The Genetic and Phylogenetic Significance of the Presence of a Third Upper Molar in a Modern Dog

Extra incisors, canines and premolars in dogs probably are usually retained deciduous teeth, or, occasionally', may develop from split tooth germs. The extra molars of Otocyon appear to be due to the reduplication of the first upper and lower molars. The occasional appearance of the third upper molar in modern dogs is a genuine case of reversion to an ancestral stage, due to some one of the indicated genetic mechanisms. This illustrates the fact that the so-called law of the irreversibility of evolution cannot be considered as a law of the mechanism of evolution, but only as a summarization of the results usually observed.