MARYLAND WEATHEK SERVICE 93 



ward along the newly emerged Coastal Plain. Among the numerous 

 species endemic to the pine barrens of the southern Coastal Plain are 

 doubtless many which have originated since the Glacial Epoch.* 



The present limit of distribution of species coextensive with the 

 Coastal Plain is determined by their manner and rate of migration 

 quite as much as by any existing external factors. The sharpness 

 with which these species are limited by the "Fall-Line" points to 

 their northward migration since Glacial time. The limitation of 

 species by climatic factors is largely, though not solely, a matter of 

 temperature. The gradually decreasing temperature of the growing 

 season encountered in passing northward in the Coastal Plain is 

 paralleled by the gradual disappearance of species.' In spite of the 

 importance of the temperature element of the climate in relation to 

 the general area of distribution of plants, it is not possible to ascribe 

 the limiting of the Coastal Zone species to this or to any combination 

 of climatic factors. The edaphic conditions in the Coastal Zone and 

 the Midland Zone are in sharp contrast owing to the presence of 

 underlying rock in the former, the greater predominance of sands 

 and sandy loams in the former and of loams and clays in the latter, 

 and the totally different topography in the two. 



The facts already related as to the importance of edaphic factors 

 in conditioning the occurrence of Coastal species in the Midland 

 Zone and rice versa, are not without bearing on the case of the species 

 entirely restricted to the Midland and Coastal Zones. The complete 

 limitation of species to the Coastal Zone is but a step further in the 

 operation of the same laws that limit the Coastal species to restricted 

 habitats when they occur in the Midland. The broader lines of 

 distribution are drawn by historical and climatic factors, the minor 

 details of the limitation of distribution, as well as of local occurrence, 

 are due to edaphic conditions. Where a sharp line of demarcation 

 occurs between regions of different edaphic character, as the "Fall- 



*The criteria which serve to indicate the place of origin and paths of 

 migration of species have heen well summed up by Adams, C. C, The South- 

 eastern United States as a centre of Geographical Distribution of Flora and 

 Fauna, Biological Bulletin, Vol. III., No. 3, 1902, pp. 115-134. 



tFor exact data as to the northern limit of distribution of southern 

 Coastal Plain species see: Kearney, Thomas H., Report on a Botanical Sur- 

 vey of the Dismal Swamp Region. 1901, pp. 451-457. 



