ars;
even if we suppose that the mammal mainly attacked the eggs and the
young. We may very well believe that more powerful mammals than the
primitive Mesozoic specimens were already developed in some part of the
earth--say, Africa--and that the rise of the land gave them a bridge
across the Mediterranean to Europe. Probably this happened; but the
important point is that the reptiles were already almost extinct. The
difficulty is even greater when we reflect that it is precisely the
most powerful reptiles (Deinosaurs) and least accessible reptiles
(Pterosaurs, Ichthyosaurs, etc.) which disappear, while the smaller land
and water reptiles survive and retreat southward--where the mammals are
just as numerous. That assuredly is not the effect of an invasion of
carnivores, even if we could overlook the absence of such carnivores
from the record until after the extinction of the reptiles in most
places.

I have entered somewhat fully into this point, partly because of
its great interest, but partly lest it be thought that I am merely
reproducing a tradition of geological literature without giving due
attention to the criticisms of recent writers. The plain and common
interpretation of the Cretaceous revolution--that a fall in temperature
was its chief devastating agency--is the only one that brings harmony
into all the facts. The one comprehensive enemy of that vast reptile
population was cold. It was fatal to the adult because he had a
three-chambered heart and no warm coat; it was fatal to the Mesozoic
vegetation on which, directly or indirectly, he fed; it was fatal to his
eggs and young because the mother did not brood over the one or care
for the other. It was fatal to the Pterosaurs, even if they were
warm-blooded, because they had no warm coats and did not (presumably)
hatch their eggs; and it was equally fatal to the viviparous
Ichthyosaurs. It is the one common fate that could slay all classes.
When we find that the surviving reptiles retreat southward, only
lingering in Europe during the renewed warmth of the Eocene and Miocene
periods, this interpretation is sufficiently confirmed. And when
we recollect that these things coincide with the extinction of the
Ammonites and Belemnites, and the driving of their descendants further
south, as well as the rise and triumph of deciduous trees, it is
difficult to see any ground for hesitating.

But we need not, and must not, imagine a period of cold as severe,
prolonged,