254 THE FITNESS OF THE ENVIRONMENT 



In setting out to consider physical and 

 chemical properties we may perhaps begin 

 with chemical phenomena in the narrowest 

 sense. Such phenomena depend, according 

 to the atomic theory, upon rearrangements 

 of atoms within molecules. They result in 

 the conversion of individual substances into 

 one another, and they are accompanied by 

 rearrangements of energy. 



In the first place, it is to be noted that 

 enormous quantities of carbon, hydrogen, 

 and oxygen, as water and carbonic acid, are, 

 during a very long period of time, apparently 

 inevitable constituents of the atmosphere of 

 an astronomical body of sufficient size, after 

 cooling has led to the formation of a crust. 

 Further, it has been shown that in number, 

 variety, complexity of forms and changes, and 

 in the magnitude of the accompanying trans- 

 formations of energy the known substances 

 made up of carbon and hydrogen, and those 

 made up of carbon, hydrogen, and oxygen far 

 surpass the compounds of any other elements. 

 Likewise the known compounds of oxygen and 

 hydrogen with other elements are the most 

 numerous and important among inorganic 

 substances. Two peculiarities of the carbon 

 compounds, the formation and properties of 

 the carbohydrates, and the nature of the pro- 



