DISTRIBUTION 317 



The harlequin cabbage bug (Murgantia histrionica) has spread from 

 Central America into California and Nevada, and has steadily progressed 

 in the Mississippi basin as far north as Illinois, Indiana and Ohio, though 

 it appears to be unable to maintain itself in the northern parts of these 

 states. This insect required about twenty-five years to pass from Loui- 

 siana (1864) to Ohio, spreading through its own efforts and not commer- 

 cially to any great extent. 



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

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

 existence. Thus Iphidides ajax occasionally reaches Massachusetts 

 as a visitor and a visitor only; Lcertias 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, some- 

 times 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 (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 origi- 

 nated 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 stability, 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 modifica- 



