DISTRIBUTION 335 



The Austral region "covers the whole of the United States and 

 [exico, except the Boreal mountains and the Tropical lowlands." It 

 :omprises three transcontinental belts: (i) the Transition zone, in 

 rhich the Boreal and the Austral overlap; (2) the Upper Austral; (3) 

 the Lower Austral. The butterfly Eudamus proteu's (Fig. 300) is re- 

 stricted, generally speaking, 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 mam- 

 mals, 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 hetero- 

 geneous assemblage of species derived from several sources, and the 

 foreign element of this fauna we shall consider 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 all directions and does 

 so spread until its further progress is prevented, in one way or another. 

 The paths along which a species spreads are determined, then, by the 

 absence of barriers. The diffusion of insects in our own country has 

 received much attention from entomologists, especially in the case of 

 such insects as are important from an economic standpoint. TJie ac- 

 cessions to our insect fauna have arrived chiefly from Asia, Central and 

 South America, and Europe. 



Webster, our foremost student of this subject, to whom the author- 

 is indebted for most of his facts, names four paths along which insects 

 have made their way into the United States: (i) Northwest Northern 

 Asia into Alaska and thence south and east; (2) Southwest Central 

 America through Mexico; (3) Southeast West Indies into Florida; (4) 

 Eastern from Europe, commercially. 



Northwest The northern parts of Europe, Asia and North America 

 have in common very many identical or closely allied species, whose 

 distribution is accounted for if, as geologists assure us, Asia and North 

 America were once connected, at a time when a subtropical climate 

 prevailed within the Arctic Circle; in fact, the distribution is scarcely 

 explicable upon any other theory. Curiously enough, the trend of 



