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Descent of Man [ 1871]

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Bullseye
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« Reply #75 on: February 09, 2009, 01:30:18 pm »

"In some brains it appears simply as an indentation of the margin of
the hemisphere, but, in others, it tends 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:*

  * "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 platyrhine apes. In
fact, while the temporo-occipital is one of the most constant of sulci
in the catarhine, or Old World, apes, it is never very strongly
developed in the New World apes; it is absent in the smaller
platyrhine; rudimentary in Pithecia;* and more or less obliterated
by bridging convolutions in Ateles.

  * FIower, "On the Anatomy of Pithecia Monachus," Proceedings of
the Zoological Society, 1862.

  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 theEuropean 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:*

  * 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, Cereopithecus, 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:*

  * 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
platyrhine 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.*

  * "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"* (loc. cit., p. 39), and the other of a human foetus at the
22nd or 23rd week of utero-gestation, 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."

  * 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."

  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,* and more particularly
by Ecker,*(2) whose work is not only the latest, but by far the most
complete, memoir on the subject.

  * Uber die typische Anordnung der Furchen und Windungen auf den
Grosshirn-Hemispharen des Menschen und der Affen," Archiv fur
Anthropologie, iii., 1868
  *(2) "Zur Entwicklungsgeschichte der Furchen und Windungen der
Grosshirn-Hemispharen im Foetus des Menschen." Archiv fur
Anthropologie, iii., 1868.

  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 utero-gestation. 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 tab. 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 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 Platyrhini 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 Platyrhini 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 Platyrhini. But, it by no means follows, that the
rule which may hold good for the Platyrhini extends to the
Catarhini. We have no information whatever respecting the
development of the brain in the Cynomorphia; 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 show 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.*

  * 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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