24 THE LESSON OF EVOLUTION 



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 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 atmo- 

 sphere for the plants, but it must not be so abundant 

 as to poison the animals ; and calcium is necessary 

 for the skeletons of animals, without which they could 

 not have grown to any size. Too much lime, however, 

 would have taken all the carbon out of the atmo- 

 sphere, 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 tem- 

 perature 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 



