ADAPTATION. 187 



habitual exertion gives to mental faculties, needs no illustra- 

 tion : every person of education has personal experience 

 of it. Even from the osseous structures, evidence 



may be drawn. The bones of men accustomed to great mus- 

 cular action, are more massive and have more strongly 

 marked processes for the attachment of muscles, than the 

 bones of men who lead sedentary lives ; and a like contrast 

 holds between the bones of wild and tame animals of the 

 same species. Adaptations of another order, in which there 

 is a qualitative rather than a quantitative modification, arise 

 after certain accidents to w^hich the skeleton is liable. When 

 the hip- joint has been dislocated, and long delay has made it 

 impossible to restore the parts to their proper places, the 

 head of the thigh-bone, imbedded in the surrounding muscles, 

 becomes fixed in its new position by attachments of fibrous 

 tissue, which aflford support enough to permit a halting walk. 

 But the most remarkable modification of this order occurs in 

 ununited fractures. ^^ False joints'^ are often formed — ■ 

 joints which rudely simulate the hinge structure or the ball- 

 and-socket structure, according as the muscles tend to pro- 

 duce a motion of flexion and extension or a motion of rota- 

 tion. In the one case, according to Rokitansky, the two ends 

 of the broken bone become smooth and covered with perios- 

 teum and fibrous tissue, and are attached by ligaments that 

 allow a certain backward and forward motion; and in the 

 other case, the ends, similarly clothed with the appropriate 

 membranes, become the one convex and the other concave, 

 are inclosed in a capsule^ and are even occasionally supplied 

 with synovial fluid ! 



The general truth that extra function is followed by extra 

 growth, must be supplemented by the equally general truth, 

 that beyond a limit, usually soon reached, very little, if an}% 

 further modification can be produced. The experiences from 

 which w^e draw the one induction thrust the other upon us. 

 After a time, no training makes the pugilist or the athlete 

 any stronger. The adult gymnast at last acquires the powet 



