MURIDAE—SIGMODONTES—HESPEROMYS LEUCOPUS. 53 
oceupies from one-third to nearly or quite one-half of the cireumference— 
generally about two-fifths. But, in many cases, the line of separation is 
obscure, and then the tail is simply paler below than above. Even some other- 
wise typical Massachusetts specimens show this last condition. 
The variation in absolute and relative length of the tail is greater than in 
any other dimension. It ranges from obviously longer than the head and 
body to about equal to the body alone. This, it should be observed, is inde- 
pendent of locality, and exclusive of what we have admitted further on as a 
variety sonoriensis. Mr. J. A. Allen, who has paid particular attention to the 
variability of feral animals, finds that in Massachusetts specimens alone the 
proportion of tail to trunk may be as 1.18: 1.00, or as 0.69: 1.00—a difference 
of over fifty per cent. of the mean; and that the number of caudal vertebrae 
themselves ranges from twenty-four to thirty or more. But this ceases to be 
remarkable when we recollect that it is purely a matter of what has been aptly 
called “vegetative repetition”. It seems to be a well-nigh universal law that 
those parts or organs that are the least specialized,—. ¢., those of which several 
have the same or corresponding character and function,—are liable to be pro- 
duced with a high degree of irregularity as regards their number; and the 
more such there are the wider are the limits of variation apt to be. In this 
species, one of our longest-tailed rodents, the law is perfectly illustrated. 
Since none of the nominal species that have been erected upon the yari- 
ations of dewcopus depend in any way upon cranial characters; and since the 
valid species of Vesperimus likewise offer no noticeable cranial features beyond 
slight differences in dimensions (greater in californicus and less in michigan- 
ensis for example), the skulls of all our North American Hesperomys, except 
those of the subgenera Onychomys and Oryzomys, may be most conveniently 
examined collectively. 
