370 Dr. W. Kiikenthal on the Origin and 



rent functions, and the second dentition which was originally 

 present ia still developed rudimentarily in the embryo, but no 

 longer cuts the gum. The similarity between the dentition of 

 these two orders of placental mammals and that of the marsu- 

 pials is therefore due to the persistence of the first dentition ; 

 but the great difference is that in the case of the marsupials 

 the second dentition does not appear, because the teeth of the 

 first have become highly specialized, while in that of the 

 edentates and toothed whales the same phenomenon is occa- 

 sioned by a degeneration produced by a diminution of the 

 functions. 



If we therefore consider impartially the groups which we 

 have been discussing, not allowing ourselves to be prejudiced 

 by phylogenetic hypotheses, we see that in Theromorpha, 

 marsupials, and placental mammals the original condition of 

 the dentition was that of polyphyodontism in the case of the 

 first-mentioned group and diphyodontism in that of the two 

 latter. But we further find that in consequence of the same 

 cause, specialization of the individual teeth, in the Thero- 

 morpha all dentitions except the first were suppressed, while 

 in the marsupials at least one tooth of the second dentition 

 became functional ; but in the placental mammals, in spite of 

 the specialization, both dentitions appeared. 



In the three groups of the Theromorpha, Marsupialia, and 

 Placentalia we thus have three stages in dental development 

 which differ in height and which have been developed according 

 to the same laws, but from a successively higher basis. 



The impression is produced upon us that the height of the 

 development of the dentition always corresponds to the degree 

 to which the organization of the groups of animals in question 

 has advanced, an idea which is rendered perfectly probable 

 owing to the principle of correlation of organs. This is as 

 much as to say that the similarities which we find in the three 

 differently advanced forms of dentition depend upon pheno- 

 mena due to convergence, and cannot be employed to set up 

 phylogenetic connexions. As a matter of fact we see that 

 the dentition of the Theriodontia really resembles that of the 

 predaceous Marsupialia and predaceous Placentalia, but not 

 that of the lowest mammals, with which we are acquainted 

 owing to the discoveries of paleeontology, and to the considera- 

 tion of which we will now proceed. 



The oldest known remains of mammals come from the Trias 

 and exhibit a wide geographical distribution, sinceisolated teeth 

 or incomplete skulls have been found in Swabia, in North 

 Carolina, in Basutoland, and at the Cape. This by itself is 

 an argument in favour of greater antiquity for the mamma- 



