7O CHAS. C. ADAMS. 



Further light is to be thrown upon the interpretation of these 

 centers of dispersal and their biotic types by taking into account 

 the successional relation of the biota, as correlated with changes 

 of the environment. The habitat relations of organisms show 

 that they do not occur promiscuously mixed, even within a small 

 area, but that their relations are orderly and definite. In addi- 

 tion to the general successional relation attending changes of the 

 environment, attention is called to the different kinds of organisms 

 in different biotic regions, which make up this succession. This 

 habitat uniqueness of the biota in different regions favors the in- 

 dependent formation or association of similar habitat types from 

 very diverse kinds of biota. 



With these sources of Postglacial supply, their routes of dis- 

 persal, and their definite habitat relations fresh in mind, it be- 

 comes very evident that these factors must greatly influence our 

 interpretation of life areas. These facts strongly suggest that the 

 present conditions of life cannot be expected to fully explain the 

 present distribution, and clearly emphasize that the historical fac- 

 tor must be dynamically considered. 



UNIVERSITY MUSEUM, 



UNIVERSITY OF MICHIGAN, 



ANN ARBOR, MICH., August, 1904. 



5. REFERENCES. 



Adams, Charles C. 



'or Baseleveling and its Faunal Significance, with Illustrations from South- 

 eastern United States. American Naturalist, Vol. XXXV., pp. 839-852. 

 '02 a. Southeastern United States as a Center of Geographical Distribution of 



Flora and Fauna. Biological Bulletin, III., pp. 115-131. 

 '02 I'. Postglacial Origin and Migration of the Life of Northeastern United 



States. Journal of Geography, Vol. I., pp. 303-310, 352-357- 

 Cockerell, T. D. A. 



'98 Tables for the Determination of New Mexico Bees. Bulletin Sci. Lab. 



Denison Univ., Vol. XI. 

 Coville, F. V. 



'93 Botany of the Death Valley Expedition. Cont. U. S. Nat. Herbarium, 



Vol. IV. 

 Coville, F. V., and Macdougal, D. T. 



'03 Desert Botanical Laboratory of the Carnegie Institution. Washington. 

 Dixon, Chas. 



'95 The Migration of British Birds. London. 



