,000 feet of
water-made rock which yields no fossils--no remains of living things,
though living things were certainly there! Our little layer of
Tertiary strata on the top is, however, very important. It took
several million years in forming, although it is only one-fortieth of
the whole thickness of aqueous deposit on the crust of the earth. We
divide it into Pliocene, Miocene, and Eocene, and each of these into
upper, middle, and lower, the Eocene being the oldest. Our London clay
and Woolwich sands are lower Eocene; there is a good deal of Miocene
in Switzerland and Germany, whilst the Pliocene is represented by
whole provinces of Italy, parts of central France, and by the White
and Red "crags" of Suffolk.[5]

[Illustration: Fig. 9.--Skeleton of the Indian elephant. Only four
toes are visible, the fifth concealed owing to the view from the
side.]

It is during this Tertiary period that the mammals--the warm-blooded,
hairy quadrupeds, which suckle their young--have developed (they had
come into existence a good deal earlier), and we find the remains of
ancestral forms of the living kinds of cattle, pigs, horses,
rhinoceroses, tapirs, elephants, lions, wolves, bears, etc., embedded
in the successive layers of Tertiary deposits. Naturally enough, those
most like the present animals are found in late Pliocene, and those
which are close to the common ancestors of many of the later kinds are
found in the Eocene, whilst we also find, at various levels of the
Tertiary deposit, remains of side-branches of the mammalian pedigree,
which, though including very powerful and remarkable beasts, have left
no line of descent to represent them at the present day. We have been
able to trace the great modern one-toed horses, zebras, and asses,
with their complicated pattern of grinding-teeth back by quite gradual
steps (represented by the bones and teeth of fossil kinds of horses),
to smaller three-toed animals with simpler tuberculated teeth, and
even, without any marked break in the series, to a small Eocene animal
(not bigger than a spaniel) with four equal-sized toes on its front
foot, and three on its hind foot. We know, too, a less direct series
of intermediate forms leading beyond this to an animal with five toes
on each foot and "typical" teeth. In fact, no one doubts that
(leaving aside a few difficult and doubtful cases) all such big
existing mammals, as I mentioned above, as well as monkeys and man,
are derived from small