America,
which was cut off from the North and its invading Carnivores during
the Eocene and Miocene--developed into large sloths, armadilloes, and
anteaters. The reconnection with North America in the Pliocene allowed
the northern animals to descend, but gigantic sloths (Megatherium) and
armadilloes (Glyptodon) flourished long afterwards in South America.
The Megatherium attained a length of eighteen feet in one specimen
discovered, and the Glyptodon often had a dorsal shield (like that of
the armadillo) from six to eight feet long, and, in addition, a stoutly
armoured tail several feet long.

The richness and rapidity of the mammalian development in the Tertiary,
of which this condensed survey will convey some impression, make it
impossible to do more here than glance over the vast field and indicate
the better-known connections. It will be seen that evolution not only
introduces a lucid order and arrangement into our thousands of species
of living and fossil mammals, but throws an admirable light on the
higher animal world of our time. The various orders into which the
zoologist puts our mammals are seen to be the branches of a living tree,
approaching more and more closely to each other in early Tertiary times,
in spite of the imperfectness of the geological record. We at last trace
these diverging lines to a few very primitive, generalised, patriarchal
groups, which in turn approach each other very closely in structure,
and plainly suggest a common Cretaceous ancestor. Whether that common
ancestor was an Edentate, an Insectivore, or Creodont, or something more
primitive than them all, is disputed. But the divergence of nearly all
the lines of our mammal world from those patriarchal types is admirably
clear. In the mutual struggle of carnivore and herbivore, in adaptation
to a hundred different environments (the water, the land, and the air,
the tree, the open plain, the underground, the marsh, etc.) and forms
of diet, we find the descendants of these patriarchal animals gradually
developing their distinctive characters. Then we find the destructive
agencies of living and inorganic nature blotting out type after type,
and the living things that spread over the land in the later Tertiary
are found to be broadly identical with the living things of to-day. The
last great selection, the northern Ice-Age, will give the last touches
of modernisation.

CHAPTER XVIII. THE EVOLUTION OF MAN

We have reserved for a close