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 which 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 afford support enough to permit a halting walk. 

 But the most remarkable modification of this order occurs in 

 ununited fractures. &quot; False joints 5 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 any, 

 further modification can be produced. The experiences from 

 which we 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 power 



