GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 401 
humid, as indicated in defining their courses. As a rule, the former 
consist of treeless plains, deserts, and barren mountains, while the lat- 
ter are bountifully clothed with forests. Most of the humbler forms of 
vegetation are different in the two sub-divisions, and differences exist 
also among the mammals, birds, and reptiles, but the great majority 
of these dissimilarities are not of the same kind as those that distin- 
guish one zone from another. Most of them are specific, not generic, 
and the number of distinctive groups of high order is very much less. 
This may be made clear by selecting the distinctive elements of the 
arid Sonoran (which has the largest number of peculiar forms) in com- 
parison with those of the humid Sonoran (or Austro-riparian) and con- 
trasting them numerically with the distinctive elements of the Sonoran 
as a whole compared with those of the Boreal as a whole.* Among 
non-pelagic mammals the arid Sonoran has one family (Antilocapride) 
and only ten generat not known to inhabit the humid Sonoran or Aus- 
tro-riparian, and the latter has but one family (Didelphide) and four 
genera Didelphis, Oryzomys, Scalops, and Nycticejus) not found inthe arid 
Sonoran (and the family and one of the genera are intrusions from the 
Tropical region), while thirteen families and twenty-seven genera are 
common to both arid and humid subdivisions.¢ 
Among birds the arid Sonoran has no family and only twenty-four 
genera not inhabiting the humid Sonoran, and the latter has no family 
and but seven genera not found in the arid, while twelve families and 
thirty-one genera are common to the two divisions. 
Contrasting the Sonoran as a whole with the Boreal as a whole, it 
appears that there are no less than eight families and forty-one genera 
of mammals and ten families and about one hundred genera of birds 
distinctive of the Sonoran, and six families and thirty genera of mam- 
mals and three families and about forty genera of birds distinctive of 
the Boreal zone. In other words, taking mammals and birds together, 
the arid Sonoran has one peculiar family and only thirty-four distinc- 
tive genera, and the humid Sonoran one family and eleven genera (of 
which the family (Didelphide) and several of the genera are clearly 
intrusions from the tropical region), while the Sonoran as contrasted 
with the Boreal has eighteen distinetive families and one hundred and 
forty-one distinctive genera, and the Boreal has nine distinctive fami- 
lies and seventy distinctive genera. 
Only eight families and eight genera of mammals are common to the 
Boreal and Sonoran regions. The common families are: Cervida, 
Muride, Sciuride, Leporide, Mustelide, Canidae, Felide, and Soricide, 
*The intrusive tropical genera are here treated as Sonoran. 
tThese genera are: Antilocapra, Cynomys, Onychomys, Thomomys, Dipodomys, Pero- 
dipus, Microdipodops, Perognathus, Bassariscus, and Antrozous. 
t The newly discovered genus of Chiroptera, Euderma, is here omitted because only 
a single specimen is known, and it can not yet be satisfactorily assigned to its proper 
faunal position. 
H. Mis. 334, pt. 1——26 
