THE TEETH OF MARSUPIALIA. 433 



than by calling the reader's attention again to the character of 

 the Marsupial fauna : this microcosm, in which every place 

 is filled by a Marsupial that mimics the placental Mammal 

 which it represents, for nowhere can we more plainly see 

 the workings of natural selection than in areas thus isolated 

 and deprived of immigrant creatures for countless ages. 



In the foregoing pages much stress has been laid upon 

 the variability of animals, and the agencies by means of 

 which the variations have been preserved and intensified, 

 so to speak, so that ultimately permanent hereditary modi- 

 fications have been the result ; and it is possible that in 

 laying this aspect of the matter prominently before the 

 reader an impression of too great instability may have been 

 conveyed ; and thus the forms of creatures made to appear 

 more plastic and more shifting than they really are, for it 

 is hardly possible to realize the enormous lengths of time 

 during which the agencies have been at work, and without 

 which they would have been powerless to produce profound 

 alterations. 



The process which we term inheritance is constantly 

 reproducing animals which are minute copies of their 

 parents ; copies which are even more exact than we can at 

 first sight realise. 



Thus., even amongst different species of the same genus, 

 whose teeth are apparently quite similar so far as their 

 number and pattern goes, differences exist which are con- 

 stant for the species, and which may be brought into 

 prominence by any method of investigation which is 

 sufficiently accurate. 



A plan of representing in the form of diagrams certain of 

 the characteristics of an animal's dentition, by means of 

 which differences of proportion so slight as to be barely 

 recognisable by an inspection of the teeth are brought con- 

 spicuously into view, has been devised by Mr. Busk ("Proc. 

 Roy. Soc. 1870 "). 



P F 



