DISTRIBUTION 3^3 



the cultivated Aristolochla. P. thoas, one of the pests of 

 the orange tree in the South, is highly prized as a rarity by 

 New England collectors and is able to perpetuate itself in the 

 Middle States on the prickly ash- (Xanthoxylum) . The 

 strong-winged grasshopper, Schistocerca 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 mode- 

 rate 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 for- 

 mer paths of diffusion can no longer be ascertained. The 

 army worm (Heliophila unipuncta) , feeding on grasses, and 

 occurring all over the United States south of Lat. 44 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 : 



1. Location of greatest differentiation of a type. 



2. Location of dominance or great abundance of individuals. 



3. Location of synthetic or closely related forms (Allen). 



4. Location of maximum size of individuals (Ridgway- 

 Allen). 



5. Location of greatest productiveness and its relative sta- 

 bility, in crops (Hyde). 



6. Continuity and convergence of lines of dispersal. 



7. Location of least dependence upon a restricted habitat. 



8. Continuity and directness of individual variations or 

 modifications radiating from the center of origin along the 

 highways of dispersal. 



9. Direction indicated by biogeographical affinities. 



