178 CLASSIFICATION OF SERES. 



problem of climax areas. The various divisions of North American vegetation 

 by Gray (1878), Engler (1879), Sargent (1880), Drude (1887), Merriam (1898), 

 Clements (1904), Harshberger (1911), and others, have either been based more 

 or less completely upon the basic climax units, here regarded as formations, or 

 at least represent them in some degree; Thus, while there is the usual diver- 

 gence of view as to the basis, relationship, and terminology of the various 

 subdivisions, there is necessary agreement as to the actual existence of a more 

 or less definite number of distinct vegetation areas. Few attempts have been 

 made to investigate these as climaxes and to determine their limits, relations, 

 and development. Cowles (1899, 1901) and Whitford (1901) have considered 

 the general relation of development to climax in the forested region of Illinois 

 and Michigan (pp. 187, 203, 205). Adams (1902 : 128) has sought to lay 

 down general rules for the study of life centers, in connection with a study 

 of the southeastern United States as a center of dispersal and origin: 



' ' First. In general the fauna and flora of northern United States east of the 

 Great Plains are geographically related to those of the Southeast and this 

 geograjjhical relationship points to an origin in the direction of the Southeast 

 except in the case of the distinctly boreal forms. 



■ "Second. The abundance and diversity of Ufe in the Southeast indicate 

 that it has been, and now is, a center of dispersal. 



"Third. The relicts indicate that the Southeast has been a center of preser- 

 vation of ancient types, and the endemism shows that it has been a center of 

 origin of types. 



" Fourth. There are two distinct southern centers of dispersal in temperate 

 United States; one in the moist Southeast, and the other in the arid Southwest. 



"Fifth. Ten criteria, aside from fossil evidence, are recognized for deter- 

 mining the center of origin or the locality of dispersal: 



" 1. Location of the 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 stabiUty, in crops. 

 (Hyde). 



"6. Continuity and ccmvergence 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 annual migration in birds. (Palmdn.) 

 " Sixth. There are three primary outlets of dispersal from the Southeast: 



"1. The Mississippi Valley and its tributaries. 



"2. The Coastal Plain. 



"3. The Appalachian Mountains and adjacent plateaus. 



"The first two have also functioned for tropical types, and the third for 

 boreal forms. Dispersal is both forward and backward along these highways. 



"Seventh. The individual variations of animals and plants, such as size, 

 productiveness, continuity of variation, color variation, and change of habit 

 and habitats, should be studied along their lines of dispersal and divergence 

 from their center of origin. Life areas should be studied as centers of dispersal 

 and origin, and hence dynamically and genetically." 



