1879.] NATURAL SCIENCES OF PHILADELPHIA. 49 



exerted and repeated millions, perhaps billions of times in the 

 course of generations, be effectual in transmitting the simple pri- 

 mary form to a more complex later one, as I have tried to show 

 more fully in the paper already cited. That a brittle inflexible 

 substance like marble, when in the form of thin, rectangular slabs 

 may be bent by the force of gravity acting upon it persistently 

 whilst lying horizontally for a long time and only supported at 

 two of its corners diagonally opposite each other, is proved by an 

 old marble gravestone very much bent from this cause and now 

 belonging to the Academy. This phenomenon it seems to me is 

 no harder to explain than the morphological phenomena presented 

 by the teeth of mammals ; for my part, I believe that both the 

 phenomena in question will most probably bear a similar inter- 

 pretation. 



I now propose to offer some new evidence based upon more 

 accurate observations of the mode in which herbivorous ungulates 

 crush and masticate their food. A large living male rhinoceros 

 has afforded me the opportunity to make the observations. I 

 distinctly saw this creature crush its food by sweeping the lower 

 molars of the side about to be brought into action , from without in- 

 wards against the upper ones; meanwhile those on the other side 

 of the head were of course scarcely in contact, provided a con- 

 siderable amount of food was being acted upon by the side in use 

 at the instant. This, I concluded, for obvious reasons, was defi- 

 nitely the mode in which the jaws of ungulates were used which 

 were moved in both lateral and vertical directions in the act of 

 chewing; in other words, it seems to be the manner in which 

 chewing is effected in all anisognathous selenodont mammals, 

 which can be definitely traced to a bunodont ancestry. 



It will be readily understood that the above observations in 

 some measure modify the conclusions reached in my former paper, 

 in which the belief was entertained that the motion which did the 

 crushing was outwards instead of inwards. From facts which I 

 have gathered, it now seems strange to me that I should have fallen 

 into this misapprehension, since the true method promises to yield 

 even a better interpretation of the true philosophy of tooth-modi- 

 fication by mechanical agencies than that first ottered. It must, 

 however, here be stated that, in no essential particular except 

 one, do I alter my former views. I still hold the mandibular 



