nisi i:ii'.i 1 1 339 



and has even made its \\ Britain, only to 



be Stamped out with commendable em-r^v. 'I he bo.\ rider bu<{ 

 tocoris trhitt(ttus) is similarly working eastward, having now reached 

 Ohi<. formerly the 1\<>< ky Mountain \o(\\>\ periodically mi crated east- 

 ward, hut always met a chirk in the moist valley of tin- Mi i~>ippi. 



The cliincli butf ( /*//.v.v//\ lcmof)tcrus], the di>t ribut ion of whit h 

 been traced by Webster, ha> >prcad from Central Amerit a and M- 

 northward alon<^ the Gulf coast into the I'nited States, following three 

 paths: (i) along the Atlantic coast to Cape Breton; 2) along the 

 Mi>sissippi valley and northward into Manitoba; (3) along the western 

 coast of Central America and Mexico into California and other Western 

 states. Everywhere this insect has found wild grasses upon which to 

 feed, but has readily forsaken these for cultivated grasses upon 

 occasion. 



Every year some of the southern butterflies reach the Northern states, 

 where they die without finding a food plant, or else maintain a precari- 

 ous existence. Thus Iphiclides ajax occasionally reaches Massachusetts 

 as a visitor and a visitor only; L&rtias philenor, however, finds a limited 

 amount of. food in the cultivated Aristolochia. P. thoas, one of the pests 

 of the orange tree in the South, is highly prized as a rarity by New Eng- 

 land collectors and is able to perpetuate itself in the Middle States on the 

 prickly ash (Xanthoxylum). The strong- winged grasshopper, Schisto- 

 cerca americana, belonging to a genus the center of whose dispersion is 

 tropical America, ranges freely over the interior of North America, 

 sometimes in great swarms, and its nymphs are able to survive in 

 moderate numbers in the southern parts of Illinois, Ohio and other 

 states of as high latitude, while the adults occasionally reach Ontario, 

 Canada. 



Many species are now so widely distributed that their former paths 

 of /diffusion can no longer be ascertained. The army worm (Cirphis 

 unipuncta), feeding on grasses, and occurring all over the United States 



south of Lat. 45 23' N., is found also in Central America, throughout 



South America, and in Europe, Africa, Japan, China, India, etc.; in 



short, it occurs in all except the coldest parts of the earth, and where it 



originated no one knows. 



Determination of Centers of Dispersal. In accounting for the 



present distribution of life, naturalists employ several kinds of evidence. 



Adams recognizes ten criteria, aside from palaxmtological evidence, for 



determining centers of dispersal: 



i. Location of greatest differentiation of a type. 



