reconcilable with each other, even when the distribution of fossils
was taken into account, but it pointed to the absolute necessity of
searching for former land-connections regardless of their extent and the
present depths to which they may have sunk.

That the key to the present distribution lies in the past had been felt
long ago, but at last it was appreciated that the various classes of
animals and plants have appeared in successive geological epochs and
also at many places remote from each other. The key to the distribution
of any group lies in the configuration of land and water of that epoch
in which it made its first appearance. Although this sounds like a
platitude, it has frequently been ignored. If, for argument's sake,
Amphibia were evolved somewhere upon the great southern land-mass of
Carboniferous times (supposed by some to have stretched from South
America across Africa to Australia), the distribution of this developing
class must have proceeded upon lines altogether different from that of
the mammals which dated perhaps from lower Triassic times, when the old
south continental belt was already broken up. The broad lines of this
distribution could never coincide with that of the other, older class,
no matter whether the original mammalian centre was in the Afro-Indian,
Australian, or Brazilian portion. If all the various groups of animals
had come into existence at the same time and at the same place, then it
would be possible, with sufficient geological data, to construct a map
showing the generalised results applicable to the whole animal kingdom.
But the premises are wrong. Whatever regions we may seek to establish
applicable to all classes, we are necessarily mixing up several
principles, namely geological, historical, i.e. evolutionary, with
present day statistical facts. We might as well attempt one compound
picture representing a chick's growth into an adult bird and a child's
growth into manhood.

In short there are no general regions, not even for each class
separately, unless this class be one which is confined to a
comparatively short geological period. Most of the great classes have
far too long a history and have evolved many successive main groups.
Let us take the mammals. Marsupials live now in Australia and in both
Americas, because they already existed in Mesozoic times; Ungulata
existed at one time or other all over the world except in Australia,
because they are post-Cretaceous; In