DISTRIBUTION OF FLORA AND FAUNA. \2J 



exist between the terminal branches on which we are to focus 

 our attention, but to those affinities which exist along the high- 

 ways toward the center of origin. The double origin of the 

 North American fauna and flora will illustrate the present stand- 

 point. The lines of dispersal may be represented by two trees, 

 one with its roots in the North, and the other with its roots in the 

 South, and whose branches intermingle. From these two centers, 

 life has radiated in all possible directions, but on account of the 

 North and South trend of the topography the lines of dispersal 

 have, in the main, been North and South. For this reason the 

 general lines of kinship are likely to be most strongly expressed 

 in these same directions. 



It is a very fundamental law that most forms of life are con- 

 fined to restricted areas and only a small number have extensive 

 distribution. Thus, from the centers of origin there is a constant 

 decrease, or attenuation in the number of forms which have been 

 able to depart far from the original home. From this standpoint, 

 therefore, the question arises as to what criteria are to be used in 

 the determination of life areas. Should more stress be laid upon 

 the concentric relations, or degree of attenuation, from the cen- 

 ter of dispersal, or, on the other hand, upon the genetic affinities, 

 such as exist along the lines of dispersal ? Are the natural 

 affinities better expressed zonally or dendritically ? In one case 

 the tree is horizontally truncated and in the other the branches 

 are torn apart. 



Surely the genetic and dynamic standpoint, which attempts to 

 explain the life of an area in terms of its origin, favors, as a work- 

 ing hypothesis, a trial of the dendritic method. The method of 

 work here suggested will put faunal studies upon a distinctly 

 genetic basis. 



From the relationship which exists along the lines of dispersal, 

 we draw an important practical conclusion. In collecting a series 

 of animals for the study of variation, one of the first things to be 

 done is to determine its means and highways of dispersal and 

 then secure material from along these lines. This will greatly 

 simplify the study of land forms, because in this manner we have 

 a clue to determine which localities are likely to be the most 

 important, and upon these we may concentrate our attention. 



