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The Descent of Man and Selection in Relation to Sex

C >> Charles Darwin >> The Descent of Man and Selection in Relation to Sex




Thus we can understand how it has come to pass that man and all other
vertebrate animals have been constructed on the same general model, why
they pass through the same early stages of development, and why they retain
certain rudiments in common. Consequently we ought frankly to admit their
community of descent: to take any other view, is to admit that our own
structure, and that of all the animals around us, is a mere snare laid to
entrap our judgment. This conclusion is greatly strengthened, if we look
to the members of the whole animal series, and consider the evidence
derived from their affinities or classification, their geographical
distribution and geological succession. It is only our natural prejudice,
and that arrogance which made our forefathers declare that they were
descended from demi-gods, which leads us to demur to this conclusion. But
the time will before long come, when it will be thought wonderful that
naturalists, who were well acquainted with the comparative structure and
development of man, and other mammals, should have believed that each was
the work of a separate act of creation.


CHAPTER II.

ON THE MANNER OF DEVELOPMENT OF MAN FROM SOME LOWER FORM.

Variability of body and mind in man--Inheritance--Causes of variability--
Laws of variation the same in man as in the lower animals--Direct action of
the conditions of life--Effects of the increased use and disuse of parts--
Arrested development--Reversion--Correlated variation--Rate of increase--
Checks to increase--Natural selection--Man the most dominant animal in the
world--Importance of his corporeal structure--The causes which have led to
his becoming erect--Consequent changes of structure--Decrease in size of
the canine teeth--Increased size and altered shape of the skull--Nakedness
--Absence of a tail--Defenceless condition of man.

It is manifest that man is now subject to much variability. No two
individuals of the same race are quite alike. We may compare millions of
faces, and each will be distinct. There is an equally great amount of
diversity in the proportions and dimensions of the various parts of the
body; the length of the legs being one of the most variable points. (1.
'Investigations in Military and Anthropological Statistics of American
Soldiers,' by B.A. Gould, 1869, p. 256.) Although in some quarters of the
world an elongated skull, and in other quarters a short skull prevails, yet
there is great diversity of shape even within the limits of the same race,
as with the aborigines of America and South Australia--the latter a race
"probably as pure and homogeneous in blood, customs, and language as any in
existence"--and even with the inhabitants of so confined an area as the
Sandwich Islands. (2. With respect to the "Cranial forms of the American
aborigines," see Dr. Aitken Meigs in 'Proc. Acad. Nat. Sci.' Philadelphia,
May 1868. On the Australians, see Huxley, in Lyell's 'Antiquity of Man,'
1863, p. 87. On the Sandwich Islanders, Prof. J. Wyman, 'Observations on
Crania,' Boston, 1868, p. 18.) An eminent dentist assures me that there is
nearly as much diversity in the teeth as in the features. The chief
arteries so frequently run in abnormal courses, that it has been found
useful for surgical purposes to calculate from 1040 corpses how often each
course prevails. (3. 'Anatomy of the Arteries,' by R. Quain. Preface,
vol. i. 1844.) The muscles are eminently variable: thus those of the foot
were found by Prof. Turner (4. 'Transactions of the Royal Society of
Edinburgh,' vol. xxiv. pp. 175, 189.) not to be strictly alike in any two
out of fifty bodies; and in some the deviations were considerable. He
adds, that the power of performing the appropriate movements must have been
modified in accordance with the several deviations. Mr. J. Wood has
recorded (5. 'Proceedings Royal Society,' 1867, p. 544; also 1868, pp.
483, 524. There is a previous paper, 1866, p. 229.) the occurrence of 295
muscular variations in thirty-six subjects, and in another set of the same
number no less than 558 variations, those occurring on both sides of the
body being only reckoned as one. In the last set, not one body out of the
thirty-six was "found totally wanting in departures from the standard
descriptions of the muscular system given in anatomical text books." A
single body presented the extraordinary number of twenty-five distinct
abnormalities. The same muscle sometimes varies in many ways: thus Prof.
Macalister describes (6. 'Proc. R. Irish Academy,' vol. x. 1868, p. 141.)
no less than twenty distinct variations in the palmaris accessorius.

The famous old anatomist, Wolff (7. 'Act. Acad. St. Petersburg,' 1778,
part ii. p. 217.), insists that the internal viscera are more variable than
the external parts: Nulla particula est quae non aliter et aliter in aliis
se habeat hominibus. He has even written a treatise on the choice of
typical examples of the viscera for representation. A discussion on the
beau-ideal of the liver, lungs, kidneys, etc., as of the human face divine,
sounds strange in our ears.

