almost complete series of
gradations from brains little higher than that of a Rodent, to brains
little lower than that of Man. And it is a remarkable circumstance that
though, so far as our present knowledge extends, there 'is' one true
structural break in the series of forms of Simian brains, this hiatus
does not lie between Man and the man-like apes, but between the lower
and the lowest Simians; or, in other words, between the old and new
world apes and monkeys, and the Lemurs. Every Lemur which has yet been
examined, in fact, has its cerebellum partially visible from above, and
its posterior lobe, with the contained posterior cornu and hippocampus
minor, more or less rudimentary. Every Marmoset, American monkey,
old-world monkey, Baboon, or Man-like ape, on the contrary, has its
cerebellum entirely hidden, posteriorly, by the cerebral lobes, and
possesses a large posterior cornu, with a well-developed hippocampus
minor.

[Illustration: FIG. 20.--Drawings of the internal casts of a Man's and
of a Chimpanzee's skull, of the same absolute length, and placed in
corresponding positions. 'A'. Cerebrum; 'B'. Cerebellum. The former
drawing is taken from a cast in the Museum of the Royal College of
Surgeons, the latter from the photograph of the cast of a Chimpanzee's
skull, which illustrates the paper by Mr. Marshall 'On the Brain of the
Chimpanzee' in the 'Natural History Review' for July, 1861. The sharper
definition of the lower edge of the cast of the cerebral chamber in the
Chimpanzee arises from the circumstance that the tentorium remained in
that skull and not in the Man's. The cast more accurately represents the
brain in Chimpanzee than in the Man; and the great backward projection
of the posterior lobes of the cerebrum of the former, beyond the
cerebellum, is conspicuous.]

In many of these creatures, such as the Saimiri ('Chrysothrix'), the
cerebral lobes overlap and extend much further behind the cerebellum, in
proportion, than they do in man (Fig. 16)--and it is quite certain that,
in all, the cerebellum is completely covered behind, by well-developed
posterior lobes. The fact can be verified by every one who possesses the
skull of any old or new world monkey. For, inasmuch as the brain in all
mammals completely fills the cranial cavity, it is obvious that a cast
of the interior of the skull will reproduce the general form of the
brain, at any rate with such minute and, for the present purpose,
utterly unimport