animals and plants are descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless, all living things have much
in common, in their chemical composition, their cellular structure,
their laws of growth, and their liability to injurious influences.
We see this even in so trifling a fact as that the same poison often
similarly affects plants and animals; or that the poison secreted by
the gall-fly produces monstrous growths on the wild-rose or oak-tree.
With all organic beings, excepting, perhaps, some of the very lowest,
sexual reproduction seems to be essentially similar. With all, as far
as is at present known, the germinal vesicle is the same; so that
all organisms start from a common origin. If we look even to the two
main divisions--namely, to the animal and vegetable kingdoms--certain
low forms are so far intermediate in character that naturalists have
disputed to which kingdom they should be referred. As Professor Asa
Gray has remarked, “the spores and other reproductive bodies of many of
the lower algæ may claim to have first a characteristically animal, and
then an unequivocally vegetable existence.” Therefore, on the principle
of natural selection with divergence of character, it does not seem
incredible that, from some such low and intermediate form, both animals
and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on
this earth may be descended from some one primordial form. But this
inference is chiefly grounded on analogy, and it is immaterial whether
or not it be accepted.
* * * * *
[Page 420.]
On the view of each organism with all its separate parts having been
specially created, how utterly inexplicable is it that organs bearing
the plain stamp of inutility, such as the teeth in the embryonic calf,
or the shriveled wings under the soldered wing-covers of many beetles,
should so frequently occur! Nature may be said to have taken pains to
reveal her scheme of modification, by means of rudimentary organs,
of embryological and homologous structures, but we are too blind to
understand her meaning.
LETTERS RETAINED IN THE SPELLING BUT USELESS IN PRONUNCIATION.
[Origin of
Species,
page 401.]
There remains, however, this difficulty. After an organ has ceased
being used, and has become in consequence much reduced, how can it be
still further reduced in size until the merest vestige is left; and
how can it be finally quite obliterated? It is scarcely possible that
disuse can go on producing any further effect after the organ has
once been rendered functionless. Some additional explanation is here
requisite which I can not give. If, for instance, it could be proved
that every part of the organization tends to vary in a greater degree
toward diminution than toward augmentation of size, then we should
be able to understand how an organ which has become useless would
be rendered, independently of the effects of disuse, rudimentary,
and would at last be wholly suppressed; for the variations toward
diminished size would no longer be checked by natural selection. The
principle of the economy of growth, explained in a former chapter, by
which the materials forming any part, if not useful to the possessor,
are saved as far as is possible, will perhaps come into play in
rendering a useless part rudimentary. But this principle will almost
necessarily be confined to the earlier stages of the process of
reduction; for we can not suppose that a minute papilla, for instance,
representing in a male flower the pistil of the female flower, and
formed merely of cellular tissue, could be further reduced or absorbed
for the sake of economizing nutriment.
Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a
former state of things, and have been retained solely through the
power of inheritance, we can understand, on the genealogical view of
classification, how it is that systematists, in placing organisms in
their proper places in the natural system, have often found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clew for its derivation.
On the view of descent with modification, we may conclude that the
existence of organs in a rudimentary, imperfect, and useless condition,
or quite aborted, far from presenting a strange difficulty, as they
assuredly do on the old doctrine of creation, might even have been
anticipated in accordance with the views here explained.
MAN’S DEFICIENCY IN TAIL.
[Descent of
Man, page 58.]
According to a popular impression, the absence of a tail is eminently
distinctive of man; but, as those apes which come nearest to him are
destitute of this organ, its disappearance does not relate exclusively
to man. The tail often differs remarkably in length within the same
genus: thus in some species of _Macacus_ it is longer than the whole
body, and is formed of twenty-four vertebræ; in others it consists of
a scarcely visible stump, containing only three or four vertebræ. In
some kinds of baboons there are twenty-five, while in the mandrill
there are ten very small stunted caudal vertebræ, or, according to
Cuvier, sometimes only five. The tail, whether it be long or short,
almost always tapers toward the end; and this, I presume, results from
the atrophy of the terminal muscles, together with their arteries
and nerves, through disuse, leading to the atrophy of the terminal
bones. But no explanation can at present be given of the great
diversity which often occurs in its length. Here, however, we are
more specially concerned with the complete external disappearance of
the tail. Professor Broca has recently shown that the tail in all
quadrupeds consists of two portions, generally separated abruptly
from each other; the basal portion consists of vertebræ, more or
less perfectly channeled and furnished with apophyses like ordinary
vertebræ; whereas those of the terminal portion are not channeled, are
almost smooth, and scarcely resemble true vertebræ. A tail, though not
externally visible, is really present in man and the anthropomorphous
apes, and is constructed on exactly the same pattern in both. In the
terminal portion the vertebræ, constituting the _os coccyx_, are quite
rudimentary, being much reduced in size and number. In the basal
portion, the vertebræ are likewise few, are united firmly together,
and are arrested in development; but they have been rendered much
broader and flatter than the corresponding vertebræ in the tails of
other animals; they constitute what Broca calls the accessory sacral
vertebræ. These are of functional importance by supporting certain
internal parts and in other ways; and their modification is directly
connected with the erect or semi-erect attitude of man and the
anthropomorphous apes. This conclusion is the more trustworthy, as
Broca formerly held a different view, which he has now abandoned. The
modification, therefore, of the basal caudal vertebræ in man and the
higher apes may have been effected, directly or indirectly, through
natural selection.
