GENERAL STRUCTURE. 126 1 



habiting that area, yet as we ascend in the geological scale we trace 

 a gradual approximation to the existing fauna ; and in the Pleisto- 

 cene we find the characteristic features of such fauna distinctly 

 marked out, although many of the generic types, especially those 

 including forms of large corporeal bulk, may be different. Ex- 

 amples of these features are afforded by the Marsupials of the 

 Pleistocene of Australia, and the numerous Edentates of the same 

 period in South America ; although it is practically certain that in 

 the former case the fauna has always been Marsupial, and therefore 

 more or less closely related to the present one. That many of the 

 existing Mammalian faunas now characteristic of particular regions 

 were not thus circumscribed till a late period is, however, shown by 

 the occurrence of Baboons, Hippopotami, Giraffes, and African 

 types of Antelopes in the Pliocene and Pleistocene of India and 

 parts of Europe ; and equally by the Mammals of the Pleistocene 

 of India being in many cases specifically identical with those of 

 Africa, while the number of common species is now very few indeed. 

 Again, many forms like the Rhinoceroses and Horses, which are 

 now exclusively Old World types, formerly wandered over the plains 

 of America, and thus point to a more uniform distribution of types 

 than now exists. There are, however, indications of certain groups 

 of Mammals having always been restricted to one hemisphere. 

 Thus we have no evidence of the existence of Apes, Hyaenas, or 

 Civets at any epoch in the New World ; and neither are there any 

 traces of the Dinocerata or Titci7iotheriidce in the Old World. 



It was considered probable some years ago that the Mammalia 

 were directly descended from some primitive Amphibian types, and 

 that they stood altogether apart from the Reptiles. The striking 

 resemblance of the pectoral girdle of the oviparous Monotremes to 

 that of many Reptiles, and more especially the Anomodonts, to- 

 gether with the remarkable approximation to a low Mammalian 

 type presented by the skeleton of the latter, renders it, however, 

 more probable that Mammals are a divergent branch of the same 

 Amphibian stock which gave origin to the last-mentioned group, 

 if indeed they be not the direct descendants of the earlier forms 

 of that group. Dr Baur, who considers that Mammals were de- 

 veloped from true Reptiles allied to the primitive Rhynchocephalia 

 and Sauropterygia, has proposed that these early hypothetical forms 

 should be termed Sauromammalia. Professor Mivart has, indeed, 

 suggested that Mammals may have had a dual origin ; and that 

 while the Monotremes may have been derived at a comparatively 

 late date from the Anomodonts or kindred types, the Marsupials 

 may have originated at an earlier epoch from a totally distinct and 

 perhaps Amphibian stock. Many objections have, however, been 

 raised against this view ; and Mr POulton expresses his opinion that 



