DISTRIBUTION 383 
the cultivated Aristolochia. 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 (Xanthorylum). The 
strong-winged grasshopper, Sc/ustocerca 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 Hlinois, 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 
paleontological 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. 
g. Direction indicated by biogeographical affinities. 
