No. 610] MUTATION THEORY AND SPECIES-CONCEPT 589 



gradual and progressive change can be overdone when it 

 is attempted to correlate the alterations observed with 

 climatic or other environmental features. Thus the pro- 

 gressive lightening in color from kennicottii to xantusi, 

 first by lightening and restriction of the brown until it 

 practically all disappears and then by paling and diminu- 

 tion of the gray, is believed to be associated with the de- 

 creasing moisture in the northern part of the range and 

 the increasing aridity in the south. There are of course 

 many well-known cases of paler races of birds and mam- 

 mals occupying desert areas. Yet it is not clear that the 

 coastal region of Oregon, where the less dense brown 

 hreivsteri occurs, is any less humid than the^ correspond- 

 ing part of Washington where kennicotti is found. Simi- 

 larly xantusi on the peninsula of Lower California can 

 not be supposed to exist in a drier habitat than gilmani or 

 cineraceus. Of course in none of these cases is it known 

 just what feature in the environment acts as the critical 

 factor nor how the race responds to it. The experimental 

 studies of Tower^^* and others show that a race may re- 

 spond in the same way {i. e., by showing the same vari- 

 ations) to different environmental stimuli or in different 

 ways to the same stimulus. But studies of this character 

 are still too few to funiisli a basis for interpreting these 

 reactions on the part of spccic^s ot' tlio liiiilior animals. 

 The experiments beiii,u- can iiMl on 1)y Siiiiiiu'i-^^ with the 

 white-footed mouse, Pcroini/scus Dniuiculdtus may be ex- 

 pected to throw further light on this important question 

 of the origin of local subspecies. 



Again, it is not certain that such races as kennicottii, 

 hreivsteri and bendirei form an absolutely graded series 

 with all intermediates. On the contrary there appears to 

 be some evidence that although their boundaries are con- 

 tiguous there are definite though small steps from one to 



