92 THE PLANT LIFE OF MARYLAND 



quently in favorable habitats in the Coastal Zone. In Charles and 

 St. Mary's counties, about the head of the Wicomico River, are 

 narrow ravines in which Susquehanna gravel is underlaid by an 

 extremely compact and impermeable gray clay of Miocene age, 

 analagous to underlying rock in its relation to soil water movement. 

 This causes the soil conditions to resemble closely those of ravines 

 in the Midland Zone, and here may be found such species as Cercis 

 canadensis, Hamamelis virginiana, Hydrangea arborescens, Cimici- 

 fuga racemosa, Heuchera americana and Carex plalypliylla, all of 

 which are rare in the Coastal Zone. These facts indicate that the 

 texture of the soil and its topographic position largely determine the 

 occurrence of species which, were it not for tlie existence of these 

 favoring habitats of restricted area, might be presumed to be absent 

 from extended districts. 



Any study of the bounds of species must keep in view the three 

 sets of factors which are operative in determining the general area 

 of distribution of plants — namely the historical, climatic and 

 edaphic, the last comprising all the factors immediately determined 

 by the physical and chemical characteristics of the soil. The his- 

 torical considerations which have to do witli the range of a species 

 relate to its phylogenetic origin, its migration and its distribution at 

 successive geological periods. The nature of the area of distribution 

 of a number of the Coastal species of Maryland at the present day 

 is such as to indicate that they have migrated northward in the 

 ( Joasta'l Plain from the South Atlantic states since the most recent 

 elevations of the Coastal Plain. These are species which had their 

 origin in the South Atlantic states or had been driven thither during 

 the Glacial Epoch. The genus Liquidambar was represented during 

 pre-glacial time by several species of wide distribution through the 

 northern hemisphere, yet Liquidambar styraciflua is at present con- 

 fined to the Coastal Plain, at least east of the Alleghany Mountains. 

 Glethra alnifolia is known from the Interglacial beds of the Don 

 Kiver Valley, ( lanada, which points to the more extensive distribu- 

 tion of this species during the era of mild climate between the two 

 periods of maximum glaciation. than at present. Liquidambar and 

 Clethra are examples of species which sought a retreat in the soiith- 

 eastern states during Glacial time and have since migrated north- 



