No. 587] 



CALIFORNIA DEER-MICE 



697 



high up in the mountains of California and Nevada. 

 These points are continuous with the main habitat of the 

 subspecies in the desert lowlands and plateaus, in the 

 sense that no abrupt barriers intervene, but they present 

 very great differences in climate and vegetation. 



Facts of this sort — " natural experiments," as we may 

 call them — seem to show that these subspecific differ- 

 ences manifest themselves in a large degree independ- 

 ently of climatic conditions, in other words, that they are 

 of germinal rather than of somatic origin. 



But these 1 'natural experiments" are not entirely con- 

 clusive, for we can never be quite certain what the actual 

 condition are which Nature has imposed in a given case. 

 Granting that these darker song-sparrows in the desert 

 are actually invaders from the coastal plain, we have no 

 means of knowing how long they have been exposed to 

 the desert conditions. Also is it definitely known that 

 their restricted habitat in certain portions of the desert 

 does not agree with their original habitat in respect to 

 those factors which are really essential in determining 

 their characteristic coloration? 



My own first attempt at transplantation consisted in 

 bringing a considerable number of specimens of P. m. 

 sonoriensis from the vicinity of Victorville to Berkeley. 18 

 At the latter point, the mice were kept in cages, freely 

 exposed to the air, and under atmospheric conditions as 

 nearly natural as possible. For control, numbers of the 

 Berkeley race were reared in neighboring cages. The 

 result of this experiment I can state briefly: Neither 

 the originally introduced animals nor their offspring, 

 nor their grandchildren, have thus far shown any per- 

 ceptible approach to the local type. They are still 

 obviously of the sonoriensis race. If there is any tend- 



cated, to a certain extent, in the foregoing table. But this does not show 



diurnal fluctuations of temperature and humidity in the desert, as com- 

 pared with those in the coastal region. 



