The evolution of the molar cusps in the Mammalia, is a broad and complex one, and one, moreover, upon which volumes have already been written without arriving a t any generally accepted or wellestablished final conclusions regarding the matter. While I have not devoted much time to the subject for a number of years past, it is my opinion, however, that the science has been in a condition for many years to indicate or outline some general conclusions in the evolutiou and morphology of the molar cusps in certain groups of the mammals, a t least, that are not very likely to be changed or modified by future discovery, and that may be regarded, therefore, in the light of ~t certain degree of fixity and finality.
The first of these conclusions to be enumerated, is to the effect that of the forces and factors concerned in the process of molar and premolar cusp addition and modification, those of stress, strain and impact have been in large part instrumental in producing the results accomplished. Such stresses, strains and impacts, have, without much doubt, furnished the causes or stimuli for favorable variations in the tooth crowns, which have been seized upon and perpetuated by selection in such a manner as to register a steady advance in molar and premolar evolution. This doctrine was first enunciated and promulgated by the late Professor John A. Ryder in a notable paper on "The Mechanical Genesis of Tooth Forms," in the Proceedings of the Philadelphia Academy for 1880. He may, therefore, be regarded as the great pioneer in this subject in this country. This hypothesis, while not susceptible of final proof, like so many other problems in evolution, is, nevertheless, supported and sustained by such an overwhelming array of evidence, from both the fossil and living Mammalia, as to be little short of axiomatic in its nature. It may, therefore, be arcepted, in my judgment, as not far wide of the truth. 1This c,oiiirnunicat.ion, received in December, 1920, was sent to the Editor by Dr. Wortmnn in nnswer to a request for a statement as to Dr. Wonman's Iresent, views on the subject of the evolution of the cusps in the human and ~risiiitnalian molars. ED.
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