The variability or diversity of the mental faculties in men of the same
race, not to mention the greater differences between the men of distinct
races, is so notorious that not a word need here be said. So it is with
the lower animals. All who have had charge of menageries admit this fact,
and we see it plainly in our dogs and other domestic animals. Brehm
especially insists that each individual monkey of those which he kept tame
in Africa had its own peculiar disposition and temper: he mentions one
baboon remarkable for its high intelligence; and the keepers in the
Zoological Gardens pointed out to me a monkey, belonging to the New World
division, equally remarkable for intelligence. Rengger, also, insists on
the diversity in the various mental characters of the monkeys of the same
species which he kept in Paraguay; and this diversity, as he adds, is
partly innate, and partly the result of the manner in which they have been
treated or educated. (8. Brehm, 'Thierleben,' B. i. ss. 58, 87. Rengger,
'Saugethiere von Paraguay,' s. 57.)

I have elsewhere (9. 'Variation of Animals and Plants under
Domestication,' vol. ii. chap. xii.) so fully discussed the subject of
Inheritance, that I need here add hardly anything. A greater number of
facts have been collected with respect to the transmission of the most
trifling, as well as of the most important characters in man, than in any
of the lower animals; though the facts are copious enough with respect to
the latter. So in regard to mental qualities, their transmission is
manifest in our dogs, horses, and other domestic animals. Besides special
tastes and habits, general intelligence, courage, bad and good temper,
etc., are certainly transmitted. With man we see similar facts in almost
every family; and we now know, through the admirable labours of Mr. Galton
(10. 'Hereditary Genius: an Inquiry into its Laws and Consequences,'
1869.), that genius which implies a wonderfully complex combination of high
faculties, tends to be inherited; and, on the other hand, it is too certain
that insanity and deteriorated mental powers likewise run in families.

With respect to the causes of variability, we are in all cases very
ignorant; but we can see that in man as in the lower animals, they stand in
some relation to the conditions to which each species has been exposed,
during several generations. Domesticated animals vary more than those in a
state of nature; and this is apparently due to the diversified and changing
nature of the conditions to which they have been subjected. In this
respect the different races of man resemble domesticated animals, and so do
the individuals of the same race, when inhabiting a very wide area, like
that of America. We see the influence of diversified conditions in the
more civilised nations; for the members belonging to different grades of
rank, and following different occupations, present a greater range of
character than do the members of barbarous nations. But the uniformity of
savages has often been exaggerated, and in some cases can hardly be said to
exist. (11. Mr. Bates remarks ('The Naturalist on the Amazons,' 1863,
vol. ii p. 159), with respect to the Indians of the same South American
tribe, "no two of them were at all similar in the shape of the head; one
man had an oval visage with fine features, and another was quite Mongolian
in breadth and prominence of cheek, spread of nostrils, and obliquity of
eyes.") It is, nevertheless, an error to speak of man, even if we look
only to the conditions to which he has been exposed, as "far more
domesticated" (12. Blumenbach, 'Treatises on Anthropology.' Eng.
translat., 1865, p. 205.) than any other animal. Some savage races, such
as the Australians, are not exposed to more diversified conditions than are
many species which have a wide range. In another and much more important
respect, man differs widely from any strictly domesticated animal; for his
breeding has never long been controlled, either by methodical or
unconscious selection. No race or body of men has been so completely
subjugated by other men, as that certain individuals should be preserved,
and thus unconsciously selected, from somehow excelling in utility to their
masters. Nor have certain male and female individuals been intentionally
picked out and matched, except in the well-known case of the Prussian
grenadiers; and in this case man obeyed, as might have been expected, the
law of methodical selection; for it is asserted that many tall men were
reared in the villages inhabited by the grenadiers and their tall wives.
In Sparta, also, a form of selection was followed, for it was enacted that
all children should be examined shortly after birth; the well-formed and
vigorous being preserved, the others left to perish. (13. Mitford's
'History of Greece,' vol. i. p. 282. It appears also from a passage in
Xenophon's 'Memorabilia,' B. ii. 4 (to which my attention has been called
by the Rev. J.N. Hoare), that it was a well recognised principle with the
Greeks, that men ought to select their wives with a view to the health and
vigour of their children. The Grecian poet, Theognis, who lived 550 B.C.,
clearly saw how important selection, if carefully applied, would be for the
improvement of mankind. He saw, likewise, that wealth often checks the
proper action of sexual selection. He thus writes:

"With kine and horses, Kurnus! we proceed
By reasonable rules, and choose a breed
For profit and increase, at any price:
Of a sound stock, without defect or vice.
But, in the daily matches that we make,
The price is everything: for money's sake,
Men marry: women are in marriage given
The churl or ruffian, that in wealth has thriven,
May match his offspring with the proudest race:
Thus everything is mix'd, noble and base!
If then in outward manner, form, and mind,
You find us a degraded, motley kind,
Wonder no more, my friend! the cause is plain,
And to lament the consequence is vain."

