 

 are in no wav inconsistent with it. 



