336 ALTERNATE GLACIAL PERIODS [Chap. XII. 



extended all round the northern and southern hemispheres — from 

 the period having heen in a geological sense recent in both hemi- 

 spheres—from its having lasted in both during a great length of 

 time, as may be inferred from the amount of work effected — and 

 lastly from glaciers having recently descended to a low level along 

 the whole line of the Cordillera, it at one time appeared to me that 

 we could not avoid the conclusion that the temperature of the 

 whole world had been simultaneously lowered during the Glacial 

 period. But now Mr. Croll, in a series of admirable memoirs, has 

 attempted to show that a glacial condition of climate is the result 

 of various physical causes, brought into operation by an increase in 

 the eccentricity of the earth's orbit. All these causes tend towards 

 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 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 showm 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 moun- 

 tains of Brazil, some few temperate European, some Antarctic, and 

 some Andean genera were found by Gardner, which do not exist 

 4n the low intervening hot countries. On the Silla of Caraccas, 



