464 PROCEEDINGS OF THE ACADEMY OF [April, 



POST-GLACIAL NEARCTIC CENTRES OF DISPERSAL FOR REPTILES. 



BY ARTHUR ERWIN BROWN. 



If lasting values are to be reached in the study of geographical dis- 

 tribution, its conclusions must accord with all else that is know^^ of 

 the dynamics of evolution. Or, stated with more- exact reference to 

 its especial problems, the areas marked off by it must correspond to 

 the distributional relations of genetically connected species and, at 

 the least, must not be at fault with possible Imes of dispersal from 

 centres of development by which they may have come to occupj^ their 

 present range. What is proposed here is an attempt to determine such 

 centres for Reptilia in North America. 



The scarcity of Tertiary remains of reptiles belonging to existing 

 groups confines the student of geographical distribution in the main 

 to post-glacial conditions, with little more knowledge of those pre- 

 ceding than the practical certainty that genera now widely ranging 

 are of great antiquity and were likewise wide ranging in earlier periods, 

 and that a former north Atlantic land connection between Europe 

 and America must have coincided with a climate of sufficient warmth 

 to serve for the passage of reptiles belonging to genera now common 

 to both. 



In other ways the problem is simplified by the absence in later periods 

 of a circumpolar reptilian fauna, a consequence of which is that the 

 chief zoological bond of connection between the eastern and western 

 continents is wanting, and for reptiles a "Holarctic" region or an 

 "Arctogsean" realm can not be said to exist, the families and genera 

 common to more southern portions of both being insufficient to 

 link them. For this reason Mr. Sclater's term ''Nearctic" is used 

 here. It should be said, however, that if for purposes of convenience or 

 uniformity it be desired to retain these later and broader generaliza- 

 tions, no harm is likely to ensue if it be remembered that they do not 

 express the exact facts of present reptilian distribution, whatever they 

 may have been during the Tertiary. 



A further simplification results in northern regions from this same 

 absence of boreal reptiles, for it eliminates the meeting ground of a 

 northern with a southern migration, which constitutes the "Transition" 



