ay have been related to another
as New Zealand is to Australia, or as Australia is to India, at the
present day. Analogy seems to me to be rather in favour of, than
against, the supposition that while only Ganoid fishes inhabited the
fresh waters of our Devonian land, _Amphibia_ and _Reptilia_, or even
higher forms, may have existed, though we have not yet found them. The
earliest Carboniferous _Amphibia_ now known, such as _Anthracosaurus_,
are so highly specialized that I can by no means conceive that they
have been developed out of piscine forms in the interval between the
Devonian and the Carboniferous periods, considerable as that is. And I
take refuge in one of two alternatives: either they existed in our own
area during the Devonian epoch and we have simply not yet found them;
or they formed part of the population of some other distributional
province of that day, and only entered our area by migration at the
end of the Devonian epoch. Whether _Reptilia_ and _Mammalia_ existed
along with them is to me, at present, a perfectly open question, which
is just as likely to receive an affirmative as a negative answer from
future inquirers.

Let me now gather together the threads of my argumentation into the
form of a connected hypothetical view of the manner in which the
distribution of living and extinct animals has been brought about.

I conceive that distinct provinces of the distribution of terrestrial
life have existed since the earliest period at which that life is
recorded, and possibly much earlier; and I suppose, with Mr. Darwin,
that the progress of modification of terrestrial forms is more rapid
in areas of elevation than in areas of depression. I take it to be
certain that Labyrinthodont _Amphibia_ existed in the distributional
province which included the dry land depressed during the
Carboniferous epoch; and I conceive that, in some other distributional
provinces of that day, which remained in the condition of stationary
or of increasing dry land, the various types of the terrestrial
_Sauropsida_ and of the _Mammalia_ were gradually developing.

The Permian epoch marks the commencement of a new movement of upheaval
in our area, which attained its maximum in the Triassic epoch, when
dry land existed in North America, Europe, Asia, and Africa, as it
does now. Into this great new continental area the Mammals, Birds, and
Reptiles developed during the Palaeozoic epoch spread, and formed the
great Triassic Arc