GEOGRAPHICAL DISTRIBUTION 169 



tempted to show that a glacial condition of climate is 

 the result of various physical causes, brought into oper- 

 ation by an increase in the eccentricity of the earth's 

 orbit. All these causes tend toward the same end; but 

 the most powerful appears to be the indirect influence 

 of the eccentricity of the orbit upon oceanic currents. 

 According to Mr. Croll, cold periods regularly recur 

 every ten or fifteen thousand years; and these at long 

 intervals are extremely severe, owing to certain contin- 

 gencies, of which the most important, as Sir C. Lyell 

 has shown, is the relative position of the land and water. 

 Mr. Croll believes that the last great Glacial period oc- 

 curred about 240,000 years ago, and endured with slight 

 alterations of climate for about 160,000 years. With re- 

 spect to more ancient Glacial periods, several geologists 

 are convinced from direct evidence that such occurred 

 during the Miocene and Eocene formations, not to men- 

 tion still more ancient formations. But the most impor- 

 tant result for us, arrived at by Mr. Croll, is that when- 

 ever the northern hemisphere passes through a cold 

 period the temperature of the southern hemisphere is 

 actually raised, with the winters rendered much milder, 

 chiefly through changes in the direction of the ocean- 

 currents. So conversely it will be with the northern 

 hemisphere, while the southern passes through a Glacial 

 period. This conclusion throws so much light on geo- 

 graphical distribution that I am strongly inclined to trust 

 m it; but I will first give the facts, which demand an 

 explanation. 



In South America, Dr. Hooker has shown that besides 

 many closely allied species, between forty and fifty of 

 the flowering plants of Tierra del Fuego, forming no in- 



