THE LESSON OF EVOLUTION 23 



the sun. For when it has cooled sufficiently to make 

 the formation of protoplasm possible, the temperature 

 of its surface will be rapidly reduced to a point below 

 which protoplasm could not live, so that there would 

 be no time for life to develop. From this we learn 

 that biological evolution can only proceed on a cool 

 body, the surface temperature of which is kept nearly 

 equable by radiation from another hot body. As these 

 conditions must last for a long time, the hot body 

 must be large and at a proper distance from the cool 

 body. But much more than this is required for the 

 development of life. If living organisms were in- 

 tended to progress from the ocean to the land, in 

 the way I have already mentioned, provision must be 

 made for the continuous existence of land from the 

 close of the Cambrian period, and this land must be 

 well watered. Consequently the surface of the earth 

 must consist partly of land and partly of water, in due 

 proportion ; and the actual amount of water necessary 

 will depend upon the size of the earth. The rain 

 falling on the land constantly washes it down into the 

 sea, and some agency must exist for renewing the 

 land by elevation. This elevation depends upon the 

 mobility of the crust, which again depends upon the 

 internal temperature of the earth. This, therefore, it 

 is necessary to conserve. Again, the mass of the earth 

 must be sufficiently great to retain on its surface, by 

 gravitation, the water-vapour which would fly off and 

 leave the world dry if the mass were too small. And, 

 once more, the materials necessary for supporting life 

 and building up organisms must be present. 



From these considerations it follows that, to secure 

 a long development of life, the mass of the earth must 



