248 THE LESSON OF EVOLUTION 



intended 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 for many 

 millions of years, 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 de- 

 pend 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 sup- 

 porting 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 be considerable, and that the cooled crust must 

 be a bad conductor of heat. That is, it must be 

 formed of oxides, and not of unoxidised metals. 

 There must also be a certain relationship between 

 the quantities of the several elementary substances 

 of which organisms are composed. It is necessary 

 that there should be a certain quantity of hydrogen 

 for the water not too much nor too little as well 

 as what was required for the tissues of plants and 

 animals. Silicon and aluminium are necessary to 



