m THE NORTH AND SO UTH. 397 



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 toward the same 

 end; but the most powerful appears to be the indirect in- 

 fluence of the eccentricity of the orbit upon oceanic cur- 

 rents. According to Mr. Croll, cold periods reguhirly recur 

 every ten or fifteen thousand years; and tliese at long in- 

 tervals are extremely severe, owing to certain contingen- 

 cies, 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 altera- 

 tions of climate, for about 160,000 years. With respect to 

 more ancient glacial periods, several geologists arc con- 

 vinced, 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 converselv it will 

 be with the northern hemisphere, while the southern passes 

 through a glacial period. This conclusion throws so much 

 light on geographical distribution that I am strongly in- 

 clined 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 inconsid- 

 erable part of its scanty flora, are common to North 

 America and Europe, enormously remote as these areas in 

 opposite liemispheres 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 (iardner 

 which do not exist in the low intervcTiing hot countries. 

 On the Silla of Caraccas the illustrious Humboldt long ago 

 found species belonging to genera characteristic of the 

 Cordillera. 



In Africa, several forms characteristic of Europe, and 



