132 PROBLEMS OF GENETICS 



conclusions drawn from actual evidence, for only rarely can they 

 be translated into more precise language. So thoroughly have 

 the biological sciences become permeated with the belief that all 

 distinctions are dependent upon adaptation, that the mere 

 existence of definite distinctions is felt by many to be sufficient 

 ground to warrant an assumption that these distinctions are 

 directly or indirectly due to special local conditions. For 

 example, Dr. J. A. Allen, who has done so much careful and valu- 

 able work in delimiting the local forms of the United States 

 fauna, writes of the Ground Squirrels (Tamias) 16 as follows: — 



"From the extreme susceptibility of this plastic group to 

 the influences of environment, it is one of the most instructive 

 and fascinating groups among North American mammals. No 

 one can doubt its comparatively recent differentiation from a 

 common stock, and its dispersion from some common centre. 

 Whether the type originated at some point in North America, 

 or in the Northern part of Eurasia, it is perhaps idle to speculate, 

 but that it has increased, multiplied, spread, and become differ- 

 entiated to a wonderful degree in North America is beyond 

 question; as it is found from the Arctic regions to the high 

 mountain ranges of Central Mexico, and has developed some 

 twenty to thirty very palpable local phases." 



"Some of them easily take rank as species, others as sub- 

 species. Probably a more striking illustration of evolution by 

 environment cannot be cited." 



He proceeds to point out that the habits of these creatures 

 are such as lead to isolation. This may well be admitted, and 

 indeed no exception can possibly be taken to the passage as a 

 whole, save in the one respect that there is no real proof that 

 the local diversity is due to "evolution by environment" or an 

 indication of "susceptibility to the influences of environment." 



Dr. Allen does indeed adduce the fact that California "ex- 

 tending through 800 miles of latitude, with numerous sharply 

 contrasted physiographic regions, has apparently no less than 

 six strongly differentiated forms, while the region east of the 

 Rocky Mountains from a little below the northern boundary of 



« Allen, J. A., Bull. Amer. Mus. N. H., Ill, 1891, pp. 51-54. 



