VARIED EFFECTS OF USE. 135 



bone lengthens as well as thickens from carry- 

 ing a greater weight, while tension (as seen in 

 sailors' arms, which are used in pulling) appears to 

 have an equally marked effect in shortening bones 

 (Descent of Man, p. 32). Thus different kinds of 

 use may produce opposite results. The cumu- 

 lative inheritance of such effects would often be 

 mischievous. The limbs of the sloth and the 

 prehensile tail of the spider monkey would 

 continually grow shorter, while the legs of the 

 evolving elephant or rhinoceros might lengthen 

 to an undesirable extent. Such cumulative 

 tendencies of use-inheritance, if they exist, are 

 obviously well kept under by natural selection. 



Although the ultimate effect of use is generally 

 growth or enlargement through increased flow of 

 blood, the first effect usually is a loss of substance, 

 and a consequent diminution of size and strength. 

 When the loss exceeds the growth, use will 

 diminish or deteriorate the part used, while disuse 



