Chap. XE] IN THE NOKTH AND SOUTH. 161 



regularly recur every ten or fifteen thousand years ; and 

 these at long intervals are extremely severe, owing to 

 certain contingencies, 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 occurred about 240,000 years ago, and endured 

 with slight alterations of climate for about 160,000 

 years. With respect to more ancient Glacial periods, 

 several geologists are convinced from direct evidence 

 that such occurred during the Miocene and Eocene 

 formations, not to mention still more ancient formations. 

 But the most important result for us, arrived at by Mr. 

 Croll, is that whenever 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, whilst the southern passes though 

 a Glacial period. This conclusion throws so much light 

 on geographical distribution that I am strongly inclined 

 to trust in 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 inconsiderable part of its scanty flora, are common 

 to North America and Europe, enormously remote as 

 these areas in opposite hemispheres are from each 

 other. On the lofty mountains of equatorial America 

 a host of peculiar species belonging to European 

 genera occur. On the Organ mountains of Brazil, 

 some few temperate European, some Antarctic, and 

 some Andean genera were found by Gardner, which do 



