402 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
The commmon genera are: Sitomys, Sciurus, Sciuropterus, Spermophi- 
lus, Lepus, Lutra, Canis, and Lynx. Several others inhabit limited 
parts of both regions, but are not common to these regions as a whole. 
With the possible exception of the gray wolf, not a single species of 
mammal ranges throughout the Sonoran and Boreal zones, though a 
number are common to the Upper Sonoran and Lower Boreal (Cana- 
dian); and in the case of the wolf it is almost certain that comparison 
of specimens will show the animal of the southern United States and 
Mexico to be perfectly distinct from that of Arctic America. The 
ermine is another species of exceptional though less extensive range, 
if it is really true that the weasel inhabiting the shores and islands of 
the Polar Sea is specifically identical with that found in the more ele- 
vated parts of the Southern States,—an assumption I can not for a 
moment entertain. 
In the case of land birds, eighteen genera are common to the Boreal 
and Sonoran regions. The number of common families is relatively 
large as would be expected from the wide dispersal of most families of 
birds. For instance, the Turdide or thrushes inhabit North and South 
America, Eurasia, Africa, India, and Australia; the Paride or titmice 
inhabit North and South America, Eurasia, Africa, India, Australia, 
and New Zealand; the Cinclide or dippers inhabit North and South 
America, Eurasia, India, and the Austro-Malayan region; the Troglo- 
dytide or wrens inhabit North and South America, Eurasia, India, 
Africa, and the Austro-Malayan region; the Corvide or crows, mnagpies 
and jays, are found in every part of the world, and so on. 
Number of distinctive families and genera of Mammals and Birds of the arid Sonoran 
compared with the humid Sonoran, and of the Sonoran as a whole compared with the 
Boreal as a whole. 
Total. 
Mammals. Birds. 
| Family. | Genera. | Family. | Genera. | Family. | Genera. 
— ead ak ea a 
| 
| 
Arid Sonoran distinguished from humid 
Sonoraniby eases bea a a eee | 1 | 10 | 0) 24 | 1 34 
Humid Sonoran distinguished from arid | | | 
Sonoran by7ss-ee a. see ee eee eee | 1 | 4 | 0 (| 1) VW 
Common to both arid and humid sono- 
PAD) eee ecco seine eie es eee eee eee ee | 13 | 27 | 12 31 25 | 58 
Sonoran as a whole distinguished from | | 
Boreal sD yc -.c seme aes eee eee eee 8 | *4] 10 | 100 18 | 141 
Boreal as a whole distinguished from | 
SONOLAMN DY cca cac esse oes meee eter 6 | 730 3 40 9) 70 
Common to Boreal and Sonoran. .....---- 8 | Bulsasecses 2 189 ae eteeeee 26 
| | | | 
*Sitomys and Lynx are omitted because they range over most of the forested part of the Boreal 
region. 
| Putorius is omitted because it ranges over much of the Sonoran region. 
Descending to the species the contrast is even more marked. 
