ce:nter of origin and dispkrsal. 9 



GEOGRAPHICAL DISTRIBUTION AND DISPERSION OF THE GENUS. 

 THE CENTER OF ORIGIN AND DISPERSION. 



In any study of the evolution of organisms it is important to know the 

 origin of the more recent species and to trace as far as is possible their his- 

 tory. By this means one is better able to determine whether the variations 

 and modifications which are observed to arise or which produced experi- 

 mentally are new or simply repetitions of stages in the phylogeny of the 

 material studied. 



To determine the point of origin in a group of organisms already well 

 established over a wide area is by no means an easy task, and only an 

 approximately correct conclusion can be arrived at by indirect methods. 



With the animals under discussion all evidence points strongly to the con- 

 clusion that there has been but a single center of origin for the genus. The 

 place of origin, or " center of dispersal" (Adams, 1902), or ''center of 

 adaptive radiation" (Osborn, 1900), can best be determined by the use of 

 the criteria given by Adams. Aside from the evidence afforded by fossils, 

 evidence wholly lacking in this case, ten criteria are given by this author 

 for the determination of the center of origin, which are more effective than 

 any method of solution hitherto proposed. We will discard at once the 

 notion that because these insects are tropical they necessarily came from South 

 America. This theory of the origin of the fauna of North America deserves 

 less general application. Other areas, as those in Central America and 

 Mexico, have been dry land as long as much of South America and were 

 equally well-adapted to become centers of dispersal. In fact, I suspect that 

 when we know better the fauna of Mexico and Central America we shall 

 find abundant evidence that these regions have been strong centers of origin 

 and dispersal, and that they have supplied both North and South America 

 with many of their characteristic forms. 



The ten criteria for the determination of the center of origin are: 



(i) lyocation of the greatest differentiation of a type. 



(2) lyocation of dominance or greatest abundance of individuals. 



(3) Location of synthetic or closely related forms. (Allen.) 



(4) lyocation of maximum size of individuals. (Ridgway, Allen.) 



(5) lyocation of greatest stability and productiveness in crops. (Hyde.) 



(6) Continuity and convergence of lines of dispersal. 



(7) lyocation of least dependence upon a restricted habitat. 



(8) Continuity and directness of individual variation or modification 

 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. (Palmen.) 



