126 ADAMS. 



streams, we are furnished with a good working hypothesis, since 

 the convergence of these lines points to a center of dispersal. 



4. The study of variation is essentially a study in biological 

 dynamics, because the lack of stability is the very characteristic 

 considered. The close relation between centers of origin and 



o 



highways of dispersal emphasizes the desirability of considering 

 faunal and floral areas from the same dynamic standpoint. By 

 this method we can understand the relationships and form of these 

 areas much better. Such areas are dynamic centers from which 

 tension lines and zones radiate ; and the genetic relationships of 

 these areas can only be determined by a study of the present and 

 past dynamic relations. An important step was made in this 

 direction when Gray and Merriam (Merriam, '90, p. 24) recognized 

 that our North American flora and fauna are composed of two 

 elements, boreal and austral, each tending to move from its center 

 of origin. 



From a dynamic and genetic standpoint, the limits and signifi- 

 cance of life areas take on a new meaning and importance when, 

 as it were, we stand at the center or focus of dispersal, and look 

 outward. From this point of view w r e look upon the life of a 

 region as constantly diverging or radiating from its original home 

 and the parental stock, encountering new conditions of environ- 

 ment and becoming modified in both habits and structure. The 

 continuity in variations, which is one of the most marked char- 

 acteristics in the geographical variations, can be safely determined 

 only by the study of the form along its highways. The paths of 

 dispersal are the lines along which we may expect the hereditary 

 factors to show their influence most distinctly. Again, these 

 paths bear the same relation to each other as the branches of a 

 phylogenetic tree, and may therefore be compared with such 

 branches. In phylogenetic studies the most fruitful results do 

 not come from a comparison of the tips of the different branches 

 but from a study and comparison of forms along the same branch 

 and therefore of common descent. 



We may with advantage carry this comparison still further, and 

 consider the genetic relationships which faunal areas bear to each 

 other. The lines of kinship are shown primarily along the lines 

 of dispersal. Here, as before, it is not to the relationships which 



