EARTH 9 



of the thousands of invertebrates that now exist in the 

 sea, because their evolution has been conditioned by the 

 continuous accumulation in sea water of the large quan- 

 tities of salts leached out of ninety-five vertical miles of 

 the earth's crust. It is even less probable that it would 

 ever have taken the direction of vertebrate evolution and 

 produced the air-breathing fishes, the terrestrial four- 

 footed animals, or the warm-blooded birds and mam- 

 mals. Except for mountains (and perhaps the moon) fife, 

 if any, would be very different— how different can be 

 left to the speculations of the more imaginative writers 

 of science fiction. 



We can confidently say that earth history would have 

 pursued its course without the aid of any hving organ- 

 isms. But the history of living organisms has been shaped 

 at every turn by earth's vicissitudes, because every geo- 

 logic upheaval, by causing profound changes in the dis- 

 tribution of land and sea, has had profound effects on 

 the climates of both, and hence on the patterns of Hfe 

 in both. 



Climate has always been much as it is now— better 

 and worse. It is determined by many complexly interact- 

 ing factors, among which winds and oceanic ciurents are 

 probably the most important. Astronomical factors, such 

 as periodic changes in the earth's orbit and angle of ro- 

 tation, have played a part, but these are short-term fac- 

 tors and at their peak probably have contributed less 

 to climatic changes than have the great cycles of con- 

 tinental elevation. By cutting off moistmre-laden winds, 

 by diverting oceanic currents, and in many other ways, 

 these mountain-building episodes have repeatedly modi- 

 fied the climate of almost the entire surface of the globe. 

 Periods of maximal continental elevation have generally 

 coincided with periods of maximal glaciation, in which 

 the thick ice extended in places almost down to the sea 

 in what are now temperate or even tropical latitudes- 

 periods which were accompanied by a decrease in the 



