ready been shown in a previous chapter, and
which serves to illustrate the successive geological forms of _Paludina_
from the Tertiary beds of Slavonia, as depicted by Prof. Neumayr of
Vienna. (Fig. 1, p. 19.)

CHAPTER VI.

GEOGRAPHICAL DISTRIBUTION.

The argument from geology is the argument from the distribution of
species in time. I will next take the argument from the distribution of
species in space--that is, the present geographical distribution of
plants and animals.

Seeing that the theory of descent with adaptive modification implies
slow and gradual change of one species into another, and progressively
still more slow and gradual changes of one genus, family, or order into
another genus, family, or order, we should expect on this theory that
the organic types living on any given geographical area would be found
to resemble or to differ from organic types living elsewhere, according
as the area is connected with or disconnected from other geographical
areas. For instance, the large continental islands of Australia and New
Zealand are widely disconnected from all other lands of the world, and
deep sea soundings show that they have probably been thus disconnected,
either since the time of their origin, or, at the least, through immense
geological epochs. The theory of evolution, therefore, would expect to
find two general facts with regard to the inhabitants of these islands.
First, that the inhabitants should form, as it were, little worlds of
their own, more or less unlike the inhabitants of any other parts of the
globe. And next, that some of these inhabitants should present us with
independent information touching archaic forms of life. For it is
manifestly most improbable that the course of evolutionary history
should have run exactly parallel in the case of these isolated oceanic
continents and in continents elsewhere. Australia and New Zealand,
therefore, ought to present a very large number, not only of peculiar
species and genera, but even of families, and possibly of orders. Now
this is just what Australia and New Zealand do present. The case of the
dog being doubtful, there is an absence of all mammalian life, except
that of one of the oldest and least highly developed orders, the
Marsupials. There even occurs a unique order, still lower in the scale
of organization--so low, in fact, that it deserves to be regarded as but
nascent mammalian: I mean, of course, the Monotremata. As regards Bir