No. 3.] A STUDY IN FOOT STRUCTURE. 309 



and cuneiform. Thus placed, the only conformation of surface 

 that would allow movement is that of two truncated cones placed 

 base to base. This is to all appearance the case, the axis of the 

 figure lying at an angle with the face of the bone in the axis drawn. 

 The metacarpals appear to move somewhat independently of 

 one another. Their proximal surfaces are quickly curved off 

 behind, and with the exception of metacarpal V, very little sur- 

 face is bearing during flexure. Movement takes place chiefly 

 by sliding between the surfaces, as in the intercarpal joint, and 

 is hardly more extensive than in that joint. 



The Foot of Menodus. 



Menodus was a heavily built animal with slow gait and short 

 step. When compared with that of Palaeosyops, its manus 

 exhibits the following changes in proportion : — 



(i) Metacarpals are shorter, broader, and more erect. 



(2) Metacarpal V is longer in proportion to the others. 



(3) The carpals are thinner proximo-distally. 



(4) Surfaces which take up lateral thrust are smaller. 



Theoretical Applications. 



Recent American Palaeontology has been largely identified 

 with Lamarckianism. It has claimed to show that change of 

 structure has been the result of changed function and condi- 

 tions, directly, not selectively. Inferring inheritance from the 

 marked changes produced and the length of time involved, it 

 is stated as the law of evolution of bone structures that growth 

 and atrophy, following lines natural to them under changing 

 mechanical relations, produce and perfect those structures. 



This is the general thesis. Before examining it further, I 

 feel justified in saying that the discussion on the Lamarckian 

 side has been loosely conducted. There has often been a great 

 want of clearness, while some facts have been adduced as evi- 

 dence which, from their lack of self-consistency, seem to dis- 

 credit the theory which they were cited to support. Thus when 

 Professor Cope ^ attributes to the longitudinal impact of running 

 the lengthening of the limb bones of many groups of mammals, 



1 Jottrnal Morphology, Vol. Ill, pp. 149-154. 



