No. 487] PLEISTOCEHE FLORA OF CANADA 451 



IX, 35) states that it is most abundant and of largest size in south- 

 ern Arkansas and adjacent parts of Texas. It is therefore evi- 

 dent that in spite of its wide distribution and high northern range, 

 it is essentially a southern type, and the evidence it affords is 

 therefore in direct accord with that offered by those other repre- 

 sentatives of the Don flora now recognized for the first time. 



With respect to Acer plcistocenicum, very little of a definite 

 character can be said since we know nothing of it except through 

 its occurrence in the Don clays; but its very definite association 

 with a warm climate flora leads to the conclusion that it also 

 must bear the same relations to meteorological conditions, and 

 that it must of necessity be a southern type. 



Acer torontoniensis is similarly unknown beyond the Don clays, 

 but the same evidence which applies to A. plei.siocenicum must 

 lead to similar conclusions with respect to its climatic relations. 

 If this species is to be regarded as the actual progenitor of the 

 sugar maple, it is perhaps somewhat diflficult to explain satis- 

 factorily how a southern type, or at least a type with a far southern 

 extension, can have become so altered as to constitute an exclu- 

 sively northern type, since the converse would be susceptible of a 

 more ready explanation. If on the contrary, this be regarded 

 as a distinct species with adaptation to a more southern climate, 

 it becomes quite easy to understand how it was obliterated from 

 the Toronto region by the southward movement of the ice sheet, 

 in precisely the same manner that other species were driven out 

 of the same area and ultimately confined to more southern localities. 



The present studies serve to give renewed emphasis to the idea 

 which has now passed beyond the limits of a working hypothesis, 

 that successive northerly and southerly movements of the con- 

 tinental ice sheet, involving corresponding movements in vegeta- 

 tion, were profluctive not merely of plant migrations from north 

 to south and vice versa, but that they established conditions which 

 permanently eliminated those species which, we may suppose, 

 occupied a somewhat unstable position in the flora and were there- 

 fore susceptible to a relatively slight change of surroundings. 

 This conception is in exact accord with the present status of the 

 genus Sequoia which, from a very wide distribution extending 

 over the entire northern half of the continent as far as Alaska and 

 Greenland, has become restricted to a verv limited area on the 



