378 FRANCIS B. SUMNER 



shown, the racial differences under consideration do not depend 

 upon single Mendelian factors. 



As to those cases in which two characters are positively cor- 

 related in the individual (e.g., tail and foot), this may be due to 

 some sort of "linkage" in inheritance, but may likewise be due 

 to parallel modification by environmental agencies. For the 

 white mouse I demonstrated many years ago that the length 

 of both tail and foot was to a considerable extent dependent 

 upon the temperature of the atmosphere in which the animals 

 were reared, and in the case of Peromyscus I have more than 

 once pointed out that cage-born animals tend to have both of 

 these parts shortened. Such facts as these point to the possi- 

 bility that the correlation of these parts in nature may be due 

 to the varying incidence of external modifying agencies of some 

 sort. 



I shall now pass to the second phase of these studies to be 

 reported upon in the present paper — that, namely, which con- 

 cerns itself with the crossing of different geographic races. It is 

 significant that the word 'genetics' has, to an increasing extent, 

 come to mean the experimental study of Mendelian unit fac- 

 tors — real or alleged — as revealed by hybridization, ^"^ And 

 indeed to genetics, in this unwarrantably restricted sense of the 

 word, we are ourselves giving considerable attention in our work 

 with Peromyscus. We have followed the inheritance of several 

 different color mutations, and obtained fairly typical mono- 

 hybrid and dihybrid ratios, along with some cases which still 

 puzzle us. Some of these results I have already published in 

 preliminary form, and a further report will probably be made 

 during the next few months by Mr. Collins and myself. 



My only quarrel is with the contention that all inheritance is 

 Mendelian, whether it seems so or not, and with the endless 

 creation of hypothetical 'unit factors,' to explain every de- 

 parture from the expected manner of transmission. In this 

 latter class I include the so-called 'multiple factor' explanation 

 of blended inheritance and of the modification of characters 

 through selection. 



1° I recall finding this meaning explicitly given to the word in a recent work. 



