INTRODUCTION 9 



tive cone. This hypothesis accords with the embryological theory and 

 the premolar analogy theory. 



It is quite possible that the trikibercular or triangular stage arose 

 independently in different groups of animals, by two or possibly three 

 different modes of origin, as outlined in the four sugg* \stions above 

 advanced, on the principle of convergence of similar forms from dis- 

 similar beginnings. Tin's, however, does not invalidate the theory of 

 the passage of the majority if not of all the higher mammalia through 

 the tritubercular stage, however arrived at. 



SUMMARY. 



To sum up, it must be clearly re-stated that the four great principles 

 of molar evolution do not stand or fall tocjctln-r. The first or primitive 

 trituberculy principle is now almost undeniable for the majority of 

 mammals ; entirely apart from the disputed question of the original 

 homology of the cusps of the upper and lower teeth, there is no 

 question whatever as to the beautiful and almost incredible homo- 

 logies between the cusps of the molar teeth in the most diverse orders 



pa* 



FIG. 2. Two divergent derivatives of the tritubercular molar pattern. Right-hand 

 figure, a grinding molar of the modern horse ; left-hand figure, a sectorial molar of a Flesh- 

 eating mammal (Oxynena) of the Eocene Period. 



of mammals. The upper carnassial of the Carnivora and the upper 

 molars of the Equida?, for example, are types adaptively so far apart, 

 it is small wonder the older odontologists did not even suspect the 

 existence of hornogeny or common derivation, through which we can now 

 compare cusp for cusp. The reptilian cone origin theory is next in 

 order of demonstration and acceptance : it has recently gained strength 

 by the very general admission that the Theriodont reptiles are at 

 least nearly ancestral to the mammals. The cusp addition theory also 

 finds more advocates at present than the concrescence theory, and rests 

 upon indisputable evidence. Finally, the greatest conflict of evidence 

 exists between the Cope-Osborn palpeontological and the embryological 

 plus premolar-analogy theories of the homologies of the upper and 

 lower cusps (see pp. 208-227). 



