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TITLE: T.H. HUXLEY ON THE BRAIN FROM DESCENT OF MAN
AUTHOR: CHARLES DARWIN
EBOOK: E02556 (O'Briens Book Cellar)

NOTE ON THE RESEMBLANCES AND DIFFERENCES IN THE STRUCTURE AND THE
DEVELOPMENT OF THE BRAIN IN MAN AND APES

BY

PROFESSOR T.H. HUXLEY, F.R.S.

[This essay is taken from 'The Descent of Man and Selection in
relation to Sex' by Charles Darwin where it appears at the end of
Chapter VII which is also the end of Part I.  Footnotes are
numbered as they appear in 'The Descent of Man.']

The controversy respecting the nature and the extent of the
differences in the structure of the brain in man and the apes,
which arose some fifteen years ago, has not yet come to an end,
though the subject matter of the dispute is, at present, totally
different from what it was formerly.  It was originally asserted
and re-asserted, with singular pertinacity, that the brain of all
the apes, even the highest, differs from that of man, in the
absence of such conspicuous structures as the posterior lobes of
the cerebral hemispheres, with the posterior cornu of the lateral
ventricle and the hippocampus minor, contained in those lobes,
which are so obvious in man.

But the truth that the three structures in question are as well
developed in apes' as in human brains, or even better; and that
it is characteristic of all the Primates (if we exclude the
Lemurs) to have these parts well developed, stands at present on
as secure a basis as any proposition in comparative anatomy. 
Moreover, it is admitted by every one of the long series of
anatomists who, of late years, have paid special attention to the
arrangement of the complicated sulci and gyri which appear upon
the surface of the cerebral hemispheres in man and the higher
apes, that they are disposed after the very same pattern in him,
as in them.  Every principal gyrus and sulcus of a chimpanzee's
brain is clearly represented in that of a man, so that the
terminology which applies to the one answers for the other.  On
this point there is no difference of opinion.  Some years since,
Professor Bischoff published a memoir (70.  'Die Grosshirn-
Windungen des Menschen;' 'Abhandlungen der K. Bayerischen
Akademie,' B. x. 1868.) on the cerebral convolutions of man and
apes; and as the purpose of my learned colleague was certainly
not to diminish the value of the differences between apes and men
in this respect, I am glad to make a citation from him.

"That the apes, and especially the orang, chimpanzee and gorilla,
come very close to man in their organisation, much nearer than to
any other animal, is a well known fact, disputed by nobody. 
Looking at the matter from the point of view of organisation
alone, no one probably would ever have disputed the view of
Linnaeus, that man should be placed, merely as a peculiar
species, at the head of the mammalia and of those apes.  Both
shew, in all their organs, so close an affinity, that the most
exact anatomical investigation is needed in order to demonstrate
those differences which really exist.  So it is with the brains. 
The brains of man, the orang, the chimpanzee, the gorilla, in
spite of all the important differences which they present, come
very close to one another" (loc. cit. p. 101).

There remains, then, no dispute as to the resemblance in
fundamental characters, between the ape's brain and man's:  nor
any as to the wonderfully close similarity between the
chimpanzee, orang and man, in even the details of the arrangement
of the gyri and sulci of the cerebral hemispheres.  Nor, turning
to the differences between the brains of the highest apes and
that of man, is there any serious question as to the nature and
extent of these differences.  It is admitted that the man's
cerebral hemispheres are absolutely and relatively larger than
those of the orang and chimpanzee; that his frontal lobes are
less excavated by the upward protrusion of the roof of the
orbits; that his gyri and sulci are, as a rule, less
symmetrically disposed, and present a greater number of secondary
plications.  And it is admitted that, as a rule, in man, the
temporo-occipital or "external perpendicular" fissure, which is
usually so strongly marked a feature of the ape's brain is but
faintly marked.  But it is also clear, that none of these
differences constitutes a sharp demarcation between the man's and
the ape's brain.  In respect to the external perpendicular
fissure of Gratiolet, in the human brain for instance, Professor
Turner remarks:  (71.  'Convolutions of the Human Cerebrum
Topographically Considered,' 1866, p. 12.)

"In some brains it appears simply as an indentation of the margin
of the hemisphere, but, in others, it extends for some distance
more or less transversely outwards.  I saw it in the right
hemisphere of a female brain pass more than two inches outwards;
and on another specimen, also the right hemisphere, it proceeded
for four-tenths of an inch outwards, and then extended downwards,
as far as the lower margin of the outer surface of the
hemisphere.  The imperfect definition of this fissure in the
majority of human brains, as compared with its remarkable
distinctness in the brain of most Quadrumana, is owing to the
presence, in the former, of certain superficial, well marked,
secondary convolutions which bridge it over and connect the
parietal with the occipital lobe.  The closer the first of these
bridging gyri lies to the longitudinal fissure, the shorter is
the external parieto-occipital fissure" (loc. cit. p. 12).

