24 DOCTRINE OF EVOLUTION 



the formula 012^22^11- Starch is again a typical carbo- 

 hydrate, and its formula is CgHj^Og, or some multiple of 

 this. One sees at a glance that these substances agree 

 in having twice as many hydrogen atoms as there are 

 oxygen atoms, the same proportion that the hydrogen 

 bears to the oxygen in the compound water, a 

 characteristic which makes it easy to remember the 

 general constitution of carbohydrate as compared 

 with the protein. The substances of this second class 

 are obviously much less complex, both as regards the 

 different kinds of atoms and in respect to the numbers 

 of each kind that enter into the formation of a single 

 molecule. Therefore the carbohydrates do not possess 

 so much power or energy as the protein molecule ; 

 in short, they are not such good fuels for the living 

 mechanism. 



Finally, we find almost always in protoplasm other 

 substances composed of carbon and hydrogen and oxygen 

 which are called hydrocarbons, distinguished from carbo- 

 hydrates by the fact that the number of oxygen atoms is 

 less than half the number of hydrogen atoms. These 

 substances are the fats and oils of various kinds, less 

 powerful sources of energy than the proteins, but they 

 contain more potential energy than the carbohydrates 

 because they are more oxidizable. 



Besides the characteristic substances of these three 

 classes, protoplasm contains certain other chemical 

 compounds, like the various salts of sodium, chlorine, 

 magnesium and potassium, and a few others, which 

 bring the list of chemical elements to the number twelve. 

 We have already noted how strikingly small and 

 restricted is the hst of elements composing living 



