414 EVOLUTION AND GENETICS 



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First law: In every animal which has not exceeded the term 

 of its development, the more frequent and sustained use of any 

 organ gradually strengthens this organ, develops and enlarges it,, 

 and gives it a strength proportioned to the length of time of such 

 use, while the constant lack of use of such an organ imperceptibly 

 weakens it, causing it to become reduced, progressively diminishes 

 its faculties, and ends in its disappearance. 



"Second law: Everything which nature has caused individuals 

 to acquire or lose by the influence of the circumstances to which 

 their race may be for a long time exposed, and consequently by 

 the influence of the predominant use of such an organ, or by that 

 of the constant lack of use of such part, it preserves by heredity and 

 passes on to the new individuals which descend from it, provided 

 that the changes thus acquired are common to both sexes, or to 

 those which have given origin to these new individuals." 



To these laws he added later the idea that necessity in the 

 organism gives rise to new organs. Other corollaries expressed 

 his belief in various modifying factors, but essentially his theory 

 involves the belief that change springs from the action of the 

 environment upon organisms. He supposed that this action was 

 direct in the case of plants, but that in animals it involved response 

 of the organisms to the environmental condition. His belief in 

 the inheritance of such changes as are obviously produced in 

 correlation with environment is the point which has always been 

 the chief subject for opposition. 



Criticism of Lamarck's Laws. Use and Disuse. Lamarck's 

 first law concerning the results of use and disuse in organisms is 

 apparently sound. Every individual during his life furnishes an 

 example of the effects of functional activity. If we exercise a 

 muscle repeatedly it gains in development, usually to a visible 

 degree, and whatever may be the effect on its bulk, the power 

 exerted by its contraction is certain to increase. Conversely 

 failure to use muscles results in diminution of muscular strength, 

 A man who leads a sedentary life is not expected to have the 

 powerful muscles of a laborer, and a muscle which for any reason 

 cannot be used at all, may atrophy to an extreme degree. 



In considering the effects of use and disuse we need not confine 

 ourselves to the obvious cases of physical activity. Tolerance for 

 poisons is an expression of an obscure functional power of the 

 body, but it is no less subject to increase or decrease. Increase 