(The Works of J. Hookham Frere, vol. ii. 1872, p. 334.))

If we consider all the races of man as forming a single species, his range
is enormous; but some separate races, as the Americans and Polynesians,
have very wide ranges. It is a well-known law that widely-ranging species
are much more variable than species with restricted ranges; and the
variability of man may with more truth be compared with that of widely-
ranging species, than with that of domesticated animals.

Not only does variability appear to be induced in man and the lower animals
by the same general causes, but in both the same parts of the body are
affected in a closely analogous manner. This has been proved in such full
detail by Godron and Quatrefages, that I need here only refer to their
works. (14. Godron, 'De l'Espece,' 1859, tom. ii. livre 3. Quatrefages,
'Unite de l'Espece Humaine,' 1861. Also Lectures on Anthropology, given in
the 'Revue des Cours Scientifiques,' 1866-1868.) Monstrosities, which
graduate into slight variations, are likewise so similar in man and the
lower animals, that the same classification and the same terms can be used
for both, as has been shewn by Isidore Geoffroy St.-Hilaire. (15. 'Hist.
Gen. et Part. des Anomalies de l'Organisation,' in three volumes, tom. i.
1832.) In my work on the variation of domestic animals, I have attempted
to arrange in a rude fashion the laws of variation under the following
heads:--The direct and definite action of changed conditions, as exhibited
by all or nearly all the individuals of the same species, varying in the
same manner under the same circumstances. The effects of the long-
continued use or disuse of parts. The cohesion of homologous parts. The
variability of multiple parts. Compensation of growth; but of this law I
have found no good instance in the case of man. The effects of the
mechanical pressure of one part on another; as of the pelvis on the cranium
of the infant in the womb. Arrests of development, leading to the
diminution or suppression of parts. The reappearance of long-lost
characters through reversion. And lastly, correlated variation. All these
so-called laws apply equally to man and the lower animals; and most of them
even to plants. It would be superfluous here to discuss all of them (16.
I have fully discussed these laws in my 'Variation of Animals and Plants
under Domestication,' vol. ii. chap. xxii. and xxiii. M. J.P. Durand has
lately (1868) published a valuable essay, 'De l'Influence des Milieux,'
etc. He lays much stress, in the case of plants, on the nature of the
soil.); but several are so important, that they must be treated at
considerable length.

THE DIRECT AND DEFINITE ACTION OF CHANGED CONDITIONS.

This is a most perplexing subject. It cannot be denied that changed
conditions produce some, and occasionally a considerable effect, on
organisms of all kinds; and it seems at first probable that if sufficient
time were allowed this would be the invariable result. But I have failed
to obtain clear evidence in favour of this conclusion; and valid reasons
may be urged on the other side, at least as far as the innumerable
structures are concerned, which are adapted for special ends. There can,
however, be no doubt that changed conditions induce an almost indefinite
amount of fluctuating variability, by which the whole organisation is
rendered in some degree plastic.

