958 Cretaceous Floras of Canada. 
equivalence in time of similar floras found in different latitudes. 
However equable the climate, there must have been some appre- 
ciable difference in proceeding from north tosouth. If, therefore, as 
seems in every way probable, the new species of plants originated 
on the Arctic land and spread themselves southward, this latter 
process would occur most naturally in times of gradual refrigeration 
or of the access of a more extreme climate, than is in times of the 
elevation of land in the temperate latitudes, or conversely, of local 
depression of land in the Arctic, leading to invasions of northern 
ice. Hence the times of the prevalence of particular types of plants 
in the far north would precede those of their extension to the south, 
and a flora found fossil in Greenland might be supposed to be some- 
what older than a similar flora when found farther south. It would 
seem, however, that the time required for the extension of a new 
flora to its extreme geographical limit, is so small in comparison 
with the duration of an entire geological period that practically, 
this difference is of little moment, or at least does not amount to 
antedating the Arctic flora of a particular type by a whole period, 
but only by a fraction of such period, 
It does not appear that during the whole of the Cretaceous and 
Eocene periods there is any evidence of such refrigeration as seri- 
ously to interfere with the flora, but perhaps the times of most 
considerable warnith are those of the Dunvegan group in the 
Middle Cretaceous and those of the later Laramie and Paleocene. 
It would appear, that no cause for the mild temperature of the 
Cretaceous needs to be invoked, other than those mutations of land 
and water which the geological deposits themselves indicate. 
condition for example of the Atlantic basin in which the high land 
of Greenland should be reduced in elevation and at the same time 
the northern inlets of the Atlantic closed against the invasion of 
Arctic ice, would at once restore climatic conditions allowing of ms 
growth of a temperate flora in Greenland. Dr. Brown has shown, 
and, as I have elsewhere argued, the absence of light in the am 
winter is no disadvantage, since, during the winter, the growth 0 
deciduous trees is in any case suspended, while the constant con” 
tinuance of light in the summer is, on the contrary, a very great 
stimulus and advantage. 
1 Florula Diseana. 
