112 EVOLUTION OF THE EARTH 



relative length, the slope of the tangent drawn to its peaks 

 would correspond more nearly to those of the Paleozoic. 



In the climatic line, the minor undulations indicate in a very 

 general way the climatic oscillations which must have existed 

 but of which we have as yet no very exact data; the larger 

 movements, on the other hand, are based upon observed fact. 

 The optimum embraces a normal range of temperature and 

 degree of moisture; a rising curve, increased aridity; its de- 

 pression, a lowering of temperature. Where the line is single 

 there is a supposed uniformity of conditions the world over; 

 where It splits, aridity and cold are differentiated. It Is highly 

 probable that with more detailed knowledge the divergence of 

 the moisture and temperature curves will be more extensive. 



The hfe line Is dotted where theory precedes the recorded 

 fact, solid when actual fossils attest the truth of the assumed 

 evolution. The numbers Indicate the times of these first 

 records as follows: i, first vertebrates; 2, lung-fishes; 3, verte- 

 brate footprints; 4, reptiles; 5, dinosaurs; 6, mammals; 7, 

 birds; 8, amphibious dinosaurs; 9, archaic mammals; 10, mod- 

 ernized mammals; 1 1, man. {Vide infra, page 144.) 



Crises of Evolution 

 Origin of life. The first great crisis In the evolution of 

 organic beings was the origin of life: the marvelously subtle 

 combinations attained by certain very familiar Inorganic ele- 

 ments, increasing their molecular complexity until a substance 

 was produced endowed with the attributes of life. Of this 

 momentous event we have no record, nor does the geologic 

 cause come either within the scope of our knowledge or con- 

 jecture, for the time was too remote and the first living sub- 

 stance too delicate to leave any decipherable record upon the 

 rocks. All we can say of It Is that In the fullness of time, w^hen 

 the earth had, In the course of Its physical evolution, become 

 adapted as the abode of life, living substance came Into being. 



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