oie 
GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 413 
On the other hand, out of the thirty-one Boreal genera of North Ameri- 
can mammals the following, twenty-four genera or 77 per cent, are com- 
mon to Boreal America and Boreal Eurasia: 
Cervus. Aretomys. Cuniculus. Lutreola. 
Rangfer. Castor. Lagomys. Putorius. 
Alce. Phenaconys. Vulpes. Mustela. 
Ovis. Evotomys. Ursus. Gulo. 
Bison. Arvicola. Thalarctos. Sorex. 
Tamias. Myodes. Latax. Urotrichus. * 
In addition to the foregoing genera, which are clearly of Boreal ori- 
gin, the following twelve genera of more extended range are also com- 
mon to the two continents: 
Sciuropterus. Lepus. Felis. Vespertilio. 
Sciurus. Canis. Lynx. Plecotus.t 
Spermophilus. Lutra. Vesperugo. Nyectinomus. 
Most of these genera are known to be of great antiquity, their remains 
having been found in Miocene strata, and it is probable that the others 
belong to the same category, but have thus far escaped detection, owing 
to their very small size. Allof them attain their maximum development 
and numbers in the Sonoran region in America and the analogue of the 
Sonoran in Eurasia; but by reason of the great length of time that has 
elapsed since they came into existence some of their representatives 
have become acclimated to a wide range of climatic conditions. 
Dr. John L. Le Conte, in his report on the Coleoptera of Lake Supe- 
rior, said: “The entomologist can not fail to be struck with two very 
remarkable characters displayed by the insect fauna of these northern 
regions. First, the entire absence of all those groups which are peculiar 
to the American continent [7. e., Sonoran and Tropical groups]. -— - 
The few new genera which I have ventured to establish are not to be 
regarded as exceptions. They are all closely allied to European forms, 
and by no means members of groups exclusively American. 
“Secondly, the deficiency caused by the disappearance of character- 
istic forms is obviated by a large increase of the members of genera 
feebly represented in the more temperate regions, and also by the intro- 
duction of many genera heretofore regarded as confined to the northern 
part of Europe and Asia. Among these latter are many species which 
can be distinguished from their foreign analogues only by the most 
careful examination. This parallelism is sometimes most exact, run- 
* As stated above, Flower and Lydekker do not recognize the American animal as 
generically distinct from Urotrichus. While I agree with Dobson in according it 
generic rank, it is convenient, in studying the origin of groups. to bring together 
such closely related types. 
+The American species of Plecotus are separated generally by Dr. Harrison Allen 
under the name Corynorhinus, which is adopted by the writer. The more compre- 
hensive name Plecotus is here used for the reason just stated under Urotrichus. 