But what are we to say about the rudimentary and variable vertebræ of
the terminal portion of the tail, forming the _os coccyx_? A notion
which has often been, and will no doubt again be ridiculed, namely,
that friction has had something to do with the disappearance of the
external portion of the tail, is not so ridiculous as it at first
appears. Dr. Anderson states that the extremely short tail of _Macacus
brunneus_ is formed of eleven vertebræ, including the imbedded basal
ones. The extremity is tendinous and contains no vertebræ; this is
succeeded by five rudimentary ones, so minute that together they are
only one line and a half in length, and these are permanently bent to
one side in the shape of a hook. The free part of the tail, only a
little above an inch in length, includes only four more small vertebræ.
This short tail is carried erect; but about a quarter of its total
length is doubled on to itself to the left; and this terminal part,
which includes the hook-like portion, serves “to fill up the interspace
between the upper divergent portion of the callosities”; so that the
animal sits on it, and thus renders it rough and callous.
POINTS OF RESEMBLANCE BETWEEN MAN AND MONKEY.
[Descent of
Man, page 150.]
As small unimportant points of resemblance between man and the
_Quadrumana_ are not commonly noticed in systematic works, and as,
when numerous, they clearly reveal our relationship, I will specify a
few such points. The relative position of our features is manifestly
the same; and the various emotions are displayed by nearly similar
movements of the muscles and skin, chiefly above the eyebrows and round
the mouth. Some few expressions are, indeed, almost the same, as in
the weeping of certain kinds of monkeys and in the laughing noise made
by others, during which the corners of the mouth are drawn backward,
and the lower eyelids wrinkled. The external ears are curiously alike.
In man the nose is much more prominent than in most monkeys; but we
may trace the commencement of an aquiline curvature in the nose of the
Hoolock Gibbon; and this in the _Semnopithecus nasica_ is carried to a
ridiculous extreme.
The faces of many monkeys are ornamented with beards, whiskers, or
mustaches. The hair on the head grows to a great length in some species
of Semnopithecus; and in the Bonnet monkey (_Macacus radiatus_) it
radiates from a point on the crown, with a parting down the middle.
It is commonly said that the forehead gives to man his noble and
intellectual appearance; but the thick hair on the head of the Bonnet
monkey terminates downward abruptly, and is succeeded by hair so short
and fine that at a little distance the forehead, with the exception of
the eyebrows, appears quite naked. It has been erroneously asserted
that eyebrows are not present in any monkey. In the species just
named the degree of nakedness of the forehead differs in different
individuals; and Eschricht states that in our children the limit
between the hairy scalp and the naked forehead is sometimes not well
defined; so that here we seem to have a trifling case of reversion to a
progenitor, in whom the forehead had not as yet become quite naked.
It is well known that the hair on our arms tends to converge from
above and below to a point at the elbow. This curious arrangement, so
unlike that in most of the lower mammals, is common to the gorilla,
chimpanzee, orang, some species of Hylobates, and even to some few
American monkeys. But in _Hylobates agilis_ the hair on the fore-arm
is directed downward or toward the wrist in the ordinary manner;
and in _H. lar_ it is nearly erect, with only a very slight forward
inclination; so that in this latter species it is in a transitional
state. It can hardly be doubted that with most mammals the thickness
of the hair on the back and its direction are adapted to throw off the
rain; even the transverse hairs on the forelegs of a dog may serve for
this end when he is coiled up asleep. Mr. Wallace, who has carefully
studied the habits of the orang, remarks that the convergence of the
hair toward the elbow on the arms of the orang may be explained as
serving to throw off the rain, for this animal during rainy weather
sits with its arms bent, and with the hands clasped round a branch or
over its head. According to Livingstone, the gorilla also “sits in
pelting rain with his hands over his head.” If the above explanation
is correct, as seems probable, the direction of the hair on our own
arms offers a curious record of our former state; for no one supposes
that it is now of any use in throwing off the rain; nor, in our present
erect condition, is it properly directed for this purpose.
