2io BRITISH MAMMALS 



less thickly covered with hair, and then the larger space between 

 the rows of molar teeth with a wet palate. The object of this 

 division is to enable these animals to gnaw all sorts of substances 

 with their teeth without the stuff through which they are gnaw- 

 ing their way reaching the interior of the mouth and being 

 swallowed, unless, of course, it is suitable for food. 



In the Rodentia Nature is again repeating herself, and pushing 

 to an absolute conclusion a plan which has possibly occurred 

 to her over and over again in the development of fishes and 

 reptiles, and the many diverse orders of Mammalia. In the 

 Proto-mammals, in those early groups of the Secondary Epoch 

 which, from such slight knowledge as we possess, appear to. be 

 allied to the Monotremes, we see frequent developments of forms 

 in which the incisor teeth are reduced in number and developed 

 into circular tusks with a chisel-like edge, while there is an 

 absence of canines and a diastema, or toothless space, between 

 the exaggerated incisors and the premolars. Then, again, in the 

 Marsupial order, one of its two divisions is distinguished from the 

 other by a more or less Rodent-like development of the incisor 

 teeth ; and in such a form as the wombat the resemblance to a 

 Rodent is very striking. Among the Primates (that group to 

 which man belongs) we have the aye-aye lemur, a creature with 

 Rodent-like incisors, and with all the intermediate teeth absent in 

 the adult animal. In branch after branch of Ungulates, Rodent- 

 like conditions of teeth are developed independently and repeat- 

 edly. This is the case with the elephants, to instance only one 

 of many examples. It is thought, indeed, by some geologists 

 that in their remote origin the Rodents may have been connected 

 with the primitive Ungulates. It is doubtful, however, whether 

 this was the case any nearer in time than that in which the 

 gnawing animals first differentiated from the primitive and 

 generalised Eutherian stock. The point at which they did so 

 must have been very near to the branching off of Marsupials, 

 Insectivores, and Primates. It will be noticed in all these three 

 groups that the first incisors to disappear are the side ones in 

 the lower jaw : there is more persistency in the upper incisors ; 



