12 Where did Life Begin? 



Our inquiry, therefore, is reduced to this 

 question : What part or parts of the earth's 

 surface first became sufficiently cooled by ra- 

 diation to be habitable by plants and animals ? 



A supposed case may help us in reaching 

 a correct answer to this question. Let us 

 assume, then, that the earth, at the time it was 

 a molten mass, had been and was revolving 

 in an orbit so near the sun that the amount 

 of heat it would have been receiving from the 

 sun would have just equalized the amount of 

 heat it was losing by radiation. Under these 

 conditions it would have cooled as the sun 

 cooled — neither faster nor slower. This helps 

 us to understand that the heat received by 

 the earth from the sun is, and ever has been, 

 an offset, so far as it goes, to the heat lost 

 from the earth by radiation. A statement of 

 the loss of heat from the earth during any 

 definite time may be formulated in this way : 

 From the heat lost by the earth by radiation 

 during a given period subtract the heat re- 

 ceived by the earth from the sun during the 

 same period, and the remainder will be the 

 earth's net or actual loss of heat. Sidereal 



