DISTRIBUTION 377 
The Austral region “ covers the whole of the United States 
and Mexico, except the Boreal mountains and the Tropical 
lowlands.” It comprises three transcontinental belts: (1) the 
Transition zone, in which the Boreal and the Austral overlap ; 
(2) the Upper Austral; (3) the Lower Austral. The butter- 
Fic. 293. 
Distribution of Erynnis manitoba, a Distribution in the United States of 
butterfly restricted to subarctic and sub- Eudamus proteus, primarily a_ tropical 
alpine regions.—After ScuDDER. butterfly.—After ScuDDER. 
fly Eudamus proteus (Fig. 293) is restricted, generally speak- 
ing, to the Tropical region and the warmer and more humid 
portions of the Austral. 
The Tropical region covers the southern extremity of 
Florida and of Lower California, most of Central America and 
a narrow strip along the two coasts of Mexico, the western 
strip extending up into California and Arizona. 
These divisions are based primarily upon the distribution of 
mammals, birds and plants, and the three primary divisions 
serve almost equally well for insects also. In regard to the 
zones, however, not so much can be said—for insects are to a 
high degree independent of minor differences of climate. 
Many instances of this are given beyond. 
The insect fauna of the United States is upon the whole a 
heterogeneous assemblage of species derived from several 
sources, and the foreign element of this fauna we shall con- 
sider at some length. 
Paths of Diffusion in North America.—It may be laid 
down as a general rule that every species tends to spread in 
