434 The American Naturalist. 
the same foot! In like manner it can be shown that neither 
the sesamoids, keels nor grooves of the metapodials are corre- 
lated structures, for we have seen that in the human thumb — 
there are sesamoids present but no keels nor lateral grooves. _ 
If upon the other hand we look at the problem from the — 
standpoint here advocated of the mechanical explanation, or — 
kinetogenesis, and we admit that the bone is plastic, then we 
are forced to conclude, it seems to me, that the lateral grooves _ 
and keels of the metapodials are the direct and unavoidable — 
results of pressure exerted by the sesamoids. This is proven — 
by the fact that wherever there are sesamoids present, and 
es 
ieee 
pleted keel. Up to this point the lateral grooves and keel had 
a distinct function, viz: to serve as guides for the sesamoids, 
but when the change was made in the position of the foot to 
that of the unguligrade, the lateral grooves began to disappear 
and the keel, which we must regard in the light of a necessary 
accompaniment of these grooves, began to perform a new 
function. The excavation of the phalangeal notch, its exten- 
sion into a groove, and the subsequent moulding of the me 
podial surface into a secondary keel to fit the groove were j 
as much a mechanical necessity as was the original or prim 
formation of the lateral grooves and keel. The proof of 
is to be found in the fact that in the further extension of 
keel beyond the digitigrade stage, the phalangeal groove 
the way. In the mechanical explanation therefore, we 
every condition satisfied, while upon the theory of natw 
selection of favorable variations the explanation is vag 
unsatisfactory and not in accord’ with the facts. 
; American Museum of Natural History, March 29th, 1893. 
