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 etext was prepared by Sue Asscher
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
found the following passage: "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 manlike 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 manlike 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."
This statement was a strictly accurate account of what was known
when it was made; and it does not appear to me to be more than
apparently weakened by the subsequent discovery of the relatively
small development of the posterior lobes in the Siamang and in
the Howling monkey. Notwithstanding the exceptional brevity of
the posterior lobes in these two species, no one will pretend
that their brains, in the slightest degree, approach those of the
Lemurs. And if, instead of putting Hapale out of its natural
place, as Professor Bischoff most unaccountably does, we write
the series of animals he has chosen to mention as follows: Homo,
Pithecus, Troglodytes, Hylobates, Semnopithecus, Cynocephalus,
Cercopithecus, Macacus, Cebus, Callithrix, Hapale, Lemur,
Stenops, I venture to reaffirm that the great break in this
series lies between Hapale and Lemur, and that this break is
considerably greater than that between any other two terms of
that series. Professor Bischoff ignores the fact that long
before he wrote, Gratiolet had suggested the separation of the
Lemurs from the other Primates on the very ground of the
difference in their cerebral characters; and that Professor
Flower had made the following observations in the course of his
description of the brain of the Javan Loris: (75. 'Transactions
of the Zoological Society,' vol. v. 1862.)
"And it is especially remarkable that, in the development of the
posterior lobes, there is no approximation to the Lemurine, short
hemisphered brain, in those monkeys which are commonly supposed
to approach this family in other respects, viz. the lower members
of the Platyrrhine group."
So far as the structure of the adult brain is concerned, then,
the very considerable additions to our knowledge, which have been
made by the researches of so many investigators, during the past
ten years, fully justify the statement which I made in 1863. But
it has been said, that, admitting the similarity between the
adult brains of man and apes, they are nevertheless, in reality,
widely different, because they exhibit fundamental differences in
the mode of their development. No one would be more ready than I
to admit the force of this argument, if such fundamental
differences of development really exist. But I deny that they do
exist. On the contrary, there is a fundamental agreement in the
development of the brain in men and apes.
Gratiolet originated the statement that there is a fundamental
difference in the development of the brains of apes and that of
man--consisting in this; that, in the apes, the sulci which first
make their appearance are situated on the posterior region of the
cerebral hemispheres, while, in the human foetus, the sulci first
become visible on the frontal lobes. (76. "Chez tous les singes,
les plis posterieurs se developpent les premiers; les plis
anterieurs se developpent plus tard, aussi la vertebre occipitale
et la parietale sont-elles relativement tres-grandes chez le
foetus. L'Homme presente une exception remarquable quant a
l'epoque de l'apparition des plis frontaux, qui sont les premiers
indiques; mais le developpement general du lobe frontal, envisage
seulement par rapport a son volume, suit les memes lois que dans
les singes:" Gratiolet, 'Memoire sur les plis cerebres de
l'Homme et des Primateaux,' p. 39, Tab. iv, fig. 3.)
This general statement is based upon two observations, the one of
a Gibbon almost ready to be born, in which the posterior gyri
were "well developed," while those of the frontal lobes were
"hardly indicated" (77. Gratiolet's words are (loc. cit. p. 39):
"Dans le foetus dont il s'agit les plis cerebraux posterieurs
sont bien developpes, tandis que les plis du lobe frontal sont a
peine indiques." The figure, however (Pl. iv, fig. 3), shews the
fissure of Rolando, and one of the frontal sulci plainly enough.
Nevertheless, M. Alix, in his 'Notice sur les travaux
anthropologiques de Gratiolet' ('Mem. de la Societe
d'Anthropologie de Paris,' 1868, page 32), writes thus:
"Gratiolet a eu entre les mains le cerveau d'un foetus de Gibbon,
singe eminemment superieur, et tellement rapproche de l'orang,
que des naturalistes tres-competents l'ont range parmi les
anthropoides. M. Huxley, par exemple, n'hesite pas sur ce point.
Eh bien, c'est sur le cerveau d'un foetus de Gibbon que Gratiolet
a vu LES CIRCONVOLUTIONS DU LOBE TEMPORO-SPHENOIDAL DEJA
DEVELOPPEES LORSQU'IL N'EXISTENT PAS ENCORE DE PLIS SUR LE LOBE
FRONTAL. Il etait donc bien autorise a dire que, chez l'homme
les circonvolutions apparaissent d'a en w, tandis que chez les
singes elles se developpent d'w en a."), and the other of a human
foetus at the 22nd or 23rd week of uterogestation, in which
Gratiolet notes that the insula was uncovered, but that
nevertheless "des incisures sement de lobe anterieur, une
scissure peu profonde indique la separation du lobe occipital,
tres-reduit, d'ailleurs des cette epoque. Le reste de la surface
cerebrale est encore absolument lisse."
Three views of this brain are given in Plate II, figs. 1, 2, 3,
of the work cited, shewing the upper, lateral and inferior views
of the hemispheres, but not the inner view. It is worthy of note
that the figure by no means bears out Gratiolet's description,
inasmuch as the fissure (antero-temporal) on the posterior half
of the face of the hemisphere is more marked than any of those
vaguely indicated in the anterior half. If the figure is
correct, it in no way justifies Gratiolet's conclusion: "Il y a
donc entre ces cerveaux [those of a Callithrix and of a Gibbon]
et celui du foetus humain une difference fondamental. Chez
celui-ci, longtemps avant que les plis temporaux apparaissent,
les plis frontaux, ESSAYENT d'exister."
Since Gratiolet's time, however, the development of the gyri and
sulci of the brain has been made the subject of renewed
investigation by Schmidt, Bischoff, Pansch (78. 'Ueber die
typische Anordnung der Furchen und Windungen auf den Grosshirn-
Hemispharen des Menschen und der Affen,' 'Archiv fur
Anthropologie,' iii. 1868.), and more particularly by Ecker (79.
