f Teleosts, so that we may say
that before the end of the Cretaceous the waters swarmed with primitive
and patriarchal cod, salmon, herring, perch, pike, bream, eels, and
other fishes. Some of them grew to an enormous size. The Portheus,
an American pike, seems to have been about eight feet long; and the
activity of an eight-foot pike may be left to the angler's imagination.
All, however, are, as evolution demands, of a generalised and unfamiliar
type: the material out of which our fishes will be evolved.

Of the insects we have very little trace in the Cretaceous. We shall
find them developing with great richness in the following period, but,
imperfect as the record is, we may venture to say that they were checked
in the Cretaceous. There were good conditions for preserving them, but
few are preserved. And of the other groups of invertebrates we need only
say that they show a steady advance toward modern types. The sea-lily
fills the rocks no longer; the sea-urchin is very abundant. The Molluscs
gain on the more lowly organised Brachiopods.

To complete the picture we must add that higher types probably arose
in the later Cretaceous which do not appear in the records. This is
particularly true of the birds and mammals. We find them spreading
so early in the Tertiary that we must put back the beginning of the
expansion to the Cretaceous. As yet, however, the only mammal remains
we find are such jaws and teeth of primitive mammals as we have already
described. The birds we described (after the Archaeopteryx) also belong
to the Cretaceous, and they form another of the doomed races. Probably
the modern birds were already developing among the new vegetation on the
higher ground.

These are the facts of Cretaceous life, as far as the record has yielded
them, and it remains for us to understand them. Clearly there has been
a great selective process analogous to, if not equal to, the winnowing
process at the end of the Palaeozoic. As there has been a similar, if
less considerable, upheaval of the land, we are at once tempted to think
that the great selective agency was a lowering of the temperature. When
we further find that the most important change in the animal world is
the destruction of the cold-blooded reptiles, which have no concern for
the young, and the luxuriant spread of the warm-blooded animals, which
do care for their young, the idea is greatly confirmed. When we add
that the powerful Molluscs which are slain, w