Evolution of Mammalian Molar Teeth, to and from the Triangular Type

Abstract

AQUARTER of a century ago next April, the late Prof. E. D. Cope, struck by the remarkable fact that the molar teeth, both upper and lower, of all the mammals from the Puerco or lowest Eocene horizon of North America carry three cusps arranged in triangular form, proposed what was practically the germ of the now well-known tritubercular theory. Briefly stated, this theory is to the effect that in the ancestors of all existing mammals each upper molar carried a triangle of cusps with the apex pointing inwards, while the corresponding lower teeth had a similar triangle with the apex directed outwards; and that from this primitive type have been evolved all the various modifications of molar structure, in most cases by the addition, but in certain instances by the subtraction, of cusps. The exponent, and to a great degree author, of the theory in its present form is Prof. Osborn, who has done well in laying before the scientific world the evidence for and against this fascinating doctrine.

Evolution of Mammalian Molar Teeth, to and from the Triangular Type.

(Biological Studies and Addresses, vol. i.) By H. F. Osborn; edited by W. K. Gregory. Pp. ix + 250; illustrated. (New York: The Macmillan Co.; London: Macmillan and Co., Ltd., 1907.)