312 CARV. [Vol. VII. 



this strain, but they are not in accordance with this principle. 

 Later on the same is applied to variation and correlation. 



A third law has been announced as operative in osteological 

 evolution. Instancing the production of new joint sockets and 

 tendinal grooves in cases of dislocation, the formation of joint 

 surfaces phylogenetically, the production of trochlear crests, the 

 sculpturing of tooth walls ^ is thought due to a similar process 

 re-enforced by heredity. Here bone tissue yields to mechanical 

 force, either physiologically or by molecular processes. This 

 is, then, contradictory to the principle last dealt with. If cer- 

 tain teeth have developed opposing cusps and ridges, and other 

 teeth are mutually arranged like shears with cusps shoved out 

 of the way of direct impact,^ the two structures may be equally 

 adapted to their respective uses ; they have not behaved the 

 same under their mechanical relations. 



The last principle stated involves plasticity in bone, either 

 physiological or molecular. As applied to the bones or the 

 teeth, it must be supposed to be operative within limits imposed 

 by the hardness of the structure, its rate of growth and meta- 

 bolism, its hereditary form. I propose to enter with it into the 

 structure of the foot under consideration, having special refer- 

 ence to the phenomena of variation and correlation. This 

 matter of adjustment and correlation through the foot seems 

 to me the hardest thing in osteological structure to account for 

 without the aid of some form of mechanical evolution. 



The results of the geometrical study of this foot, though the 

 problem does not permit of exactness, point toward the con- 

 clusion that among those surfaces on which the weight of the 

 body is thrown, pressure is equal. The diagram, representing 

 the head of metacarpal IV, will illustrate this. The amount of 

 thrust to be taken on surface a or b \s determined by their 

 inclination to the axis of the bone. A with its great obliquity 



1 See papers of Cope and Ryder on subjects mentioned. 



2 Cope, Proc. Am. Ass. Adv. Set., 1887, p. 256; ^oiir. Morph., Vol. Ill, p. 233. 

 Professor Cope, in these papers and in " The Origin of the Fittest," reiterates that 

 wear is shown on the inner face of the upper sectorial of the Carnivora, on the outer 

 face of the lower sectorial; also that the anterior internal cusp of the upper sectorial, 

 supported by but one root, has been shoved out and forward by the opposing cusp 

 of the lower sectorial which is supported by two roots. Compared with the facts 

 cited by Professor Osborne, diverse effects are seen to have been assigned to similar 

 causes. 



