Nov424.], LATENT OR POTENTIAL HOMOLOGY. 267 
they can barely be perceived, so that it is difficult to theoreti- 
cally assign them a survival value in the struggle for existence; 
that the mechanical or Lamarckian explanation is the only one 
which can be offered!; I laid the chief stress, however, not 
upon the mechanical explanation, but upon definite or determi- 
nate origin, and this has been confirmed by the subsequent 
study of thousands of teeth in different families of mammals, 
The still more significant fact that this definite and determi- 
nate evolution was proceeding independently in à great many 
different families of mammals did not at the time impress itself 
so strongly upon my mind. 
If molar teeth are found independently evolving in exactly 
similar ways in such remote parts of the world as Switzerland, 
Wyoming, and Patagonia, it is obvious that the process is not 
governed by chance but represents the operation of some 
similar or uniform law deduced from the four following 
considerations: 
Firstly, the teeth differ from all the other tissues and organs 
of the body in being preformed, beneath the gum.? Unlike all 
other organs they are not modified, improved, or rendered more 
adaptive by use; on the contrary, after the first stage of wear, 

1 
> 
Fic. 6. — Superior molar of Merychippus, showing styles fs, ms, mits, and conules pl, mi, 
h lastic with th f th wholly 1 lp i t Fig. 5; E. 


homopias 
the longer they are used the more useless and less adaptive 
they become. Thus, new structures in the teeth do not first 
appear as modifications (as distinguished from congenital 
1 Ryder and Cope confidently advanced the mechanical explanation ; it is not 
without grave difficulties, owing to the lack of an heredity theory. 
of this fact was first pointed out to me by Prof. E. B. 
Poulton of Oxford. sitaitiug: vigiiniebal s1 MIU 
