68 ROYAL SOCIETY OF CANADA 



knowledge of the succession of the floras attendant upon movements 

 of the ice sheet, and it is in this connection that the present specimen 

 acquires additional interest. The Toronto deposits show three well 

 defined phases im the migration of plants during glacial times. In 

 the first period the vegetation of the entire region was of a much more 

 southern type than that which now flourishes in the same region. It 

 WHiS, in fact, such as now characterizes the Middle United States, 

 embracing such types as Asimina triloba and Madura aurantiaca. As 

 the ice sheet subsequently moved southward it drove before it all these 

 mild climate forms, and brought about the introduction of more boreal 

 forms, in which, the northern poplars and spruces and a variety of mosses 

 were especially prominent, so that the vegetation as a whole was like 

 that of northern Quebec and Labrador as we know it to-day. Finally 

 the ice sheet once more retreated to the position it now occupies, and 

 in so doing it favoured a northward migration of the vegetation, which 

 was once more restored to a condition about midway between that of 

 the first and second periods, thus giving the flora which has continued 

 to the present day. 



One of the best evidences of the extent to which this northward 

 migration developed is to be found in the re-e&tablishment of the com- 

 mon papaw or Asimina triloba along the shores of Lake Ontario. Dur- 

 ing the first or Don period, characterized by a warm climate, this shrub 

 was a somewhat common elemenit of the flora. In the second or 

 Scarborough period, characterized by a cold climate, it seems to have 

 disappeared completely, and we as yet have no record of its occurrence 

 in the third or Greenes Oreek period, characterized by a mild climate, 

 though it does now occur somewhat sparingly in the same region where 

 it reaohes its highest northern limit of distribution. From the shores 

 of Lake Ontario the papaw ranges southward through New York and 

 far beyond, with a continuity which does not afford amy means of 

 determining the actual extent of the migration. 



Directing our attention to the osage orange — Madura aurantiaca — 

 it is possible to gain somewhat more definite information as to the 

 range of such migrations. This shrub was a well-defined element of 

 the Toronto flora in the Don period, and it therefore flourished abun- 

 dantly in latitude 43° 15' N". Taking the present northern limit of 

 distribution as defined by the northern limit of Kansas, in latitude 40° 

 IST., there is an actual interval of about 224 miles between such northern 

 extension as represented in Pleistocene time and now. This, then, 

 gives an approximate measure of the extent of the migrations attendant 

 upon movements of the ice sheet, but only approximately, because there 

 is some reason for the belief that the range of the osage orange may 



