352 GEORGE E. NICHOLS 



such as lakes and ponds, also progress toward mesophytism." 

 The formations of these two series are further grouped as pri- 

 mary and secondary. 



Considered next with reference to the larger topographic 

 features^ of the region, the edaphic formations of the xerarch 

 series are divided into three groups: (1) The formation-types of 

 ordinary uplands; (2) The formation-types of uplands along 

 streams; (3) The formation-types of uplands along the seacoast. 

 The edaphic formations of the hydrarch series are similarly 

 grouped under two heads: (1) The formation-types of inland 

 lakes and swamps; (2) The formation-types of lakes and swamps 

 along the seacoast. They might equally well be divided into 

 four groups: (1) The formation-types of glacial lakes and 

 swamps ; (2) The formation-types of sink-hole lakes and swamps ; 

 (3) The formation-types of river lakes and swamps; (4) The 

 formation-types of coastal lakes and swamps; except that river 

 lakes and swamps are scarcely represented in this region. 



Finally, the association-complexes which comprise the vari- 

 ous edaphic formations are further grouped with reference to 

 whatever specific physiographic unit areas it seems best to dis- 

 tinguish. Under this head are considered the character and 

 the successional relations of the individual associations and 

 association-types. This latter part of the scheme in particular 

 is so elastic and so capable of modification that it may be readily 

 adapted to the special requirements of any region and to the 

 individual views of any investigator. 



(1) Adams, C. C, An outline of the relations of animals to their inland en- 



vironments. Bull. 111. State Lab. Nat. Hist. 11: 1-32. 1915. 



(2) Brockmann-Jerosch, H. and Rxjbel, E., Die Einteilung der Pflanzen- 



gesellschaften nach okologisch-physiognomischen Gesichtspunkten, 

 pp. 1-72. /. 1. Leipzig. 1912. 



(3) Clements, F. E., The development and structure of vegetation. Bot. 



Surv. Nebraska, Rep. 7, pp. 1-175. Lincoln. 1904. 



(4) Clements, F. E., Plant succession. Carnegie Inst. Wash., Publ. 242. 



pp. I-XIII + 1-511. PI. 1-61. 1916. 



2 A similar physiographic classification on the basis of soil might be made, 

 should this be considered of more importance than topography. 



