Shaler.] 180 [October 6, 



earth's conditions, we need only changes which would not materially 

 alter the brilliancy of our sun, as seen from the distance at which we 

 behold the fixed stars. The heat of the sun could be increased by 

 twenty-five per cent, without materially changing the magnitude of 

 the sun as a star. 



In a previous communication to this Society, 1 I have called atten- 

 tion to this cause as a possible source of the climatic changes of the 

 glacial period, the invasion of ice being brought about by the sudden 

 increase of the precipitation of water in high latitudes, due to an 

 increase of heat and a consequent increase of rainfall. If we take 

 this view of the cause of the glacial conditions, then the existence of 

 evidence of the diminution of rainfall during the time since the close 

 of the glacial period, becomes a matter of the greatest interest. 

 While the whole question is involved in the greatest doubt, as I have 

 tried to show in the first part of these notes, I am inclined to think 

 that there is some evidence to be drawn from the physical record left 

 in our salt lake basins to indicate the great probability of a diminu- 

 tion of rainfall since the last ice time. 



Besides this physical evidence of the change in rainfall, the palae- 

 ontolooical record supplies us with some evidence of a valuable kind, 

 looking in the same direction. Whenever we trace back the history 

 of any of our land mammals, we generally find the variety of repre- 

 sentative species which was in existence during, or just at the close 

 of the o-lacial period, showing by its size or by its distribution that the 

 conditions of environment were those which gave a very abundant 

 supply of food. These conditions could not have been those brought 

 about by greater mean annual cold, but must have been the result of 

 climatic conditions, such as would be caused by greater rainfall, and 

 less rano-e of temperature between winter and summer. As I pro- 

 pose to extend these considerations in a special paper on the subject, 

 I will not cite the instances which support this opinion. 



In various discussions of this subject, I have attributed the great 

 transportation of water from the equatorial to the polar regions, as- 

 sumed to have occurred during that period, to the increased difference 

 of temperature between these regions during, at least, the first stages 

 of a o-lacial period, and the consequent increase in the activity of the 

 trade winds; it being assumed that, owing to the formation of a 

 cloud-wrap about each pole, the equator would gain more in heat 

 from an increase in the heat of the sun, than the circumpolar regions. 



1 See Memoirs of the Boston Society of Xatirral History, Tol. II, Pt. in, No. 3. 



