262 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



does not affect the remaining conclusions of this vokime. To a less degree the same is 

 true of our hypotheses as to the exact mechanism by which the conditions of one climatic 

 zone may be shifted into another. 



All these hypotheses with their varying degrees of certainty are subsidiary to one 

 main conclusion. They fall if it proves untrue, but if they prove untrue its position remains 

 unchanged, for it does not depend upon them. That upon which it does depend is the 

 convergence of a large number of lines of evidence upon the single point of whether the 

 climate of the earth has changed appreciably during the past few thousand years since 

 history began. All the evidence seems to unite in indicating that such a change has taken 

 place, and that it has been of a pulsatory nature. This, then, is our main conclusion, the 

 one point around which all else centers. Doubtless the details as to the time of changes, 

 and especially as to their relation to one another in different parts of the world, will require 

 modification, for we have been able to gather only a small part of the facts. This matters 

 little, however, provided we are headed in the right direction. The only essential is that 

 each new venture shall advance us one short step on that most wonderful of roads which 

 leads to the knowledge of what some men call the law of the universe, and others, more 

 deeply thinking, call the law of God. 



