THE CAUSE OF THE GLACIAL PERIOD. 465 



creased the absorption of carbonic dioxide from the atmos- 

 phere, through the growth of vegetable and animal life. 

 This is shown by the coal and lignite deposits, and by the 

 vast beds of limestone and other carbonates. Through this 

 means it is suggested, the temperature of the whole earth 

 was so lowered that glaciers began to form in the higher lati- 

 tudes and on the higher mountains. Coincident with this 

 general lowering of temperature the water of the ocean, 

 especially in the higher latitudes, would begin to absorb a 

 greater proportion of the carbon dioxide and carry it down. 



The theory in full is much more complicated than this, 

 and loses its value largely from this fact. But the most 

 obvious objection to it is that the operation of the forces in- 

 volved must be so slow that it could not fit into the rapid 

 succession of events which crowded in upon one another 

 during the last glacial period, which was characterized 03^ 

 numerous episodes of advance and recession of the ice-sheet 

 and all within a very limited period even as geologists reckon 

 time. 



The fifth theory and the sixth naturally go together. 

 That there may be variations in the temperature of space is 

 entirely within the realm of possibility, and that the sun 

 may be a variable star is a statement which can not be proved 

 absolutely false. Indeed, the hypothesis that the heat of the 

 sun is kept up by a bombardment of meteoroids is defended 

 by eminent astronomers. In case this were true, a known 

 natural cause for the production of variability is certainly in 

 the field, since the cometary bodies, which are circulating 

 irregularly through space, are probably themselves but ganglia 

 of meteoroids which may readily get entangled within the 

 predominant sphere of the sun's attraction, and become a 

 means of increasing for a long period the amount of the sun's 

 heat. This theory can not be positively affirmed to be true ; 

 but, as long as it can not be disproved by astronomical con- 

 siderations, it remains in the field to diminish the confidence 

 with which we support other hypotheses. 



