are confined to the
hotter portions of the earth. For the most part, the reptiles now play
an insignificant and unobtrusive part. The little molelike creatures,
practically unnoticed between their feet in the later Mesozoic, have
come to supplant them entirely, and almost to rival them in size.
While the reptiles have grown steadily smaller, the mammals have
steadily become larger.

While there is no land mammal to-day as big as the heaviest of the
reptiles in the Mesozoic, the whale, which is one of the mammals that
has again taken to the ocean, surpasses in size even those gigantic
creatures. There never lived in the world before a creature quite so
big as the biggest of our whales. Size, however, is not the most
important point in any animal. Speed, sagacity, variability, and power
of adaptation, these are the qualities which the world prizes, and
these the new mammals possessed.

The next geological era is the Cenozoic, or period of modern life.
This is divided into two quite distinct sections, the Tertiary and the
Quaternary. This era began about five million years ago, roughly
speaking, and is still going on. The greater half of it is known as
the Tertiary. It was during this time that the mammals came to their
own. At first these creatures belonged to what the scientist knows as
generalized types. They are jacks-of-all-trades. The student of early
animal life finds in the little Phenacodus, which was scarcely bigger
than a good-sized setter dog, the beginnings from which many forms
have subsequently developed. This creature showed points of structure
which to-day may be seen in such diversified animals as the dog, the
horse, the rabbit, and the monkey. It is not, of course, suggested
that Phenacodus was the immediate ancestor of any of these. But there
were no animals in those times more like these I have mentioned than
was Phenacodus, and from forms like it in main features all of these
other animals have since been derived, each species of animal having
become adapted to one particular kind of life. The development of
diversified situations on the earth, the varieties of climate, the
variation between marsh and upland, between valley and plateau,
furnish a complexity of environment into each niche of which a new
form of animal fitted itself.

With the increased complexity of mammals comes the submergence of the
reptiles and amphibians to-day. In all sorts of situations we find
mammals. The old-fashioned con