GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS. 123 



CAUSES OF A GLACIAL CLIMATE. 



Mr. Croll, in a series of admirable memoirs, 

 has attempted to show that a glacial condition 

 of climate is the result of rarious 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 influence 

 of the eccentricity of the orbit upon oceanic currents. 

 According to Mr. Croll, cold periods regularly recur every 

 ten to fifteen thousand years ; and these at long intervals 

 are extremely severe, owing to certain contingencies, of 

 which the most important, as Sir 0. Lyell has shown, is 

 the relative position of the land and water, Mr. CroU 

 believes that the last great Glacial period occurred about 

 two hundred and forty thousand years ago, and endured 

 with slight alterations of climate for about one hundred 

 and sixty thousand years. With respect to more ancient 

 Glacial periods, several geologists are convinced from di- 

 rect evidence that such occurred during the Miocene and 

 Eocene formations, not to mention still more ancient for- 

 mations. 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, while the southern passes 

 through a glacial period. This conclusion throws so 

 much light on geographical distribution that I am strong- 

 ly inclined to trust in it. 