* * * * *
[Page 152.]
It must not be supposed that the resemblances between man and certain
apes in the above and many other points--such as in having a naked
forehead, long tresses on the head, etc.--are all necessarily the
result of unbroken inheritance from a common progenitor, or of
subsequent reversion. Many of these resemblances are more probably due
to analogous variation, which follows, as I have elsewhere attempted
to show, from co-descended organisms having a similar constitution, and
having been acted on by like causes inducing similar modifications.
With respect to the similar direction of the hair on the fore-arms of
man and certain monkeys, as this character is common to almost all the
anthropomorphous apes, it may probably be attributed to inheritance;
but this is not certain, as some very distinct American monkeys are
thus characterized.
VARIABILITY OF MAN.
[Descent of
Man, page 26.]
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. 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. 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 1,040 corpses how often each
course prevails. The muscles are eminently variable: thus those of the
foot were found by Professor Turner 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 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
Professor Macalister describes no less than twenty distinct variations
in the _palmaris accessorius_.
CAUSES OF VARIABILITY IN DOMESTICATED MAN.
[Descent of
Man, page 28.]
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 civilized 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.
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”
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.
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.
ACTION OF CHANGED CONDITIONS.
[Page 30.]
This is a most perplexing subject. It can not 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 favor 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
organization is rendered in some degree plastic.
In the United States, above one million soldiers, who served in the
late war, were measured, and the States in which they were born and
reared were recorded. 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 shown “by the great difference between the statures of
soldiers and sailors at the ages of seventeen and eighteen years.”
Mr. B. A. Gould endeavored 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 Villermé, 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, 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 show 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 vigor.”
THE INHERITED EFFECTS OF THE INCREASED AND DIMINISHED USE OF PARTS.
[Descent of
Man, page 32.]
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. Different occupations,
habitually followed, lead to changed proportions in various parts of
the body. Thus it was ascertained by the United States commission that
the legs of the sailors employed in the late war were longer by O·217
of an inch than those of the soldiers, though the sailors were on an
average shorter men; while 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, while 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.
* * * * *
[Page 33.]
In infants, long before birth, the skin on the soles of the feet is
thicker than on any other part of the body; 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, while men living much out-of-doors, and especially
savages, are generally long-sighted. Short-sight and long-sight
certainly tend to be inherited. The inferiority of Europeans, in
comparison with savages, in eye-sight and in the other senses, is no
doubt the accumulated and transmitted effect of lessened use during
many generations.
* * * * *
[Page 35.]
Although man may not have been much modified during the latter stages
of his existence through the increased or decreased use of parts, the
facts now given show that his liability in this respect has not been
lost; and we positively know that the same law holds good with the
lower animals. Consequently we may infer that when at a remote epoch
the progenitors of man were in a transitional state, and were changing
from quadrupeds into bipeds, natural selection would probably have been
greatly aided by the inherited effects of the increased or diminished
use of the different parts of the body.
REVERSION AS A FACTOR IN THE DEVELOPMENT OF MAN.
[Descent of
Man, page 40.]
In man, the canine teeth are perfectly efficient instruments for
mastication. But their true canine character, as Owen remarks, “is
indicated by the conical form of the crown, which terminates in an
obtuse point, is convex outward and flat or sub-concave within, at the
base of which surface there is a feeble prominence. The conical form
is best expressed in the Melanian races, especially the Australian.
The canine is more deeply implanted, and by a stronger fang than the
incisors.” Nevertheless, this tooth no longer serves man as a special
weapon for tearing his enemies or prey; it may, therefore, as far as
its proper function is concerned, be considered as rudimentary. In
every large collection of human skulls some may be found, as Häckel
observes, with the canine teeth projecting considerably beyond the
others in the same manner as in the anthropomorphous apes, but in a
less degree. In these cases, open spaces between the teeth in the one
jaw are left for the reception of the canines of the opposite jaw.
An interspace of this kind in a Caffre skull, figured by Wagner, is
surprisingly wide. Considering how few are the ancient skulls which
have been examined, compared to recent skulls, it is an interesting
fact that in at least three cases the canines project largely; and in
the Naulette jaw they are spoken of as enormous.