CALIFORNIA ACADEMY OF SCIENCES. 239 



Iii searching for a common cause of variations affecting 

 the west slopes of both continents similarly, we arrive at the 

 conclusion that it is change of climate, produced b}* tertiary 

 and recent geological action, and are obliged to admit that 

 such action is still going on. 



That the Atlantic slope of the United States has not un- 

 dergone such changes recently, enough to influence the land 

 shells, is also a rational conclusion, although facts point to 

 the probable increase in number of forms by variation as 

 still going on there, since many living species are not known 

 from the post-pliocene strata. 



Prof. J. D. Whitney, in his late elaborate work on changes 

 of climate in the succession of tertiary epochs, has endeav- 

 ored to prove that the decreasing heat of the sun has been 

 the chief cause of a gradually colder and drier climate, both 

 in Europe and America. It seems however that the effects 

 of such a decrease would have affected both sides of this 

 continent more equally. The colder and drier climate of 

 the western slope may be sufficiently explained without such 

 a cause, by the gradual elevation higher and higher of the 

 interior mountains and plateaux, as well as oscillations both 

 up and down of the coast, not only within our borders, but 

 in the arctic and tropical zones. 



The influence of the vast volcanic eruptions at the end of 

 the pliocene epoch all through the western half of the con- 

 tinent, both on climate and organic life, must have been 

 immense. An idea of their action may be obtained from 

 the ingenious article by J. E. Clayton, M. E., in our Pro- 

 ceedings, Vol. VI, p. 123. 



The eastern slope, on the contrary, shows evidences of 

 changing from a colder climate to a warmer during the 

 quaternary epoch, the glacial drift southward, which pre- 

 ceded the recent period, having there followed after the 

 tropical climate of the pliocene, when the gigantic extinct 

 Edentata, etc., existed. It is not intended, however, to 

 discuss this subject here, except as relates to mollusca. 



