THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 11 



I. RANGE AND CENTER OF DISTRIBUTION. 



The term "center of distribution" may be understood in two 

 ways. It may mean, on the one hand, the region in which one or 

 more species have originated and from which they have spread; 

 or it may signify the region where a species or a group of species 

 attains its greatest development. The present study is not primarily 

 concerned with origins and migrations, and therefore the second 

 interpretation is the one used. It is true, however, that in most 

 cases, and probably in the one here considered, a given center is a 

 " center of distribution" in both senses. Transeau, in an important 

 paper (88), has expressed the concept as follows: 



"In using the term 'center of distribution' it is not implied that the plants have 

 necessarily spread from these centers, but that the complex of climatic factors most 

 favorable to the development of this type of vegetation is here localized, and that as 

 we depart from such centers we find conditions more and more unfavorable." 



Referring to a particular center he says that "within its limits, 

 the plants have a wider range of habitats, attain a greater size, 

 and are more abundant than elsewhere." 



Adams (5) gives a list of ten criteria for the determination of 

 centers of distribution, which include the three stated by Transeau. 

 Six of these are susceptible of use in the present study. They 

 are as follows: 



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. 



4. Location of maximum size of individuals. 



5. Location of least dependence upon a restricted habitat. 



6. Continuity and convergence of lines of dispersal. 



To these I would add another, stated as follows: Location of 

 greatest importance of the type in the climax community. There 

 is evidence along all of these lines in support of the conclusion about 

 to be stated. Another criterion proposed by Adams, and which 

 would be of great use were our data sufficient, is worth stating, as a 

 suggestion for future study: "Continuity and directness of indi- 

 vidual variations or modifications radiating from the center of 

 origin along the highways of dispersal." 



From the evidence about to be presented I have reached the con- 

 clusion that the center of distribution of the broad-sclerophyll 

 vegetation type of the Pacific Coast corresponds with the region 

 known popularly as "Southern California," west of the deserts, 

 including Ventura and Santa Barbara Counties, and also extending 

 an unknown distance into Lower California. The evidence can 

 best be stated under seven heads, corresponding with the six criteria 

 proposed by Adams and the seventh which I have added. At thi? 