The obliteration of the external perpendicular fissure of
Gratiolet, therefore, is not a constant character of the human
brain.  On the other hand, its full development is not a constant
character of the higher ape's brain.  For, in the chimpanzee, the
more or less extensive obliteration of the external perpendicular
sulcus by "bridging convolutions," on one side or the other, has
been noted over and over again by Prof. Rolleston, Mr. Marshall,
M. Broca and Professor Turner.  At the conclusion of a special
paper on this subject the latter writes:  (72.  Notes more
especially on the bridging convolutions in the Brain of the
Chimpanzee, 'Proceedings of the Royal Society of Edinburgh,'
1865-6.)

"The three specimens of the brain of a chimpanzee, just
described, prove, that the generalisation which Gratiolet has
attempted to draw of the complete absence of the first connecting
convolution and the concealment of the second, as essentially
characteristic features in the brain of this animal, is by no
means universally applicable.  In only one specimen did the
brain, in these particulars, follow the law which Gratiolet has
expressed.  As regards the presence of the superior bridging
convolution, I am inclined to think that it has existed in one
hemisphere, at least, in a majority of the brains of this animal
which have, up to this time, been figured or described.  The
superficial position of the second bridging convolution is
evidently less frequent, and has as yet, I believe, only been
seen in the brain (A) recorded in this communication.  The
asymmetrical arrangement in the convolutions of the two
hemispheres, which previous observers have referred to in their
descriptions, is also well illustrated in these specimens" (pp.
8, 9).

Even were the presence of the temporo-occipital, or external
perpendicular, sulcus, a mark of distinction between the higher
apes and man, the value of such a distinctive character would be
rendered very doubtful by the structure of the brain in the
Platyrrhine apes.  In fact, while the temporo-occipital is one of
the most constant of sulci in the Catarrhine, or Old World, apes,
it is never very strongly developed in the New World apes; it is
absent in the smaller Platyrrhini; rudimentary in Pithecia (73. 
Flower, 'On the Anatomy of Pithecia Monachus,' 'Proceedings of
the Zoological Society,' 1862.); and more or less obliterated by
bridging convolutions in Ateles.

A character which is thus variable within the limits of a single
group can have no great taxonomic value.

It is further established, that the degree of asymmetry of the
convolution of the two sides in the human brain is subject to
much individual variation; and that, in those individuals of the
Bushman race who have been examined, the gyri and sulci of the
two hemispheres are considerably less complicated and more
symmetrical than in the European brain, while, in some
individuals of the chimpanzee, their complexity and asymmetry
become notable.  This is particularly the case in the brain of a
young male chimpanzee figured by M. Broca.  ('L'ordre des
Primates,' p. 165, fig. 11.)

Again, as respects the question of absolute size, it is
established that the difference between the largest and the
smallest healthy human brain is greater than the difference
between the smallest healthy human brain and the largest
chimpanzee's or orang's brain.

Moreover, there is one circumstance in which the orang's and
chimpanzee's brains resemble man's, but in which they differ from
the lower apes, and that is the presence of two corpora
candicantia--the Cynomorpha having but one.

In view of these facts I do not hesitate in this year 1874, to
repeat and insist upon the proposition which I enunciated in
1863:  (74.  'Man's Place in Nature,' p. 102.)

"So far as cerebral structure goes, therefore, it is clear that
man differs less from the chimpanzee or the orang, than these do
even from the monkeys, and that the difference between the brain
of the chimpanzee and of man is almost insignificant when
compared with that between the chimpanzee brain and that of a
Lemur."

In the paper to which I have referred, Professor Bischoff does
not deny the second part of this statement, but he first makes
the irrelevant remark that it is not wonderful if the brains of
an orang and a Lemur are very different; and secondly, goes on to
assert that, "If we successively compare the brain of a man with
that of an orang; the brain of this with that of a chimpanzee; of
this with that of a gorilla, and so on of a Hylobates,
Semnopithecus, Cynocephalus, Cercopithecus, Macacus, Cebus,
Callithrix, Lemur, Stenops, Hapale, we shall not meet with a
greater, or even as great a, break in the degree of development
of the convolutions, as we find between the brain of a man and
that of an orang or chimpanzee."

To which I reply, firstly, that whether this assertion be true or
false, it has nothing whatever to do with the proposition
enunciated in 'Man's Place in Nature,' which refers not to the
development of the convolutions alone, but to the structure of
the whole brain.  If Professor Bischoff had taken the trouble to
refer to p. 96 of the work he criticises, in fact, he would have

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