Preface 



acquired characters may say: Effects of use and 

 disuse, of habit, are not inherited, and have no 

 value or effect in evolution. But use and dis- 

 use — in one word, habit — always accompany a 

 certain environment, and make life possible or 

 successful only under certain surroundings or 

 conditions. Habits and conditions thus modify 

 the trend and direction of natural selection, and 

 favor the survival of any form having a congen- 

 ital, structural variation useful to the possessor 

 of the habit. To judge from the writings of 

 the Neo-Darwinlans, congenital variation in a 

 certain direction may always be relied upon to 

 occur when needed. It is a very present help in 

 all times of trouble. 



The primitive vertebrate, to use only one Illus- 

 tration, had been driven from the bottom by 

 stronger competitors, and was compelled to 

 maintain a swimming habit. His environment 

 was totally different from that of the animals 

 creeping on a sea bottom rich In food but 

 crowded with competitors of every kind. As 

 the swimming habit was necessary, natural selec- 

 tion would repress all tendencies toward an ex- 

 ternal skeleton as long as it could hinder this 

 form of locomotion. Its appearance later In 



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