DESIGN IN NATUEE 249 



form a non-conducting crust. Oxygen is necessary 

 for the water, and to combine with the silicon and 

 aluminium, while enough must remain over for the 

 respiration of animals. Carbon must be in sufficient 

 quantity in the atmosphere for the plants, but it 

 must not be so abundant as to poison the animal's ; 

 and calcium is necessary for the skeletons of animals, 

 without which they could not have grown to any 

 size. Too much lime, how 7 ever, would have taken 

 all the carbon out of the atmosphere, and there would 

 be none left for the plants. A little more hydrogen 

 or carbon, or a little less oxygen or silicon, would 

 have rendered the earth uninhabitable. Even the 

 right proportion of the elementary substances would 

 have proved useless, if the earth had been too small, 

 or if the temperature of its surface had been much 

 hotter or colder than it is. The latter depends upon 

 the distance and temperature of the sun, and has 

 nothing to do with the size and composition of the 

 earth. Also, if man was ever to become civilised, 

 gold, copper, and other metals in accessible positions 

 were necessary, although they are of no use in the 

 economy of animals and plants. Gold, however, 

 would be almost useless to man if it was abundant, 

 while iron would be equally useless if it was as rare 

 as gold. But we know that these, as well as the other 

 substances, exist in their right proportion. 60 



We cannot believe that all these various and com- 

 plicated adjustments were brought about by a fortui- 



60 Gold is the most suitable substance in the earth for 

 coinage, as it does not oxidise. Iron is the most suitable 

 substance for the manufacture of tools, for it can be 

 hardened. 



