DAEWIN AND GEOLOGY 35 1 



muds tranquilly accumulating on the bottom. All conditions go to 

 show that the mammalia became extinct after the sea had received 

 its present fauna; and there is nothing to suggest that a period of 

 overwhelming violence swept away and destroyed the inhabitants of 

 the land; everything supports the contrary belief. The only note- 

 worthy change in conditions has been a gradual elevation of the con- 

 tinent; but that was not enough to modify the climate or to bring 

 about a change in the land fauna. 



Several of the important genera collected by Darwin had been 

 found in North America long prior to his time. This similarity of 

 the Quaternary faunas induced him to speculate on the causes which 

 had divided the American continent into two well-defined and some- 

 what contrasting zoological provinces. He does not hesitate to sug- 

 gest recent elevation of the Mexican platform or more probably, 

 recent submergence of the West Indian Archipelago as a conceivable 

 cause of this separation. It seems to him most probable that the 

 elephants, the mastodons, the horses and the hollow horned rumi- 

 nants of North America " migrated, on land since submerged near 

 Behring "Straits, from Siberia into North America, and thence, on land 

 since submerged in the West Indies, into South America, where for 

 a time they mingled with forms characteristic of that southern con- 

 tinent and have since become extinct." Had this American Museum 

 of Natural History existed in Darwin's day, study of the remarkable 

 exhibits in its Mammalian Hall would have enabled him to extend 

 his list of extinct forms common to both continents, and possibly he 

 might have anticipated some of the all-important generalizations for 

 which the world is indebted to the former president of this academy, 

 who now is president of the museum. 



Nothing in South America, east or west, escaped Darwin; from 

 glaciers to peat bogs, from earthquakes to climatal variations, every- 

 thing was important; but what impressed him most on both sides of 

 the continent were the evidences of extremely slow secular movement 

 in the earth's crust. This was the preparation for that study of the 

 coral islands which resulted in his chief contribution to philosophical 

 geology. 



Many voyagers prior to 1833 had observed and had tried to account 

 for the strange atolls, or low ring-like coral reefs, each enclosing a 

 lagoon which communicates with the sea by a narrow channel; but 

 Darwin discovered other forms of reefs which were equally perplexing. 

 Many islets of rock are fringed by coral growth, while vast barrier reefs, 

 separated from the land by channels of varying depth, extend at times 

 for hundreds of miles along coasts. All explanations by previous 

 observers were defective as they seemed to ignore these types as well as 

 other features, not less important. 



