82 THE MAMMALIA, 



II. 



SPECIAL COMPARISON OF THE LIVING 

 MAMMALIA AND THEIR ANCESTORS. 



IN entering now upon an examination of the 

 different groups of the living Mammalia according 

 to their historical or palseontological past an ex- 

 planation of that which has come to be out of that 

 which has been the method to be followed is self- 

 evident: a systematic arrangement, proceeding 

 from the lowest to the highest forms, comprising 

 the result of anatomical as well as of palaeonto- 

 logical considerations. The lower groups of Mam- 

 mals are, of course, those which have retained the 

 inherited qualities of their ancestors most distinctly, 

 and have changed least. This does not necessarily 

 include the certainty that they inhabited the earth 

 at an earlier period than all those whose skeleton, 

 brain, and fcetal development show a pre-existence 

 of the lower forms, but end with a higher result. 

 The lower Mammals may have remained as a rem- 

 nant of a group whose nearest relatives at first, 



