1893.] Evolution of Metapodial Keels of Diplarthra. 423 
of the temporary or milk teeth, then the term “ under pres- 
sure ” is especially inappropriate, since Tomes has shown’ that 
this process is entirely independent of pressure of any kind 
whatever, but is in some way connected with the presence of 
“myeloid” or “giant” cells. The only other process of 
“ absorption under pressure ” with which I am acquainted, is a 
pathological one. In this case the pressure is sufficiently 
severe to cut off entirely, or seriously affect the supply of 
nutrition, thereby causing the death of the part and its subse- 
quent removal by sloughing. This however may be very 
gradual, so gradual in fact, as to resemble a perfectly normal 
change, as is often seen in the pressure of an aneurismal sac 
upon a neigboring bone. It may be true that a process of this 
nature, having its origin in a strictly pathological cause, may 
have become normal. Indeed, Huxley has remarked’ that it 
is not always easy to draw the line between pathological and 
normal changes. Upon the whole, however, it seems to me 
that before we can admit this principle of “absorption under 
pressure ” as having played any important part in the modifi- 
cation of the skeleton, we must require further proof of its 
existence and its method of operation. 
That living bone tissue is plastic, I should say highly plastic 
and that too in the ordinary physical sense, is so abundantly 
demonstrated upon every hand, that no further argument is 
necessary to establish the truth of this proposition. It is rec- 
ognized and acted upon in everyday surgical practice, and the 
number of cases wherein entirely new joints have been formed 
in old dislocations, leaves nothing to be desired to complete 
the proof. The changes thus produced however, have all 
occurred within the lifetime of the individual and it yet 
remains to be proven that they are capable of transmission to 
succeeding generations. _ 
With reference to the production of the-metapodial keels or 
crests, Cope has explained it in the didactyle foot at follows’ : 
“ A similar cause produces a similar result in the development 
1Dental Anatomy, p. 197. 
2Address to the British Medical Association, London Lancet. 
3The Artiodactyla, American Naturalist, March 1889, p. 115. 
