86 PAST HISTORY OF MAMMALS. [CH. II 



we have a better knowledge of extinct forms than we 

 have of any other existing group. It is perfectly true 

 that this knowledge will bear increasing and that it is 

 practically limited to the extinct genera of Europe and 

 North America with certain parts of South America and 

 India. But after all compared with other groups the 

 knowledge is undoubtedly considerable. 



Though all this is perfectly true, yet the Mammalia 

 are by no means an ideally perfect group for these 

 purposes. 



In the first place they are a comparatively modern 

 group, dating back at the furthest to the triassic period. 

 They are therefore perhaps to be regarded as representa- 

 tives of the present state of affairs in land and sea. They 

 have for example in all probability never reached New 

 Zealand, between which and New Australia there may 

 very likely have been an ancient land connection some- 

 where in the early secondary period or even earlier. The 

 Mammalia also, although a comparatively modern race, are 

 a waning race ; as it has been said, we live in a world 

 which is as regards mammals zoologically impoverished. 

 This has brought about the existence (if the hibernicism 

 be allowed) of so many missing links. The Marsupials for 

 example, though called by Mr Huxley Metatheria and 

 believed by him and by others to stand midway between 

 the Monotremes and the higher Mammalia, with a more 

 near approximation to the latter, cannot be satisfactorily 

 tacked on to any particular group of the Eutheria. The 

 very fact that the living Mammalia can be so easily 

 classified, and that there is so little difference of opinion 



