No. 544] 



BOTANICAL SOCIETY OF AMERICA 



213 



conclusion is entirely without geological or paleobotan- 

 ical warrant, and that the most pronounced climatic dif- 

 ferentiation the world has known extends only from the 

 Pliocene to the present. As a matter of fact we of to-day 

 are living in the glacial epoch in what possibly is only an 

 interglacial period, and we know that the time which has 

 elapsed since the close of the last ice-invasion has been of 

 less duration than was one, and possibly two, of the Pleis- 

 tocene inter-glacial periods. We also know that the cli- 

 mate was milder during these inter-glacial intervals than 

 has obtained since the final retreat of the ice, as shown by 

 the fact that in eastern North America certain species of 

 plants then reached a point some 150 miles further north 

 in the Don Valley than they have since been able to 

 attain. The development of strongly marked climatic 

 zones, at least between the polar circles, is, then, "excep- 

 tional and abnormal, and we have no evidence that in any 

 other post-Silurian period, with the possible exception of 

 the Permo-Carboniferous period, has the climatic distri- 

 bution and segregation of life been so highly differen- 

 tiated and complicated as in post-Tertiary times." 1 



The regular and normal conditions which have existed 

 for vastly the greater part of geologic time, have been 

 marked by relative uniformity, mildness and comparative 

 equability of climate. This is abundantly shown by the 

 almost world-wide distribution and remarkable uniform- 

 ity of the older floras: When, for instance, we find the 

 middle Jurassic flora extending in practical uniformity 

 from King Karl's Land, 82° N., to Louis Philippe Land, 

 63° S., we have conditions which not only bespeak a prac- 

 tically continuous land-bridge, but exceptionally uniform 

 climatic conditions. To have made this possible there 

 could have been neither frigid polar regions nor a torrid 

 equatorial belt, such as now exist. The absence of growth- 

 rings in the stems of these plants, as well as the presence 

 of such warmth-loving forms as cycads and tree-ferns, 

 point to the absence of seasons and the presence of mild 

 and equable climatic conditions. 



