THE PROGRESS OF LIFE 49 



to give us clearer notions on this dictum of a palaeon- 

 tological base. 



We know as a fact that the earth is a hot body 

 travelling through space which is intensely cold. It 

 must, therefore, be cooling. Consequently, in the early 

 days of its history, it must have been very much 

 hotter than it is now. There are, indeed, reasons for 

 thinking that at a very remote period the earth was 

 actually molten owing to the intense heat, when, of 

 course, the whole of the water of the ocean must have 

 been in a state of vapour, and formed part of the 

 atmosphere. As the temperature lowered, this aque- 

 ous vapour would condense and fall on the surface of 

 the earth as hot rain. The first ocean would, there- 

 fore, be almost at its boiling-point, and would gradually 

 cool down ; but no life could exist in the ocean or 

 on the land while the temperature much exceeded 

 200F., which, so far as we know, is the highest tem- 

 perature in which plants can live. This period of the 

 hot ocean was, therefore, the Azoic era of the earth's 

 history, which, as the cooling progressed, passed into 

 the Protozoic and then into the Palseozoic era, which 

 includes the Cambrian period. At first the ocean 

 must have been nearly uniform in temperature from 

 the equator to the poles ; but climatic zones appear 

 to have been established in the Silurian period, if not 

 earlier. 



The pre-Cambrian rocks have received various 

 names in different parts of the world ; but, as they 

 are better developed and more easy to decipher in 

 North America than elsewhere, it is probable that, so 

 soon as the officers conducting the geological surveys 

 of Canada and the United States agree on a classifica- 



D 



