

DISTRIBUTION 339 



and has even made its way several times into Great Britain, only to 

 be stamped out with commendable energy. The box-elder bug (Lep- 

 tocoris trimttatus) is similarly working eastward, having now reached 

 Ohio. Formerly the Rocky Mountain locust periodically migrated east- 

 ward, but always met a check in the moist valley of the Mississippi. 



The chinch bug (Blissus leucopterus), the distribution of which has 

 been traced by Webster, has spread from Central America and Mexico 

 northward along the Gulf coast into the United States, following three 

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

 Mississippi 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; Lartias 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 palaeontological evidence, for 

 determining centers of dispersal: 



i. Location of greatest differentiation of a type. 



