g of the previously flat surfaces: later on this rounding
progressively increases, until eventually we get the complete
interlocking of the present time.

As regards teeth, and still confining attention to the hoofed mammals,
we find that low down in the geological series the teeth present on
their grinding surfaces only three simple tubercles. Later on a fourth
tubercle is added, and later still there is developed that complicated
system of ridges and furrows which is characteristic of these teeth at
the present time, and which was produced by manifold and various
involutions of the three or four simple tubercles of Eocene and lower
Miocene times. In other words, the principle of gradual improvement in
the construction of teeth, which has already been depicted as regards
the particular case of the Horse-family (Fig. 83), is no less apparent
in the pedigree of all the other mammalia, wherever the palaeontological
history is sufficiently intact to serve as a record at all.

Lastly, as regards the skull, casts of the interior show that all the
earlier mammals had small brains with comparatively smooth or
unconvoluted surfaces; and that as time went on the mammalian brain
gradually advanced in size and complexity. Indeed so small were the
cerebral hemispheres of the primitive mammals that they did not overlap
the cerebellum, while their smoothness must have been such as in this
respect to have resembled the brain of a bird or reptile. This, of
course, is just as it ought to be, if the brain, which the skull has to
accommodate, has been gradually evolved into larger and larger
proportions in respect of its cerebral hemispheres, or the upper masses
of it which constitute the seat of intelligence. Thus, if we look at the
above series of wood-cuts, which represents the comparative structure of
the brain in the existing classes of the Vertebrata, we can immediately
understand why the fossil skulls of Mammalia should present a gradual
increase in size and furrowing, so as to accommodate the general
increase of the brain in both these respects between the level marked
"maml" and that marked "man," in the last of the diagrams. (Fig. 87.)

[Illustration: FIG. 86.--Comparative series of Brains. (After Le
Conte.) The series reads from above downwards, and represents
diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal,
and a Man. In each case the letter A marks a side view, and the
letter B a top vi