252 THE GEOLOGICAL HISTORY OF PLANTS. 



Disco, near which are some of the most celebrated locali- 

 ties of fossil Cretaceous and Tertiary plants. Yet the 

 mouth of the McKenzie Eiver enjoys a much more favour- 

 able climate and has a much more abundant flora than 

 Disco. If north Greenland were submerged, and low 

 land reaching to the south terminated at Disco, and if 

 from any cause either the cold currents of Baffin's Bay 

 were arrested, or additional warm water thrown into the 

 North Atlantic by the Gulf Stream, there is nothing to 

 prevent a mean temperature of 45 Fahr. from prevailing 

 at Disco ; and the estimate ordinarily formed of the re- 

 quirements of its extinct floras is 50,* which is probably 

 above rather than below the actual temperature required. 



Since, then, geological facts assure us of mutations of 

 the continents much greater than those apparently re- 

 quired to account for the changes of climate implied in 

 the existence of the ancient arctic floras, it does not seem 

 absolutely necessary to invoke any others, f If, however, 

 there are other true causes which might either aid or 

 counteract those above referred to, it may be well to 

 consider them. 



Mr. Croll has, in his valuable work " Climate and 

 Time," and in various memoirs, brought forward an in- 

 genious astronomical theory to account for changes of 

 climate. This theory, as stated by himself in a recent 

 paper, J is that when the eccentricity of the earth's orbit 

 is at a high value, and the northern winter solstice is in 

 perihelion, agencies are brought into operation which 

 make the southeast trade- winds stronger than the north- 

 east, and compel them to blow over upon the northern 



* Heer. See, also, papers by Prof. Haughton and by Gardner in 

 "Nature" for 1878. 



f Sir William Thomson, " Transactions of the Geological Society of 

 Glasgow," February 22, 1878. 



\ " Cataclysmic Theories of Geological Climate," " Geological Maga- 

 zine," May, 1878. 



