ALTERNATE GLACIAL PERIODS 419 



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 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 hemi- 

 sphere passes through a cold period the temperature 

 of the southern hemisphere is actually raised, with the win- 

 ters 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 

 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 besides 

 many closely allied species, between forty and fifty of the 

 flowering plants of Tierra del Fuego, forming no inconsider- 

 able 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 not 

 exist in the low intervening hot countries. On the Silla of 

 Caraccas, the illustrious Humboldt long ago found species 

 belonging to genera characteristic of the Cordillera. 



