PALEOBOTANY AND GEOLOGV KNOWLTON. 357 



exist, are normal or essential, and that they were present without 

 marked differences during all geological ages. It is now established, 

 however, that this conclusion is entirely without geological or paleo- 

 botanical warrant, and that the most pronounced cHmatic differen- 

 tiation 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 

 Pleistocene interglacial periods. We also know that the climate was 

 milder during these interglacial intervals than has obtained since the 

 final retreat of the ice, as showTi by the fact that in eastern North America 

 certain species of plants then reached a point some 150 miles farther 

 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, "exceptional 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 dis- 

 tribution and segregation of life been so highly differentiated and 

 complicated as in post-Tertiary times." ^ 



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 uniformity of the older floras. When, for instance, we 

 fijid the middle Jurassic flora extending in practical uniformity from 

 Kmg Karls Land, 82° N., to Louis Philippe Land, 63° vS., we have 

 conditions which not only bespeak a practically 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. 



Another example of similar import is afforded by the early Penn- 

 sylvaniau flora; that is, the flora of the lower part of the Upper 

 Carboniferous. Wherever tcrrigcrous beds of this age have been 

 discovered, representatives of this peculiar flora, which includes such 

 common genera as Lepidodendron, Sigillaria, SphenophyUum, etc., 

 have been found, this distribution rangmg from wSouth Africa to 

 Brazil and Argentina, and thence over the northern hemisphere. 



vSimilarly, the iMississippian flora (Lower Carboniferous) has been 

 found in Spitzbergcn, Greenland, and arctic Alaska, and thence 



' See White and Knowlton, Science, n. s., vol. 31, 1910, p. 760. 

 85360°— SM 191:: 24 



