18*79.] NATURAL SCIENCES OF PHILADELPHIA. 4T 



FURTHER NOTES ON THE MECHANICAL GENESIS OF TOOTH-FORMS. 



BY J. A. RYDER. 



In a paper published in the Proceedings of the Academy of 

 Natural Sciences of Philadelphia, 1 in 1818, I sought to indicate 

 the modes in which the teeth of mammals were modified by means 

 of the movements of their jaws incident to mastication, through 

 long series of generations. I there reached the conclusion that 

 mechanical strains and impacts had probably been the secondary 

 causes to which the origin of the various forms of teeth might, in 

 large measure, be attributed. The teeth were supposed to be 

 plastic, or at least slightly so, in all stages, notwithstanding their 

 extreme hardness. This view was forced upon me by facts pre- 

 sented by vertebrate palaeontology, together with my observation 

 that the physiological act of mastication was progressively spe- 

 cialized, and in each case its degree of specialization was found to be 

 in correspondence with the type of molar tooth with which it was 

 associated. In the course of m}' studies it seemed clear to me that 

 the tooth-modifying capacity resided in the powers of the animals 

 themselves, and the ways in which they were compelled? accord- 

 ing to the kind of food for which they had a preference, to exert 

 their powers. I am aware that this sort of reasoning amounts to 

 saying that an animal causes its own structures to vary in form, 

 by the natural operation of its own powers in overcoming resist- 

 ances, which view, notwithstanding its seeming improbability, has 

 more in its favor than that which holds that chance variations, 

 which have been of benefit to individuals, have been preserved and 

 transmitted to offspring and developed into organs in the course 

 of generations by the operation of the law of natural selection, the 

 importance of which I would be the last to underrate. The latter 

 view gives us no causal interpretation for so-called spontaneous 



1 " On the Mechanical Genesis of Tooth-Forms," pp. 45-80. 



2 Emphatically not wholly of their own wills, because the specialization 

 of organization presupposes a certain limitation in the power to make choice 

 caused by habits, which have become physiological characteristics, so 

 that the charge made against Lamarckianism that it throws all outside 

 power out of consideration, no matter of what character, is utterly false on 

 scientific grounds alone. 