In the United States, above 1,000,000 soldiers, who served in the late war,
were measured, and the States in which they were born and reared were
recorded. (17. 'Investigations in Military and Anthrop. Statistics,'
etc., 1869, by B.A. Gould, pp. 93, 107, 126, 131, 134.) From this
astonishing number of observations it is proved that local influences of
some kind act directly on stature; and we further learn that "the State
where the physical growth has in great measure taken place, and the State
of birth, which indicates the ancestry, seem to exert a marked influence on
the stature." For instance, it is established, "that residence in the
Western States, during the years of growth, tends to produce increase of
stature." On the other hand, it is certain that with sailors, their life
delays growth, as shewn "by the great difference between the statures of
soldiers and sailors at the ages of seventeen and eighteen years." Mr.
B.A. Gould endeavoured to ascertain the nature of the influences which thus
act on stature; but he arrived only at negative results, namely that they
did not relate to climate, the elevation of the land, soil, nor even "in
any controlling degree" to the abundance or the need of the comforts of
life. This latter conclusion is directly opposed to that arrived at by
Villerme, from the statistics of the height of the conscripts in different
parts of France. When we compare the differences in stature between the
Polynesian chiefs and the lower orders within the same islands, or between
the inhabitants of the fertile volcanic and low barren coral islands of the
same ocean (18. For the Polynesians, see Prichard's 'Physical History of
Mankind,' vol. v. 1847, pp. 145, 283. Also Godron, 'De l'Espece,' tom. ii.
p. 289. There is also a remarkable difference in appearance between the
closely-allied Hindoos inhabiting the Upper Ganges and Bengal; see
Elphinstone's 'History of India,' vol. i. p. 324.) or again between the
Fuegians on the eastern and western shores of their country, where the
means of subsistence are very different, it is scarcely possible to avoid
the conclusion that better food and greater comfort do influence stature.
But the preceding statements shew how difficult it is to arrive at any
precise result. Dr. Beddoe has lately proved that, with the inhabitants of
Britain, residence in towns and certain occupations have a deteriorating
influence on height; and he infers that the result is to a certain extent
inherited, as is likewise the case in the United States. Dr. Beddoe
further believes that wherever a "race attains its maximum of physical
development, it rises highest in energy and moral vigour." (19. 'Memoirs,
Anthropological Society,' vol. iii. 1867-69, pp. 561, 565, 567.)

Whether external conditions produce any other direct effect on man is not
known. It might have been expected that differences of climate would have
had a marked influence, inasmuch as the lungs and kidneys are brought into
activity under a low temperature, and the liver and skin under a high one.
(20. Dr. Brakenridge, 'Theory of Diathesis,' 'Medical Times,' June 19 and
July 17, 1869.) It was formerly thought that the colour of the skin and
the character of the hair were determined by light or heat; and although it
can hardly be denied that some effect is thus produced, almost all
observers now agree that the effect has been very small, even after
exposure during many ages. But this subject will be more properly
discussed when we treat of the different races of mankind. With our
domestic animals there are grounds for believing that cold and damp
directly affect the growth of the hair; but I have not met with any
evidence on this head in the case of man.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS.

It is well known that use strengthens the muscles in the individual, and
complete disuse, or the destruction of the proper nerve, weakens them.
When the eye is destroyed, the optic nerve often becomes atrophied. When
an artery is tied, the lateral channels increase not only in diameter, but
in the thickness and strength of their coats. When one kidney ceases to
act from disease, the other increases in size, and does double work. Bones
increase not only in thickness, but in length, from carrying a greater
weight. (21. I have given authorities for these several statements in my
'Variation of Animals and Plants under Domestication,' vol. ii. pp. 297-
300. Dr. Jaeger, "Uber das Langenwachsthum der Knochen," 'Jenaischen
Zeitschrift,' B. v. Heft. i.) Different occupations, habitually followed,
lead to changed proportions in various parts of the body. Thus it was
ascertained by the United States Commission (22. 'Investigations,' etc.,
by B.A. Gould, 1869, p. 288.) that the legs of the sailors employed in the
late war were longer by 0.217 of an inch than those of the soldiers, though
the sailors were on an average shorter men; whilst their arms were shorter
by 1.09 of an inch, and therefore, out of proportion, shorter in relation
to their lesser height. This shortness of the arms is apparently due to
their greater use, and is an unexpected result: but sailors chiefly use
their arms in pulling, and not in supporting weights. With sailors, the
girth of the neck and the depth of the instep are greater, whilst the
circumference of the chest, waist, and hips is less, than in soldiers.

Whether the several foregoing modifications would become hereditary, if the
same habits of life were followed during many generations, is not known,
but it is probable. Rengger (23. 'Saugethiere von Paraguay,' 1830, s. 4.)
attributes the thin legs and thick arms of the Payaguas Indians to
successive generations having passed nearly their whole lives in canoes,
with their lower extremities motionless. Other writers have come to a
similar conclusion in analogous cases. According to Cranz (24. 'History
of Greenland,' Eng. translat., 1767, vol. i. p. 230.), who lived for a long
time with the Esquimaux, "the natives believe that ingenuity and dexterity
in seal-catching (their highest art and virtue) is hereditary; there is
really something in it, for the son of a celebrated seal-catcher will
distinguish himself, though he lost his father in childhood." But in this
case it is mental aptitude, quite as much as bodily structure, which
appears to be inherited. It is asserted that the hands of English
labourers are at birth larger than those of the gentry. (25.
'Intermarriage,' by Alex. Walker, 1838, p. 377.) From the correlation
which exists, at least in some cases (26. 'The Variation of Animals under
Domestication,' vol. i. p. 173.), between the development of the
extremities and of the jaws, it is possible that in those classes which do
not labour much with their hands and feet, the jaws would be reduced in
size from this cause. That they are generally smaller in refined and
civilised men than in hard-working men or savages, is certain. But with
savages, as Mr. Herbert Spencer (27. 'Principles of Biology,' vol. i. p.
455.) has remarked, the greater use of the jaws in chewing coarse, uncooked
food, would act in a direct manner on the masticatory muscles, and on the
bones to which they are attached. In infants, long before birth, the skin
on the soles of the feet is thicker than on any other part of the body;
(28. Paget, 'Lectures on Surgical Pathology,' vol. ii, 1853, p. 209.) and
it can hardly be doubted that this is due to the inherited effects of
pressure during a long series of generations.

