HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 87 



is on this ground that the primitive atmosphere is held to have been 

 vast, hot, and heavy, and to have contained all the water of the globe, 

 all the carbonic acid now in limestone, and that corresponding to the 

 carbon now in carbonaceous deposits and in organic substances, all 

 the oxygen since shut up in the rocks by oxidation, as well as that of 

 the atmosphere and of organic tissues (p. 83). The assumption back 

 of this seems to be that heat always promotes gaseous expulsion, and 

 if so, the separation of the gases from the extremely hot molten rock 

 should certainly have become most complete in the white-hot primitive 

 globe. Under this view, absorption, rather than expulsion, should 

 have been the rule in all later and colder states. This fundamental 

 conception has been widely entertained, and has found familiar expres- 

 sion in certain volcanic hypotheses, in the supposed absorption of a 

 former atmosphere and hydrosphere of the moon into that body, and 

 in well-known prophecies of a similar doom for the envelopes of the 

 earth. 



The adverse evidence of early life. — The more the physiological 

 functions and adaptations of the early life are compared with those 

 of modern life, the more nearly identical they appear to be, and the 

 more scant has become the basis for postulating profoundly different 

 atmospheric and thermal conditions in early times. This observation 

 is not confined to the discovery of air-breathing animals far back in 

 geologic history, when a prohibitive excess of carbon dioxide should, 

 theoretically, have still remained in the air, but relates as well to various 

 adaptations found in nearly all forms of life, plants not less than animals. 

 For example, the xerophytic organs of some plants that lived in the 

 later half of the Paleozoic era are perhaps as irreconcilable with a 

 vast hot vaporous atmosphere, overcharged with carbon dioxide and 

 water-vapor, as is the presence of the flying insects with their extremely 

 active organizations. 



Incongruities on the physical side.— An atmosphere so heavily sur- 

 charged with carbon dioxide and water-vapor must have been rich in 

 heat-absorbing power, and should have given a very warm, equable 

 climate to the earth, as has been rightly assumed. Warm equable 

 climates did indeed prevail in a portion of the earlier history of the 

 earth, as also in the later; but the investigations of the past two 

 decades in India, Australia, and South Africa have forced the recognition 

 of extensive glaciation on the very border of the tropics, at a period as 



