122 ADAMS. 



Northeastern United States during the Ice Age evidently could 

 have had only a boreal fauna and flora. The present distribution 

 of the boreal fauna and flora is along the northern border of the 

 United States and the higher parts of the Appalachian Moun- 

 tains. But it is not this element with which we are primarily con- 

 cerned, because we have seen that the biogeographical affinities 

 of the forms which at present dominate in the Northern United 

 States east of the Great Plains do not point to a boreal origin, 

 nor to the arid region of the plains, nor to the Southwest, but to 

 the Southeast. 



The criteria, which have been used to determine this center of 

 dispersal will next be considered. 



CRITERIA FOR THE DETERMINATION OF CENTERS OF DISPERSAL. 



In attempting to determine centers of dispersal of animals, I 

 have found it necessary to keep clearly in mind the tests which 

 are legitimate for such determinations. In a very large number 

 of cases we do not have paleoritological evidence in sufficient 

 abundance to materially aid us. For this reason I have thought 

 it desirable to state the criteria used. 



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 modifi- 

 cations 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). 



Some of these tests have long been utilized, but, so far as I 

 have been able to learn, all have not been definitely used for this 

 particular purpose. The fifth (productiveness) is very closely 

 related to the second (dominance). It is very well known that 