It is familiar to every one that watchmakers and engravers are liable to be
short-sighted, whilst men living much out of doors, and especially savages,
are generally long-sighted. (29. It is a singular and unexpected fact
that sailors are inferior to landsmen in their mean distance of distinct
vision. Dr. B.A. Gould ('Sanitary Memoirs of the War of the Rebellion,'
1869, p. 530), has proved this to be the case; and he accounts for it by
the ordinary range of vision in sailors being "restricted to the length of
the vessel and the height of the masts.") Short-sight and long-sight
certainly tend to be inherited. (30. 'The Variation of Animals under
Domestication,' vol. i. p. 8.) The inferiority of Europeans, in comparison
with savages, in eyesight and in the other senses, is no doubt the
accumulated and transmitted effect of lessened use during many generations;
for Rengger (31. 'Saugethiere von Paraguay,' s. 8, 10. I have had good
opportunities for observing the extraordinary power of eyesight in the
Fuegians. See also Lawrence ('Lectures on Physiology,' etc., 1822, p. 404)
on this same subject. M. Giraud-Teulon has recently collected ('Revue des
Cours Scientifiques,' 1870, p. 625) a large and valuable body of evidence
proving that the cause of short-sight, "C'est le travail assidu, de pres.")
states that he has repeatedly observed Europeans, who had been brought up
and spent their whole lives with the wild Indians, who nevertheless did not
equal them in the sharpness of their senses. The same naturalist observes
that the cavities in the skull for the reception of the several sense-
organs are larger in the American aborigines than in Europeans; and this
probably indicates a corresponding difference in the dimensions of the
organs themselves. Blumenbach has also remarked on the large size of the
nasal cavities in the skulls of the American aborigines, and connects this
fact with their remarkably acute power of smell. The Mongolians of the
plains of northern Asia, according to Pallas, have wonderfully perfect
senses; and Prichard believes that the great breadth of their skulls across
the zygomas follows from their highly-developed sense organs. (32.
Prichard, 'Physical History of Mankind,' on the authority of Blumenbach,
vol. i. 1851, p. 311; for the statement by Pallas, vol. iv. 1844, p. 407.)

The Quechua Indians inhabit the lofty plateaux of Peru; and Alcide
d'Orbigny states (33. Quoted by Prichard, 'Researches into the Physical
History of Mankind,' vol. v. p. 463.) that, from continually breathing a
highly rarefied atmosphere, they have acquired chests and lungs of
extraordinary dimensions. The cells, also, of the lungs are larger and
more numerous than in Europeans. These observations have been doubted, but
Mr. D. Forbes carefully measured many Aymaras, an allied race, living at
the height of between 10,000 and 15,000 feet; and he informs me (34. Mr.
Forbes' valuable paper is now published in the 'Journal of the Ethnological
Society of London,' new series, vol. ii. 1870, p.193.) that they differ
conspicuously from the men of all other races seen by him in the
circumference and length of their bodies. In his table of measurements,
the stature of each man is taken at 1000, and the other measurements are
reduced to this standard. It is here seen that the extended arms of the
Aymaras are shorter than those of Europeans, and much shorter than those of
Negroes. The legs are likewise shorter; and they present this remarkable
peculiarity, that in every Aymara measured, the femur is actually shorter
than the tibia. On an average, the length of the femur to that of the
tibia is as 211 to 252; whilst in two Europeans, measured at the same time,
the femora to the tibiae were as 244 to 230; and in three Negroes as 258 to
241. The humerus is likewise shorter relatively to the forearm. This
shortening of that part of the limb which is nearest to the body, appears
to be, as suggested to me by Mr. Forbes, a case of compensation in relation
with the greatly increased length of the trunk. The Aymaras present some
other singular points of structure, for instance, the very small projection
of the heel.

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