ogress of mammalian life in America, from the
beginning of the Tertiary to the present, is well illustrated by
the brain-growth, in which we have the key to many other
changes. The earliest known Tertiary mammals all had very small
brains, and in some forms this organ was proportionally less
than in certain reptiles. There was a gradual increase in the
size of the brain during this period, and it is interesting to
find that this growth was mainly confined to the cerebral
hemispheres, or higher portion of the brain. In most groups of
mammals the brain has gradually become more convoluted, and thus
increased in quality as well as quantity. In some also the
cerebellum and olfactory lobes, the lower parts of the brain,
have even diminished in size. In the long struggle for existence
during Tertiary time the big brains won, then as now; and the
increasing power thus gained rendered useless many structures
inherited from primitive ancestors, but no longer adapted to new
conditions."

This remarkable proof of development in the organ of the mental
faculties, forms a fitting climax to the evidence already adduced of the
progressive evolution of the general structure of the body, as
illustrated by the bony skeleton. We now pass on to another class of
facts equally suggestive of evolution.

_The Local Relations of Fossil and Living Animals._

If all existing animals have been produced from ancestral forms--mostly
extinct--under the law of variation and natural selection, we may expect
to find in most cases a close relation between the living forms of each
country and those which inhabited it in the immediately preceding epoch.
But if species have originated in some quite different way, either by
any kind of special creation, or by sudden advances of organisation in
the offspring of preceding types, such close relationship would not be
found; and facts of this kind become, therefore, to some extent a test
of evolution under natural selection or some other law of gradual
change. Of course the relationship will not appear when extensive
migration has occurred, by which the inhabitants of one region have been
able to take possession of another region, and destroy or drive out its
original inhabitants, as has sometimes happened. But such cases are
comparatively rare, except where great changes of climate are known to
have occurred; and we usually do find a remarkab