I propose to lay before you, as briefly as I can, the ideas to which a long consideration of the subject has given rise in my own mind.

If the doctrine of evolution is sound, one of its immediate consequences clearly is, that the present distribution of life upon the globe is the product of two factors, the one being the distribution which obtained in the immediately preceding epoch, and the other the character and the extent of the changes which have taken place in physical geography between the one epoch and the other; or, to put the matter in another way, the Fauna and Flora of any given area, in any given epoch, can consist only of such forms of life as are directly descended from those which constituted the Fauna and Flora of the same area in the immediately preceding epoch, unless the physical geography (under which I include climatal conditions) of the area has been so altered as to give rise to immigration of living forms from some other area.

The evolutionist, therefore, is bound to grapple with the following problem whenever it is clearly put before him:—­Here are the Faunae of the same area during successive epochs.  Show good cause for believing either that these Faunae have been derived from one another by gradual modification, or that the Faunae have reached the area in question by migration from some area in which they have undergone their development.

I propose to attempt to deal with this problem, so far as it is exemplified by the distribution of the terrestrial Vertebrata, and I shall endeavour to show you that it is capable of solution in a sense entirely favourable to the doctrine of evolution.

I have elsewhere[1] stated at length the reasons which lead me to recognize 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 Arctogaea, 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 1:  “On the Classification and Distribution of the Alectoromorphae;” 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