538 F. H. KXOWLTOX EVOLUTIOX OF GEOLOGIC CLIMATES 



tologists — in fact, I am at a loss to know how the data available can be 

 otherwise interpreted. 



Assuming, as we now seem justified in doing, that this cardinal fact 

 has been established, the human mind is so constituted that it is not satis- 

 fied to stop at this point, but it must seek, if possible, an explanation of 

 the cause or causes that gave rise to this marked difference between the 

 pre-Pleistocene and subsequent climatic distribution. In other words, 

 Why was there no climatic zoning in the age before the Pleistocene? 

 There must be a reason, if we can but find it. 



In seeking an answer to this question, I may confess that I have l^een 

 led far afield — much farther, in fact, than I had originally intended to 

 go. As present-day climatic differentiation is so dependent on atmos- 

 pheric conditions and variations, there is no logical stop, in looking back- 

 ward over the climatic conditions that have obtained in the successively 

 older and older geologic ages, short of a consideration of the original 

 atmosphere; and, having reached this point, logic urges a consideration 

 of the earth on which it rested, perhaps including the origin of the earth 

 itself. I hasten to add, however, that it is not my intention to venture a 

 full discussion or analysis of these profound problems for two good and 

 sufficient reasons, not the least of which is lack of time. 



The zonal distribution of climate as we know it today results from the 

 direct, though variously modified, control by the sun. If the earth was 

 without an atmosphere and had a homogeneous surface throughout, the 

 distribution of heat would be strictly by latitude, producing what has 

 been called solar climate. But the problem is by no means so simple, for 

 solar climate is modified and delimited by a number of important factors, 

 such as the presence of an atmosphere containing water vapor, carbon 

 dioxide and other gases in varying amounts, together with dust particles 

 from various sources. Solar climate is also further modified to a marked 

 extent by certain physical features of the earth itself, such as continents, 

 plateaus, mountains, deserts, oceans, etcetera. 



It appears to have been a pretty general assumption, especially in late 

 years, that the sun exercised approximately the same control over earth 

 atmosphere, surface temperature, or climate in past geologic ages as it is 

 kno"\^Ti to do at present. If this is true it would be impossible to escape 

 the conviction that climates should have been disposed in zones through- 

 out the whole of geologic history — that is to say, this result would have 

 followed inevitably and of necessity if the sun had dominated earth at- 

 mosphere as it now does; but, if the biologic criteria of the past have 

 been correctly interpreted, the sun did not so dominate. Or, to be on 

 the safe side, we should perhaps say that the sun's dominancy has, at 



