300 TOWARDS THE HUMAN FORM 



that probably existed during the Cretaceous Period, but which 

 has not as yet been found. 



Both cutting-edged incisors and sharply-pointed canines 

 had only one root and are but little removed from Reptilian 

 teeth. They are teeth that cut or tear, and by these actions 

 stimulate the dental germ that produced them and maintains 

 them in activity. Hence their growth was continuous, especially 

 in the case of animals which attack hard substances like wood. 

 This phenomenon had been already produced once before in 

 the case of the Marsupial Diprotodon (p. 291) . It occurred again 

 in Eocene times in Tillondontia, in whom the first and 

 second incisor (Psittacotherium) or the second only (Estonyx) 

 Tillotherium, or even the third (Stylinodon) , underwent great 

 development ; the others became smaller again or disappeared 

 (Tillotherium). In Rodents the second incisor is highly 

 developed, while the first and third attenuate or disappear. 

 In Hares, Rabbits, and analogous forms the upper jaw 

 of each side has two incisors, one large and the other small, 

 situated one behind the other. In other Rodents the small 

 incisor disappears. Toxodontia also had incisors very 

 unequally developed (Nesodon) or even reduced to two pairs 

 (Toxodon). The same phenomenon is produced in the series 

 leading up to the Proboscidea, which, in addition to a trunk, 

 possess enormous incisors constituting tusks. 



Thanks to the discoveries made in the Fayum, in Egypt, 

 twenty years ago, we can here follow this transformation step 

 by step, and determine the exact causes which have 

 produced these tusks and have led by way of repercussion to 

 the development of the trunk. In this region there lived about 

 the middle of the Eocene period Mceritherium, of the 

 dimensions of a Tapir, wdiose second pair of incisors had taken 

 a considerable development in each jaw. The large incisors of 

 one jaw came into contact at the extremity with those of the 

 other, and this reciprocal pressure tended to bring them into the 

 position of a prolongation of the jaws. The other incisors 

 and the upper canines were rudimentary ; they had already 

 disappeared from the lower jaw. The Mceritherium must have 

 possessed either a long mobile upper lip, like that of the 

 Rhinoceros, or a short trunk like that of the Tapir. A little 

 later on there lived in the same region the Palceomastodon, 

 which had only two enormous incisors in each jaw and no 



