QUATERNARY 683 



been preserved. The ginkgo tree (p. 567), for example, would have 

 been to us an extinct species if man had not preserved it by cultivation. 

 (4) Not only has his contact with the life of the world been important, 

 but his indirect effect upon inanimate nature has been stupendous. 

 The cutting and burning of forests in certain regions has resulted in the 

 rapid erosion of large areas, and the pulverization of the soil in plow- 

 ing has permitted rainwash to carry away the best of the soil. By 

 deforestation alone a single lumber merchant may in 50 years deprive 

 the human race of soil that required thousands of years to form. 

 Another effect which will eventually greatly lessen the fertility of the 

 soil is the enormous and irrecoverable loss of phosphates in the 

 sewerage of cities. As the result of these and many other effects of 

 man's supremacy, the earth has suffered a vastly greater change in 

 the past few hundred years than in many thousands of years in the 

 most destructive periods of the past. 



Future Habitability of the Earth 



Two statements are often made concerning the future of the 

 earth : one that the climate will become progressively cooler until 

 it will be unsuited for the existence of plants and animals; the 

 other, that the earth will eventually be consumed by fire. The 

 former statement is based on the assumption that the heat of the 

 sun is diminishing, and that, since the earth depends upon it for 

 its heat, a cooling of the sun will cause refrigeration. There is no 

 question but that this would be the case were the sun's heat to 

 decrease, but no such change can be detected. The present heat of 

 the sun is apparently maintained by the infalling of meteorites, as 

 well as by that given off by radioactive minerals, and, consequently, 

 sufficient heat to produce a favorable climate may exist for many 

 millions of years. A convincing proof lies in the fact that, since life 

 began, the climate has not changed sufficiently to cause a widespread 

 destruction of life. Periods of aridity, glaciation, and other climatic 

 changes have frequently occurred, but none that was universally fatal. 



The statement that the earth will eventually be consumed by fire 

 assumes that the sun or earth may collide with some other star. No 

 such catastrophe, however, has occurred in the past, none seems to 

 be impending. It seems safe, consequently, to predict that for many 

 years — hundred of thousands, perhaps millions — the conditions 

 favorable to man's existence will be present. 



