1904.] NATURAL SCIENCES OF PHILADELPHIA. 601 



THE COMPARATIVE AGE OF THE DIFFERENT FLORISTIC ELEMENTS OF 

 EASTERN NORTH AMERICA. 



BY JOHN W. HARSHBERGER, PH.D. 



The historic element must be considered in a phytogeographic study 

 of any country. Many questions concerning the present distribution 

 of plants depend upon the character and extent of the past distribu- 

 tion of the species of any region or formation. The degree of invasion 

 of new species into a region is determined by the presence or absence 

 of vegetation. If vegetation is present, then the botanist is compelled 

 to account for its presence by a consideration of the physiographic 

 history of that part of the earth's surface, the association of species 

 and the probable origin of these species, whether indigenous or derived. 

 The determination of the indigenous and derived species of a formation 

 or larger division is of the utmost importance, as it enables us to retrace 

 the steps by which the formation has reached its present condition 

 and association of species, and to reconstruct formations that have 

 long since disappeared. 



The methods which must be adopted in scrutinizing the flora of a 

 country are several: 



1. The botanist must determine the past and present physiography 

 of the region concerned. 



2. He must determine, if possible, the geologic time at which the 

 recorded physiographic changes took place. 



3. He must recognize the indigenous species by eliminating the 

 derived. 



4. A study of the distribution of species will enable him to determine 

 to some extent the age of the different fioristic elements, and the appli- 

 cation of the following criteria will also aid him in the solution of 

 questions such as are considered here. 



a. Location of greatest differentiation of type. 



b. Location of dominance or great abundance of individuals. 



c. Location of synthetic and closely related forms. 



d. Location of maximum size of individuals. 



e. Location of greatest productiveness and its relative stability. 

 /. Continuity and convergence of lines of dispersal. 



g. Location of least dependence upon a restricted habitat. 

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