374 Dr. W. Kukenthal on the Origin and 



individual elements. This, however, bj no means excludes 

 the possibility of individual teeth increasing in size, even 

 without fusion, in consequence of having an increased amount 

 of work to do ; only the teeth with a number of tips cannot 

 be thus explained. I therefore consider the original single 

 tooth of the fishes as a tooth of the first order, as opposed to 

 the teeth of the second order, which have arisen through the 

 fusion of several, as we already find them within the class of 

 fishes. With this complication there naturally takes place a 

 diminution in the number of dentitions of which rudiments 

 are formed. In fishes tootii-change as a general rule is 

 unlimited ; it already ceases, however, within the limits of 

 this class with the development of very large individual 

 teeth, therefore with commencing specialization {e. g. in 

 Chimcera or Ceratodus) . 



In reptiles also the number of dentitions is a limited one. 

 If we would compare the individual tooth of a reptile with 

 the teeth of fishes we should preferably select the teeth of the 

 second order in the case of the latter. Like these many 

 reptilian teeth also exhibit complications, which point to a 

 fusion having formerly taken place {e.g. the teeth of 8celido- 

 saurus ITamsoni, Owen [Zittel, Bd, iii. p. 741], or of Antho- 

 don or Oalesaurus among the Theromorpha). 



Yet another fusion took place on the origin of the mammals 

 from reptile-like ancestors. The mammalian molars are 

 therefore teeth of the third order, which have arisen through 

 fusion of reptilian teeth. The result of this process is seen 

 most beautifully developed in the case of the Multituberculata, 

 the oldest mammals which are as yet known. 



A simple tooth of a fish and reptile and a mammalian 

 molar are therefore not homologizable with one another ; on 

 the contrary, they represent three different stages of dental 

 development proceeding from fusion. This at the same time 

 gives us the simple mechanical cause of the gradual reduction 

 of the dentitions. 



The principle of fusion of teeth consequently explains the 

 constant increasingly higher development of the dentition 

 within the vertebrate series. A second principle, operating 

 within each individual group, is that which modifies the teeth 

 60 as to make them as efiicient as possible, and adapts them 

 in accordance with the claims of function. Function depends 

 upon the mode in which food is acquired ; this, however, 

 varies but little in the different classes of animals, and thus 

 is explained the great similarity also which exists between 

 the dentitions of many forms belonging to different classes of 

 vertebrates, such as, for instance, is found in Theriodontia, 



