272 A MANUAL OF DENTAL ANATOMY. 



have no means of gnawing through a shell or other hard 

 body. Now it happens that in three regions of the world, 

 pretty completely cut off from one another, three animals, 

 in parentage widely dissimilar, have arrived at dentitions of 

 " rodent " type. 



Thus in Australia, a region practically wholly monopo- 

 lised by Marsupials ; a marsupial, the Wombat, has a den- 

 tition very much like an ordinary placental Rodent. In the 

 island of Madagascar, one of the very few parts of the globe 

 without indigenous rodents, except a few Muridse, a Lemurine 

 animal, the Cheiromys, has a dentition modified in a similar 

 direction, though probably employed to get at a different 

 food ; and elsewhere, scattered all over the world, we have 

 the ordinary Rodents. 



In fact, three creatures, as widely different from each other 

 in parentage as they well could be, have been modified by 

 natural selection until they have dentitions, not identical, 

 but for practical purposes not unlike. 



It is impossible to conceive that these three creatures 

 have had anything in the way of common origin : their 

 ancestry must have been widely different, the regions in 

 which they live have been isolated from one another for 

 countless years, and yet they have each got to a "rodent" 

 type of dentition. Of extinct Lemurs little is known, and 

 of the ancestry of Cheiromys nothing ; but in the compact 

 group of Marsupials, still living in Australia, we are able to 

 dimly see some of the progressive steps which seem to tend 

 towards a rodent form of dentition. In Australia, roughly 

 speaking, there were nothing but Marsupials ; in Madagascar 

 more Lemurs than anything else ; and in each case out of 

 the material at hand, natural selection manufactured its 

 " rodent " dentition. 



At the same time the force of inheritance is seen in each 

 of them retaining characteristics of the groups whence they 

 have been derived, so that underlying the pritnd facie 



