re 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[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.