44 



is in every case at least a hydrophytic and a xerophytic extreme, 

 forming- two general converging lines of succession. In our present 

 knowledge of the subject, it is impossible to state whether there is 

 one definite climax association in each province; it seems probable 

 that there are several such associations, each characteristic of a lim- 

 ited portion. It is certain that in each province there is a dominant 

 forniatioji, or type of vegetation, deciduous forest, coniferous forest, 

 or prairie, as the case may be. Present evidence seems to indicate that 

 the nature of the dominant type is determined by a long chain of his- 

 torical factors (Adams, ip02, ipoj) and its present areal distribu- 

 tion by the broader existing climatic factors, notably heat and rain- 

 fall (Transeau, ipo^). 



The boundaries of the four provinces have been subject to great 

 changes in the past, both during and following the glacial period, as 

 the ice swept to the south, overthrowing the previous conditions of 

 climate, and then retreated to the north, uncovering unoccupied 

 ground and throwing it open to plant invasion. The ensuing move- 

 ments of vegetation were among the greatest in the history of the 

 continent, and have been of the greatest moment in determining the 

 present distribution of the biota. 



These movements have by no means ceased. They are merely 

 less obvious when measured in terms of years and centuries rather 

 than in geological periods. Even now a biotic migration is in prog- 

 ress, which is probably the direct continuation of early postglacial 

 movements, and is doubtless as rapid and as far-reaching in its effects 

 as any of the past. 



In the present migration the vegetation of the Deciduous Forest 

 Province is the chief factor. It is now pushing out its boundaries to 

 the north and west and enlarging its area at the expense of the 

 Northeastern Conifer Province on one side and the Prairie Province 

 on the other. Some detailed features of the northern extension have 

 been given by Whitford (igoi), Transeau (iQO^-o6), and others, 

 and summarized by Adams (ipo^^). The westward migration has 

 been mentioned by many, but scarcely described in detail. 



The actual steps in the migration of the vegetation are due to a 

 series of successions, by which associations of the prairie or of the 

 coniferous forest are replaced by others, with similar environmental 

 demands, from the deciduous forest. Some of the northern and 

 western associations are succeeded with comparative ease; others 

 resist succession for long periods of time. Because of this the forest 

 extends north and west, not in continuous masses but in long tong-ues 

 and detached bodies, while relics of the former vegetation lag be- 