'Zur Entwicklungs Geschichte der Furchen und Windungen der
Grosshirn-Hemispharen im Foetus des Menschen.' 'Archiv fur
Anthropologie,' iii. 1868.), whose work is not only the latest,
but by far the most complete, memoir on the subject.
The final results of their inquiries may be summed up as
follows:--
1. In the human foetus, the sylvian fissure is formed in the
course of the third month of uterogestation. In this, and in the
fourth month, the cerebral hemispheres are smooth and rounded
(with the exception of the sylvian depression), and they project
backwards far beyond the cerebellum.
2. The sulci, properly so called, begin to appear in the
interval between the end of the fourth and the beginning of the
sixth month of foetal life, but Ecker is careful to point out
that, not only the time, but the order, of their appearance is
subject to considerable individual variation. In no case,
however, are either the frontal or the temporal sulci the
earliest.
The first which appears, in fact, lies on the inner face of the
hemisphere (whence doubtless Gratiolet, who does not seem to have
examined that face in his foetus, overlooked it), and is either
the internal perpendicular (occipito-parietal), or the calcarine
sulcus, these two being close together and eventually running
into one another. As a rule the occipito-parietal is the earlier
of the two.
3. At the latter part of this period, another sulcus, the
"posterio-parietal," or "Fissure of Rolando" is developed, and it
is followed, in the course of the sixth month, by the other
principal sulci of the frontal, parietal, temporal and occipital
lobes. There is, however, no clear evidence that one of these
constantly appears before the other; and it is remarkable that,
in the brain at the period described and figured by Ecker (loc.
cit. pp. 212-213, Taf. II, figs. 1, 2, 3, 4), the antero-temporal
sulcus (scissure parallele) so characteristic of the ape's brain,
is as well, if not better developed than the fissure of Rolando,
and is much more marked than the proper frontal sulci.
Taking the facts as they now stand, it appears to me that the
order of the appearance of the sulci and gyri in the foetal human
brain is in perfect harmony with the general doctrine of
evolution, and with the view that man has been evolved from some
ape-like form; though there can be no doubt that form was, in
many respects, different from any member of the Primates now
living.
Von Baer taught us, half a century ago, that, in the course of
their development, allied animals put on at first, the characters
of the greater groups to which they belong, and, by degrees,
assume those which restrict them within the limits of their
family, genus, and species; and he proved, at the same time, that
no developmental stage of a higher animal is precisely similar to
the adult condition of any lower animal. It is quite correct to
say that a frog passes through the condition of a fish, inasmuch
as at one period of its life the tadpole has all the characters
of a fish, and if it went no further, would have to be grouped
among fishes. But it is equally true that a tadpole is very
different from any known fish.
In like manner, the brain of a human foetus, at the fifth month,
may correctly be said to be, not only the brain of an ape, but
that of an Arctopithecine or marmoset-like ape; for its
hemispheres, with their great posterior lobster, and with no
sulci but the sylvian and the calcarine, present the
characteristics found only in the group of the Arctopithecine
Primates. But it is equally true, as Gratiolet remarks, that, in
its widely open sylvian fissure, it differs from the brain of any
actual marmoset. No doubt it would be much more similar to the
brain of an advanced foetus of a marmoset. But we know nothing
whatever of the development of the brain in the marmosets. In
the Platyrrhini proper, the only observation with which I am
acquainted is due to Pansch, who found in the brain of a foetal
Cebus Apella, in addition to the sylvian fissure and the deep
calcarine fissure, only a very shallow antero-temporal fissure
(scissure parallele of Gratiolet).
Now this fact, taken together with the circumstance that the
antero-temporal sulcus is present in such Platyrrhini as the
Saimiri, which present mere traces of sulci on the anterior half
of the exterior of the cerebral hemispheres, or none at all,
undoubtedly, so far as it goes, affords fair evidence in favour
of Gratiolet's hypothesis, that the posterior sulci appear before
the anterior, in the brains of the Platyrrhini. But, it by no
means follows, that the rule which may hold good for the
Platyrrhini extends to the Catarrhini. We have no information
whatever respecting the development of the brain in the
Cynomorpha; and, as regards the Anthropomorpha, nothing but the
account of the brain of the Gibbon, near birth, already referred
to. At the present moment there is not a shadow of evidence to
shew that the sulci of a chimpanzee's, or orang's, brain do not
appear in the same order as a man's.
Gratiolet opens his preface with the aphorism: "Il est dangereux
dans les sciences de conclure trop vite." I fear he must have
forgotten this sound maxim by the time he had reached the
discussion of the differences between men and apes, in the body
of his work. No doubt, the excellent author of one of the most
remarkable contributions to the just understanding of the
mammalian brain which has ever been made, would have been the
first to admit the insufficiency of his data had he lived to
profit by the advance of inquiry. The misfortune is that his
conclusions have been employed by persons incompetent to
appreciate their foundation, as arguments in favour of
obscurantism. (80. For example, M. l'Abbe Lecomte in his
terrible pamphlet, 'Le Darwinisme et l'origine de l'Homme,'
1873.)
But it is important to remark that, whether Gratiolet was right
or wrong in his hypothesis respecting the relative order of
appearance of the temporal and frontal sulci, the fact remains;
that before either temporal or frontal sulci, appear, the foetal
brain of man presents characters which are found only in the
lowest group of the Primates (leaving out the Lemurs); and that
this is exactly what we should expect to be the case, if man has
resulted from the gradual modification of the same form as that
from which the other Primates have sprung.
End of T.H. Huxley's On the Brain [from Descent of Man by Charles Darwin]
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