314 



CARY. 



[Vol. VI I. 



ried a step further up the foot. A little consideration will show, 



moreover, that increase in size and thrust of metacarpal V will, 



; through plasticity, produce a relative 



increase in the size of the unciform- 



lunar surface — a state of things 



which is found in Menodus. 



Similarly as to the production of 

 curvatures. A femur, say, slipped 

 from its socket, may form a new joint 

 with its girdle. The process so far 

 forth is but a yielding of the bone to 

 the pressure and movement of the 

 femur head. If such is the physi- 

 ology of bone tissue, the process must 

 be supposed operating in the history of each animal past and 

 present, producing and perfecting the curved surfaces by which 

 bones bear and move. The process kept within limits is, so far 

 as I can see, a wholly adaptive one ; but with the marked exam- 

 ples put before us, of changes wrought during the lifetime of 

 animals, the office of heredity in the matter is not to be taken 

 for granted. The crest and grooves on the lower metacarpal 

 ends in some forms, produced apparently in relation to the sesa- 

 moid bones,! is one of the most marked examples of probable 

 mechanical evolution. But before such structures can be said 

 to prove the inheritance of acquired characters, the question 

 should be tested whether they are not produced somewhere in 

 the history of each individual by the necessary interaction of 

 parts. It is understood that many structures, such as tooth 

 forms which are cut in their adult shape, cannot be explained 

 without heredity. To this it is answered that the correspond- 

 ence of these structures to their mechanical surroundings has 

 not been so definitely shown. 



In conclusion it will be well to limit clearly the inferences 

 pointed at in the preceding discussion. 



I. Plasticity of bone, using the word plasticity not in a physi- 

 cal sense merely, but to include absorption under pressure, will 

 probably account for much structure in the foot and elsewhere, 



1 The crests in certain highly specialized forms, like the horse and deer, reach 

 round to the anterior face of the bone and apparently cannot be thus interpreted 

 or assigned to any mechanical origin that is obvious. 



