174 HABIT AND INTELLIGENCE. [chap. 



and de- destroyed, by discontinuance of the acts ; as, for instance, 



disuse. ^ when we forget how to speak a language, or to practise 



an art, which we once knew but have discontinued. 



From these two laws — that habits are strengthened by 



repetition of the acts, and are weakened by their dis- 



Strength continuance — it follows that the strength of any particiilar 



depends^ habit, other things being equal, depends on two different 



on time factors : one, the length of time during which the habit 



which it lia^s been exercised ; the other, the shorter or longer time 



has been i]^qi^ j^g^g elapsed since it has been exercised. The effect of 



exercised, '- 



and on these two factors, however, is not the same in kind. The 



it'has^been pi"^scnt strength of any particular habit depends chiefly on 



exercised, its having been recently exercised ; but the tenacity of a 



Present habit, or, in other words, the difficulty of weakening or 



of a habit, destroying it by disuse, is a different thing from its 



Tenacity present strength, and the two do not stand in any constant 



' proportion to each other. The tenacity of a habit depends 



on the length of time during which it has been exercised : 



that is to say, the longer a habit has been in forming and 



strengthening by exercise, the longer time it will take for 



it to be weakened or destroyed by disuse. These facts are 



familiar. Every one knows that habits of long standing 



are not easily lost ; and the most tenacious habits are those 



which belong to the species, and have been exercised not 



merely through a lifetime but through an unknown number 



Hereditary of generations.^ Hereditary characters, indeed, are seldom 



are thV^^ ■ — I believe never — destroyed by disuse during a single 



most_ generation, thoi;gh they may be destroyed by disuse 



during many generations : the domestic fowl and duck, 



for instance, have nearly lost the power of flight by 



long-continued disuse. Thus the law of the hereditary 



transmission of habit is equally true of its destruction 



as of its formation. 



' I am surprised that Darwin should say, " I do not wish to dispute the 

 truth of the proposition that inheritance gains strength simply through 

 long continuance, but I doubt whether it can be proved." (Variation 

 under Domestication, vol. ii. p. 26.) That inheritance should so gain 

 strength appears to me at once a necessary consequence of the laws of 

 habit, and a necessary inference from the general truth that the characters 

 of the variety are more variable than those of the species. 



