34 EVOLUTION 



and hotter than it now is. All the carbon dioxide that 

 would go to the making of the great forests of a later 

 date, and much that would be absorbed in the rocks, was 

 then held in the atmosphere. The rays of the sun (such 

 as it then was) would hardly be able directly to penetrate 

 this dense shell of cloud and gas; but, on the other 

 hand, the heat that reached the surface could with 

 difficulty radiate again into space. Long after the ocean 

 had cooled, long after heat ceased to reach the surface 

 from below (as it did cease at an early geological period) 

 the earth had a very high temperature from pole to pole. 

 Professor Sollas does, indeed, raise the speculation 

 whether, as the sun's rays could so little penetrate the 

 atmosphere, the primitive ocean, after it cooled, may not 

 have been entirely frozen until the air was sufficiently 

 cleared to admit the sunlight. For reasons into which I 

 cannot enter here, I prefer to suggest to the reader to 

 follow the general view of the earth as having, until 

 millions of years later, a very high and almost uniform 

 temperature. 



One other alternative must, however, be noted before 

 we proceed. We saw that a recent theory of the forma- 

 tion of the earth does not admit the initial incandescence 

 of our globe. This "planetesimal theory " of Professors 

 Chamberlin and Salisbury supposes a less violent aggre- 

 gation of the "planetesimals," or tiny particles, to form 

 the earth. Condensation would, of course, generate a 

 considerable heat ; but they suppose that this occurred 

 in the interior of the planet, and only showed at the 

 surface in the escape from below of molten lava. The 

 atmosphere might consist of volumes of gas making its 

 way upward through volcanic vents and porous strata, 

 and the water might be formed underground and settle 

 on the surface long before the planet was fully formed. 

 This interesting speculation has as yet found little 



