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Title: Note on the Resemblances and Differences in the Structure and the Development of the Brain in Man and Apes

Author: Thomas Henry Huxley
        Charles Darwin

Release date: October 1, 2000 [eBook #2354]
                Most recently updated: December 31, 2020

Language: English

Credits: Produced by Sue Asscher.  HTML version by Al Haines.


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  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.












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