GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA, 415 
PRINCIPLES ON WHICH BIO-GEOGRAPHIC REGIONS SHOULD BE ES® 
TABLISHED. 
Wallace, in writing of the principles on which zodlogical regions 
should be formed, expresses the opinion that ‘ convenience, intelligi- 
bility, and custom should largely guide us.” But I quite agree with 
America’s most distinguished and philosophic writer on distribution, 
Dr. J. A. Allen, that in marking off the life regions and sub-regions of 
the earth, truth should not be sacrificed to convenience; and [ see no 
reason why a homogeneous cirewn-polar fauna of great geographic 
extent should be split up into primary regions possessing comparatively 
few peculiar types simply because a water separation happens to exist 
in the present geologic period; nor is it evident why one of the re- 
sulting feeble divisions should be granted higher rank than a region of 
much less geographic extent comprising several times as many peculiar 
types. Hence the divisions here recognized, and the rank assigned 
them, are based as far as possible upon the relative numbers of distine- 
tive types of mammals, birds, reptiles, and plants they contain, with 
due reference to the steady multiplication of species, genera, and higher 
groups from the poles toward the tropics. Mammals have been chietly 
used as illustrations because they answer the purpose better than any 
other single group, and because it is clearly impossible in a brief essay 
of this character to enumerate such a multitude of forms as would be 
necessary were equal consideration accorded to each class. 
