26 BEPORT — 1886. 



quantities of it would remain undissolved in summer, and thus an accu- 

 mulation of permanent ice would take place, along the American coast 

 at first, but probably at length even on the European side. This would 

 still further chill the atmosphere, glaciers would be established on all the 

 mountains of temperate Europe and America,' the summer would be kept 

 cold by melting ice and snow, and at length all Eastern America and 

 Europe might become uninhabitable, except by arctic animals and plants, 

 as far south as perhaps 40° of north latitude. This would be simply a 

 return of the Glacial age. I have assumed only one geographical change ; 

 but other and more complete changes of subsidence and elevation might 

 take place, with effects on climate still more decisive ; more especially 

 would this be the case if there were a considerable submergence of the 

 land in temperate latitudes. 



We may suppose an opposite case. The high plateau of Greenland 

 might subside or be reduced in height, and the openings of Baffin's Bay 

 and the North Atlantic might be closed. At the same time the interior 

 plain of America might be depressed, so that, as we know to have been 

 the case in the Cretaceous period, the warm waters of the Mexican Gulf 

 would circulate as far north as the basins of the present great American 

 lakes. In these circumstances there would be an immense diminution of 

 the sources of floating ice, and a correspondingly vast increase in the 

 surface of warm water. The effects would be to enable a temperate flora 

 to subsist in Greenland, and to bring all the present temperate regions 

 of Europe and America into a condition of subtropical verdure. 



It is only necessary to add that we know that vicissitudes not dis- 

 similar from those above sketched have actually occurred in compara- 

 tively recent geological times, to enable us to perceive that we can dis- 

 pense with all other causes of change of climate, though admitting that 

 some of them may have occupied a secondary place. ^ This will give us, 

 in dealing with the distribution of life, the great advantage of not being 

 tied up to definite astronomical cycles of glaciation, which may not 

 always suit the geological facts, and of correlating elevation and sub- 

 sidence of the land with changes of climate affecting living beings. It will, 

 however, be necessary, as Wallace well insists, that we shall hold to that 

 degree of fixity of the continents in their position, notwithstanding the 

 submergences and emergences they have experienced, to which I have 

 already adverted. Sir Charles Lyell, more than forty years ago, pub- 

 lished in his ' Principles of Geology ' two imaginary maps which illus- 

 trate the extreme effects of various distribution of land and water. In 

 one all the continental masses are grouped around the equator. In the 



* According to Bonney, the west coast ofWales is about 12° above the average for 

 its latitude, and if reduced to 12° below the average its mountains would have large 

 glaciers. 



- More especially the ingenious and elaborate arguments of Croll deserve con- 

 sideration ; and, though I cannot agree with him in his main thesis, I gladly acknow- 

 ledge the great utility of the work he has done. 



