hs. 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 (possibly with 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 wide-spread 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 co