I have elsewhere[6] stated at length the reasons which lead me to recognise four primary distributional provinces for the terrestrial Vertebrata in the present world, namely,—­first, the Novozelanian, or New-Zealand province; secondly, the Australian province, including Australia, Tasmania, and the Negrito Islands; thirdly, Austro-Columbia, or South America plus North America as far as Mexico; and fourthly, the rest of the world, or Arctogoea, in which province America north of Mexico constitutes one sub-province, Africa south of the Sahara a second, Hindostan a third, and the remainder of the Old World a fourth.

[Footnote 6:  “On the Classification and Distribution of the Alectoromorphoe;” Proceedings of the Zoological Society, 1868.]

Now the truth which Mr. Darwin perceived and promulgated as “the law of the succession of types” is, that, in all these provinces, the animals found in Pliocene or later deposits are closely affined to those which now inhabit the same provinces; and that, conversely, the forms characteristic of other provinces are absent.  North and South America, perhaps, present one or two exceptions to the last rule, but they are readily susceptible of explanation.  Thus, in Australia, the later Tertiary mammals are marsupials (possibly with the exception of the Dog and a Rodent or two, as at present).  In Austro-Columbia, the later Tertiary fauna exhibits numerous and varied forms of Platyrrhine Apes, Rodents, Cats, Dogs, Stags, Edentata, and Opossums; but, as at present, no Catarrhine Apes, no Lemurs, no Insectivora, Oxen, Antelopes, Rhinoceroses, nor Didelphia other than Opossums.  And in the widespread Arctogaeal province, the Pliocene and later mammals belong to the same groups as those which now exist in the province.  The law of succession of types, therefore, holds good for the present epoch as compared with its predecessor.  Does it equally well apply to the Pliocene fauna when we compare it with that of the Miocene epoch?  By great good fortune, an extensive mammalian fauna of the latter epoch has now become known, in four very distant portions of the Arctogaeal province which do not differ greatly in latitude.  Thus Falconer and Cautley have made known the fauna of the sub-Himalayas and the Perim Islands; Gaudry that of Attica; many observers that of Central Europe and France; and Leidy that of Nebraska, on the eastern flank of the Rocky Mountains.  The results are very striking.  The total Miocene fauna comprises many genera and species of Catarrhine Apes, of Bats, of Insectivora; of Arctogaeal types of Rodentia; of Proboscidea; of equine, rhinocerotic, and tapirine quadrupeds; of cameline, bovine, antilopine, cervine, and traguline Ruminants; of Pigs and Hippopotamuses; of Viverridoe and Hyoenidoe among other Carnivora; with Edentata allied to the Aretogaeal Oryeteropus and Manis, and not to the Austro-Columbian Edentates.  The only type present in the Miocene, but absent in the existing, fauna of Eastern Arctogaea, is that of the Didelphidoe, which, however, remains in North America.