Chai. xil.j IN THE NORTH AND SOUTH. 161 



f 



regularly recur every ten or fifteen thousand years; and 

 these at long intervals are extremely severe, owing to cer- 

 tain 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 iQlacial 

 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 hemi- 

 sphere 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 north- 

 ern hemisphere, whilst the southern passes through 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 be- 

 sides many closely allied species, between forty and 

 fifty of the flowering plants of Tierra del Fuego, form- 

 ing no inconsiderable part of its scanty flora, are com- 

 mon to Forth 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 not exist 



