280 THE HISTORY OF 



distributed upon the earth in proportion to its position in 

 relation to the source of heat ; the tropics must be the 

 hottest, the poles the coldest, as has been already shown in 

 a former Lecture. But this condition did not always exist. 

 So long as the earth was yet in an incandescent fluid state, 

 and was surrounded by a dense atmosphere which would 

 transmit the sun's rays but in a slight degree, the amount 

 of heat which it obtained from the sun remained scarcely 

 appreciable, compared to that which it lost by cooling, or in 

 other words, in the first epoch of the earth's formation, the 

 source of its heat lay, to all intent, within. Here, there- 

 fore, there was no distribution of heat over the earth, 

 depending upon its position in relation to the sun, and the 

 temperature was pretty nearly equal over the whole earth. 

 A hot, moist atmosphere, at present characteristic of the 

 tropics, then prevailed all over the globe, and made the 

 polar regions to resemble tropical countries. Only by 

 degrees, as the earth gradually cooled and the atmosphere 

 precipitated its vapour more and more in the form of rain, 

 gave up its carbonic acid to the organic world, and thus 

 became lighter and more transparent, did the sun acquire 

 higher importance ; and thus the regions of the higher 

 latitudes and even the polar lands passed, in a series of 

 stages, through the climates, one after another, which we 

 now find upon the earth, side by side, in going from the 

 equator to the poles. This circumstance will prove here- 

 after very full of consequences in reference to the explana- 

 tion of the different kinds of vegetation following one 

 another upon the earth. 



As I have already said, the first germs of life probably 

 originated in water, and in agreement with this we find 

 in the oldest stratified rocks, the Grauwacke, or as the 

 English call them, the Silurian rocks, merely some few 



