214 THE FITNESS OF THE ENVIRONMENT 



In the latter case the introduction of methyl 

 groups in place of hydrogen produces no appre- 

 ciable change in the general characteristics 

 of the substances ; in the former, the succes- 

 sive introduction of hydroxyl groups forms 

 substances belonging to three different classes 

 of compounds, — alcohols, aldehydes, and acids, 

 which have nothing in common. In short, 

 variation in the number and arrangement of 

 such groups as occur in the paraffine hydro- 

 carbon, 



I I I 



C-CH 3 , -C-CH 2 -C-, )CH-C 



I I I 



-c 



is without manifest effect upon the more 

 important properties of the molecule, but 

 variation in the number and arrangement of 

 any other groups produces complete change 

 in its characteristic properties. 



