of Secular Changes of Climate. 93 



have resulted had the distribution of land and water been 

 different from what it is now, that I ought on this account to 

 be charged with undervaluing the importance of geographical 

 conditions. 



Mr. Wallace refers to one case of a difference in geogra- 

 phical conditions which he thinks might have aided glacia- 

 tion. Prof. Dana has expressed the opinion that, during the 

 height of the glacial epoch, North-eastern America was con- 

 siderably elevated, bringing the wide area of the banks of 

 Newfoundland far above water. This, Mr. Wallace thinks, 

 would reduce the southward-flowing Arctic currents, causing 

 the icebergs to hang about the American shores, chilling the 

 air so as to produce constant fogs and clouds with almost 

 perpetual snow-showers, even at midsummer. But Prof. 

 Dana has also shown that during the glacial epoch North- 

 eastern America was depressed as well as elevated. Now the 

 point is, whether the elevation was contemporaneous with the 

 cold, or with the warm periods of the glacial epoch ? Mr. 

 Wallace himself admits that depression, not elevation, of 

 the land accompanied the increased cold ; and he quotes 

 Mr. Searles V. Wood, jun., approvingly as holding the same 

 opinion (p. 115). It was quite natural for Prof. Dana to 

 suppose that the elevation to which he refers occurred at the 

 time the country was buried under ice ; for when he wrote 

 he believed the glacial epoch was chiefly due to elevation of 

 the land caused by the lateral pressure resulting from the 

 shrinking of the earth's crust. It is now, however, pretty 

 well established that the continental or elevated periods of 

 the glacial epoch, when our island was united to the main- 

 land, were warm periods ; for it was then that this country 

 was invaded by tropical and subtropical mammalia. Had the 

 climate at that time been cold, and the country even partially 

 covered with snow and ice, these animals would not have 

 made their appearance. It is therefore probable that the 

 elevation to which Prof. Dana refers may have taken place 

 during some of those warm periods. But be this as it may, 

 even were it proved that during the glacial epoch geographical 

 conditions were more favourable for the formation of ice than 

 the present, this would not affect the general conclusion at 

 which I wish to arrive. 



Trusting that these preliminary considerations may tend to 

 remove the partial confusion in which this somewhat complex 

 subject has been involved, I shall now proceed to examine Mr. 

 Wallace's main argument. I shall consider it, first, in rela- 

 tion to physical principles, and, secondly, in relation to geolo- 

 gical and paiaeontological facts. 



