3IO CARV. [Vol. VII. 



to stretching in arboreal habits the length of the fore limbs of 

 the sloths, to cross pull of the wing the great elongation of the 

 calcaneum of some bats, few, I think, will believe that the uses 

 which he points out can, in any other than a selective sense, 

 be said to condition those structures. Similar effects are here 

 attributed to the most diverse causes. To prove the Lamarckian 

 case, it is not ehough to attach to a structure its use. It must 

 be shown that use puts the bone under such physiological con- 

 ditions that by the "natural processes of growth" the structure 

 will be produced. 



The proposition is that evolution has followed natural laws of 

 growth in the individual. One of these laws formulated is that 

 already implied, that use determines growth. This is undis- 

 puted as a physiological law operative, within limits, in the life- 

 time of animals ; but it has been extended into Phylogeny. To 

 take the Lamarckians in this matter on their strongest ground, 

 it is the cause of the reduction of digits through disuse of the 

 shorter ones following erection into digitigradism and change of 

 habitat to hard ground.^ Atrophy of metacarpals has followed, 

 of carpals also except when put to use by other digits. 



The physiology is that of bone cells. Impact on the bone 

 surface stimulates its cells to more active deposit ; lack of use 

 inhibits that deposit. The same bone in a series of feet and 

 corresponding bones in the same foot are proportioned to their 

 use. In the metacarpal series of any foot, as in its carpal series, 

 there is a physiological balance of the elements with the impact 

 they receive.^ Specialization among the teeth has been explained 

 by the neo-Lamarckians in the same way. 



Now from the nature of the case it is evident that under 

 selection alone this relation would hold true approximately and 

 in the main ; but when such a constantly adjusting principle is 

 brought in, the facts must be rigidly questioned. Now in the 

 foot I have been studying the trapezoid is too small to harmon- 

 ize with this law. It is a thin bone especially. The dispropor- 



^ This for Ungulata only. 



2 The foot of Menodus furnishes an excellent illustration of the principle. The 

 assumption of a heavy body must tend to spread out the digits, and bring the shorter 

 metacarpals more into action. Thus metacarpal V is much larger in proportion in 

 Menodus than in Palfeosyops. The advantage to a heavy animal of having a broad 

 foundation is, however, perfectly evident. 



