No. 5S7] 



CALIFORNIA DEER-MICE 



701 



difference between the two races may be purely onto- 

 genetic. Indeed, Donaldson 22 seems to have demon- 

 strated the effects of exercise upon the weight of the 

 nervous system of the rat. 



By taking a known stock of wild mice and measuring 

 each successive generation reared in captivity, and by 

 being careful to avoid selection, it would seem that the 

 foregoing ambiguities of interpretation might in a large 

 degree be obviated. It is possible, therefore, that this 

 phase of the subject deserves quite as much attention as 

 the problems relating to the physical environment and 

 the distribution of subspecies. 



Hybridization has thus far failed completely between 

 rubidus and gambeli (48 matings). The Berkeley gam- 

 beli has, however, been successfully crossed with sonori- 

 ensis and some young of an F 2 generation have already 

 been obtained. Owing to the intergrading and widely 

 overlapping character of the differences between these 

 two races, it does not seem likely that they will lend them- 

 selves well to Mendelian analysis. But it would be idle 

 for me to discuss the results of these crosses at the present 

 stage of the experiments. Further attempts will, of 

 course, be made to obtain hybrids between the more widely 

 separated races. 



(Since writing the foregoing, several successful mat- 

 ings between rubidus and sonoriensis have been effected.) 



22 Journal of Comparative Neurology, Vol. 21, No. 2, 1911. 



