340 ENTOMOLOGY 



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- 

 tions radiating from the center of origin along the highways of dispersal. 



9. Direction indicated by biogeographical affinities. 



10. Direction indicated by the annual migration routes, in birds 

 (Palmen). 



2. GEOLOGICAL 



Means of Fossilizatipn. Abundant as insects are at present, they 

 are comparatively rare as fossils, the fossil species forming but one per 

 cent, of the total number of described species of insects. The absence 

 of insect remains in sedimentary rocks of marine origin is explained by 

 the fact that almost no insects inhabit salt water; and terrestrial forms 

 in general are ill-adapted for fossilization. The hosts of insects that die 

 each year leave remarkably few traces in the soil, owing perhaps, in 

 great measure, to the dissolution of chitin in the presence of moisture. 



Most of the fossil insects that are known have been found in vege- 

 table accumulations such as coal, peat and lignite, or else in ancient 

 fresh-water basins, where the insects were probably drowned and 

 rapidly imbedded. At present, enormous numbers of insects are 

 sometimes cast upon the shores of our great lakes a phenomenon which 

 helps to explain the profusion of fossil forms in some of the ancient 

 lake basins. 



Insects in rich variety have been preserved in amber, the fossilized 

 resin of coniferous trees. This substance, as it exuded, must have en- 

 tangled and enveloped insect visitors just as it does at present. Many 

 of these amber insects are exquisitely preserved, as if sealed in glass. 

 Copal, a transparent, amber-like resin from various tropical trees, par- 

 ticularly Leguminosae, has also yielded many interesting insects. 



Ill-adapted as insects are by organization and habit for the com- 

 moner methods of fossilization, the number of fossil species already 

 described is no fewer than three thousand. 



Localities for Fossil Insects. The Devonian of New Brunswick 

 has furnished a few forms, found near St. John, in a small ledge that 



