Effects of External Conditions 105 
ones which are employed by mammalologists who concern them- 
selves with rodents. Coues and Allen, for example, give the 
following external measurements for Muridz, Leporide, Sciuridz, 
etc.: From tip of nose to (1) eye, (2) ear, (3) occiput, (4) tail; 
length of tail to end of (1) vertebrz, (2) hairs; length of fore-foot, 
and hind foot “from the tuberosity of the heel to the end of the 
longest claw”; height of ear. Merriam," likewise, presents figures 
for ‘total length” (1. e., body plus tail); “tail vertebre” (i. e., 
length of the vertebral portion of the tail); “pencil” (tuft of hair 
at tip of tail); “hind foot;” “ear from crown” (sometimes from 
“anterior root” or from “‘notch’’); and occasionally some others, 
including weight. Various dental and skeletal features are of 
course included in the complete diagnosis of a species, as well 
as differences in the color, texture and length of the pelage. But 
many of these characters are such as do not lend themselves to 
measurement, while others require the preparation of cleaned 
skeletons. 
STATISTICAL METHODS 
Since it is to be presumed that many biologists are still unfa- 
miliar with the methods of biometry, the following statement as to 
those employed in the present paper may not be out of place.” 
For a really earnest and, on the whole, successful endeavor to ren- 
der this difficult subject intelligible to the non-mathematical mind, 
the reader is referred to Thorndike’s “ Introduction to the Theory 
of Mental and Social Measurements” (Science Press, 1904). 
Davenport’s “Statistical Methods” is of course indispensable to 
those who are already sufficiently familiar with the use of these 
methods. 
The mean or average of a series of values (in the present case, 
18 Monographs of North American Rodentia. Report of the U. S. Geological Survey, vol. xi, 
1877, pp. I-Io0gI. 
14North American Fauna” series, published by the Biological Survey of the Department of Agri- 
culture. 
My thanks are due to Messrs. E. L. Thorndike and R. P. Bigelow for criticising certain portions 
of my manuscript and to Prof. C. B. Davenport for important information relative to biometrical 
methods. 
