200 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
in this memoir. Some specimens are little more than half as long as others, 
and certainly have less than half the bulk. Our figures for apparently 
mature animals run from 4.50 to 8 inches for length of trunk. Now, to 
keep largely within bounds, we will strike off half an inch each way, and 
say a normal limit of variation is between 5.and 74 inches: this 24 inches 
is 50 per cent. of the minimum and 40 per cent. of the mean length of the 
animal. This great discrepancy is the more instructive, because in the case 
of xanthognathus there is no possible question of specific identity of the 
largest and smallest specimens. In some other cases, where reputed nominal 
species, based in part on dimensions, were at issue, we may possibly be 
suspected of granting improbable and undue range of variation. But here 
the matter is brought to a focus: we show, in the specimens of unquestion- 
ably a single species, as great variability in size as we have anywhere 
attempted to prove. 
And yet this difference is no greater than we believe is well known to 
occur in other species of the genus, notably the Arvicola amphibius of 
Europe. No one is surprised to kill two house-rats, one of which is twice 
as big as the other. We hold that a corresponding variability is as normal 
to some purely feral animals as to the semi-domesticated species just cited; 
and we believe that it argues a progressive increase in size, with age, over 
the stature ordinarily reached at the period of puberty—that is to say, a 
Mus or an Arvicola may be “adult” or “mature” in the sense that it has lost 
the signs of youth, gained those of adult life, and become capable of repro- 
duction, and yet, after this, may increase in length by one-third at least, and 
double its bulk in the subsequent years of its life. 
Recurring again to our measurements, we next observe that the tail of 
this animal (taking it to the end of the vertebree as a more constant and 
reliable measurement than to the tip of the hairs) ranges from 0.75 to 2.25, 
as the figures stand! and, making large allowance for erroneous elements, we 
may safely say that the tail is “aa inch or two” long, 7. e., it varies 100 per 
cent. of the minimum! What could more forcibly illustrate the instability 
that attends the dimensions of organs produced in any sense as matters of 
vegetative repetition? The measurements of other parts need not detain us. 
For several reasons, among them ease of correct measurement, the limits of 
the figures for the feet and ears do not stand quite so far apart as those for 
the body and tail do; they coincide with the results of our measurements 
