212 THE FITNESS OF THE ENVIRNOMENT 



From them other series may be derived by 

 making a substitution in the molecule. Thus 

 the substitution of a hydroxyl radical for a 

 single hydrogen atom leads from the paraf- 

 fine hydrocarbons, C n H 2n+2 , to the alcohols 

 C n II 2ri +iOH; the substitution of a carboxyl 

 radical — COOH, for the methyl group — CIT 3 , 

 leads from the paraffine hydrocarbons 

 C n H 2n+1 CH 3 to the acids C n H 2n+1 COOH. 



Moreover, the classes of compounds thus 

 defined chemically fulfill the logical require- 

 ments of a class. They are collections of 

 well-characterized and very similar individual 

 things which differ greatly, and in well-marked 

 manner, from all other things. In other 

 words, growing complexity of the molecule, 

 when it consists only in increase in complexity 

 of the simple radical comprised of carbon 

 and hydrogen, of the formula C n H 2n +u 



CH 3 -, CH 3 CH 2 -, CH 3 CH 2 CH 2 -, CH 3 CH 2 CH 2 CH 2 - 



CH 3x CH 3 . 



>CH-, >CH-CH 2 - 



ch/ ch/ 



CHs-CH. 



>CH- 

 CH/ 



CH 3 v 



CHr)C - 

 CH 3 / 



has very little effect upon the properties of 

 the molecule. Thus the compound methane, 



