284 (hi the Dentiliun of Mainmah. 



The following conclusions result from the foregoing investi- 

 gations into tlie dentitions of mammals. The rudiments of 

 luotli dentitions occur not only in the liigher mammals, but 

 also in the lower orders of j\Iar<u])ials, Edentates, Odontocetes, 

 and j\Jjstacocetes. The earliest mammaes weke dipiiyo- 

 DOKT. The monophyodont and homodont condition of many 

 mammals, e. (j. the Toothed Whales, has been secondarily 

 acquired. Within the mammalian class, ascending from the 

 lowest to the highest forms, we see how the second dentition 

 gains tlie upper hand more and more as regards form and 

 iunetion, while in the lower forms the first dentition is pre- 

 dominant. In the rudimentary stage both dentitions are of 

 equal value ; embryology gives us no support for the often- 

 expressed assertion that one of the two dental rudiments has 

 arisen in dependence upon tlie other ; they are both sisters, 

 whose mother is the simple invagination in the jaw, which we 

 term the dental fold (' Zahnleiste'). 



Now can we discover a bridge whicli connects the dentition 

 of Mammals with that of their ancestors, the Reptiles ? 



Ihere are no absolute differences between the mammalian 

 and reptilian tooth, as has already been shown by Seeley * ; 

 not one of the characters of the mammalian tooth is perfectly 

 constant ; the loss of any one of them is an approximation to 

 the reptilian tooth, and conversely reptilian teeth often 

 assumed characters belonging to those of mammals. The 

 replacement of teeth moreover occurs in reptiles to a still 

 greater extent than in mammals, since several series of teeth 

 may follow one another, the rudimiMits of which, as in the 

 case of the second dentition of mammals, are formed inter- 

 nally to the first. The idea of deriving the dentition of 

 mammals from that of reptiles therefore does not appear to 

 me to be too hazardous ; of the several series of teeth which 

 are found in reptiles, only two still persist in mammals. 



In conclusion I would subjoin the following attempt to 

 explain the origin of molar teeth in mammals, while freely 

 admitting its ])urely hypothetical nature. Owing to our 

 investigation of tooth-germs in AVhalebone Wliales, we liave 

 become acquainted with the phenomenon of the division of 

 the molars in nuimmals, whose jaws become elongated, into a 

 multitude of conically pointed structures, resembling the teeth 

 of reptiles. Conversely, have not the molars of mammals 

 also arisen in this way, in that, in consequence of the reverse 

 process, a shortening of the jaws, which the ancestors of 

 existing mammals underwent in the course of their trans- 



* H. 0. Seeley, "On tlie Nntnre and Umitt* of Ke])tiliaii Chiirncter in 

 Mainmnlinii Teeth," Vxoc. \\o\ . Sue. Lend. vol. xliv. jiji. Ii'i) 141. 



