MUEIDAE— SIGMODONTES— HESPEROMYS LEUCOPUS. 65 



It seems unnecessary for us to examine the figures of this table in detail 

 after what we have said of the Massachusetts lot. Bringing together so many 

 specimens, we find, does not appreciably affect the Jenks' average ; but it has 

 the inevitable result of spreading the extremes a little farther apart, and 

 proving the range of variation to be rather more than we allowed in the former 

 case — in fact, it demonstrates the variability of the species to be fully as great 

 as claimed by Allen. 



Contrary to our expectation, we do not find in this series any evidence 

 that latitude exerts an appreciable influence upon the absolute size or relative 

 proportion of the parts of this mouse. Nor do we observe any difference 

 with latitude in the character of the jjelage, the hairiness of the soles or tail, 

 &c. — at any rate to an appreciable extent — and certainly no such difference 

 as may be observed between summer and winter specimens from the same 

 locality (when we come, however, to bring in Arctic skins, as below, we 

 shall be able to see a difference). In the matter of color, there is positively 

 nothing in this whole series that we cannot exactly match among Massachu- 

 setts skins. And yet it is curious to observe that almost every considerable 

 geographical area within the limits represented in the table produces a slight 

 strain or breed of its white-footed mice — some difference in color indescriba- 

 ble in words, but which strikes the eye that is very familiar with the subject. 

 The Nova Scotian animal and the Virginian, the Illinois and the Kansas, are 

 always distinguishable. We venture to assert that we can distinguish in 

 North America about twenty kinds of Hesperomys leucopus upon characters at 

 least as constant, reliable, and tangible as those hitherto held to define the 

 greater part of the " species" that have been in vogue of late years. 



The first nominal species that we shall investigate is the H. "myoides" 

 of Baird, who described his animal chiefly from Vermont specimens, identify- 

 ing it with the "Cricetus myoides" of Gapper. The only characters ascribed 

 to it are: first, possession of cheek-pouches; secondly, "tail- vertebrae gen- 

 erally 0.25 of an inch longer than head and body." But we have just shown 

 that the possession of a tail a fourth (or more) of an inch longer than the 

 body has no significance whatever as a specific character ; and among the 

 specimens enumerated by Baird (and also tabulated by us) are some with the 

 tail no longer than the body, and others with the tail shorter than the body ; 

 5 M 



