186 THE POPULAR SCIENCE MONTHLY. 



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 inte- 

 rior 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 Mexi- 

 can 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 ena- 

 ble a temperate flora to subsist in Greenland, and to bring all the pres- 

 ent temperate regions of Europe and America into a condition of sub- 

 tropical 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 

 dispense 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 subsidence 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 posi- 

 tion, notwithstanding the submergences and emergences they have 

 experienced, to which I have already adverted. We can now more 

 precisely indicate this than was possible when Lyell produced his 

 " Principles," and can reproduce the conditions of our continents in 

 even the more ancient periods of their history. Some examples may 

 be taken from the history of the American Continent, which is more 

 simple in its arrangements than the double continent of Europ-Asia. 

 We may select the early Devonian or Erian period, in which the mag- 

 nificent flora of that age — the earliest certainly known to us — made 

 its appearance. 



Imagine the whole interior plain of North America submerged, so 

 that the continent is reduced to two strips on the east and west, con- 

 nected by a belt of Laurcntian land on the north. In the great Medi- 

 terranean sea thus produced, the tepid water of the equatorial cur- 

 rent circulated, and it swarmed with corals, of which we know no less 

 than one hundred and fifty species, and with other forms of life appro- 

 priate to warm seas. On the islands and coasts of this sea was intro- 

 duced the Elian flora, appearing first in the north, and with that vital- 



