KINE TO GENESIS. 379 



expressed the opinion that the shear of the sectorial 

 teeth of Carnivora was produced by lateral friction 

 during vertical movement of the lower tooth on the 

 upper. I have also asserted that the forms of facets 

 of limb articulations are due to pressure. Mr. Gary 

 sees here the attempt to explain the origin of totally 

 different structures through identical mechanical pro- 

 cesses, and believes that the attempt is a failure. 

 Were the conditions" of the problems alike, as Mr. 

 Gary thinks them to be, he would have good reason 

 for his opinion. But the conditions in the three cases 

 are entirely different, and our author's conclusion is 

 due to neglect of the elementary facts of the proposi- 

 tion. 



The development of conules at the points indicated 

 by Professor Osborn, has been supposed by him to 

 be due to friction between existing ridges of enamel 

 which cross each other when in action, at the points 

 in question. In the case of the development of the 

 sectorial shear, the faces between which the shearing 

 motion takes place are smooth, and without ridges or 

 crests. Hence the entire surface receives a homo- 

 geneous friction. In the third case, that of the foot 

 articulations, there is no friction, but there is pressure 

 which when abruptly applied in movement becomes 

 impact. There is really no parity between the three 

 cases. 



The author of this paper also thinks that the ex- 

 planation of the elongation of bones through use of 

 different kinds is not a permissible hypothesis. He 

 cites my attempt to account for the elongation of the 

 leg bones of higher mammals through impact-stimulus ; 

 and of other limb bones of other mammals through 

 stretching. But he does not prove that similar results 



