280 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



The statements set forth in the preceding paragraphs tend 

 to shew : 



(A) That the theory which regarded the conical teeth of the 

 Odontoceti or Toothed Whales as the parent form must be modi- 

 fied, and indeed there is good reason for regarding the homodont 

 haplodont dentition of those cetaceans as a secondary acquisition 1 . 



(B) That although the mammalian teeth are ultimately 

 referable to a haplodont type, yet this must be sought for in 

 ancestors more primitive than those to which the Cynodonts point, 

 and in a geological period more remote than the Trias. Possibly 

 the stage required is revealed in the Permian Cotylosauria. 

 Osborn (1907, op. cit. p. 39) states that the haplodont type has 

 not yet been discovered among the primitive mammalia. 



(C) That the Cynodontia provide evidence to the effect that 

 the distinction of incisors from canines, premolars and molars was 

 brought about in a pre-mammalian stage, so that the evolution 

 of those varieties must be sought in a correspondingly remote 

 geological period. 



the exclusion of a distinctively reptilian one. Such a view might be justified so 

 long as the mammalian characters of the Cynodontia were unknown or not realized. 

 By the recent extension of knowledge, the mammals are shewn to have emerged 

 from an ancestral stock which they shared with the Cynodontia in so remote a 

 period as the Trias. Gregory (Bulletins of the American Museum of Natural History, 

 Vol. xxvu. pp. 117 et seq., and Journal of Morphology, Vol. xxiv. 1913) has published 

 invaluable summaries of the evidence on this subject. Cf. also Re}). Brit. Assoc. 

 Adv. Sc, Portsmouth, 1911, "Discussion on the Origin of Mammals." 



1 The toothed whales (possessed of a homodont and haplodont dentition) were 

 at one time thought to lack a milk set of teeth, and were accordingly regarded as 

 monophyodont ; but Kiikenthal's researches shew that they are really heterodont 

 and also diphyodont, the first or "milk" set appearing as the normal teeth of 

 these animals, while the second set of teeth is vestigial only. Moreover, Kiikenthal 

 (cf. Denkschr. der med. naturwiss. Ges. in Jena, Band in. 1893, quoted by Schwalbe ; 

 Anat. Anz. 1894; also Jenaische Ztitschr., Band xxviii. 1893, p. 76) found that 

 even whalebone whales are provided with tooth-germs which early abort. These 

 researches, together with those of Leche, indicate that the whales are the descendants 

 of ancestors provided with more complicated teeth than those of the toothed-whales, 

 and Kiikenthal supposes that the numerous conical teeth of the latter result from 

 the splitting up of several compound teeth. This suggestion is however by no 

 means generally accepted, and in particular, Osborn and Gadow reject it. It should 

 be added that Osborn {Evolution of Mammalian Molar Teeth, 1907) is quite ready 

 to assent to the view that the haplodont dentition of the Toothed Whales is the 

 result of secondary modifications in teeth of more complicated form. 



