Darwinism (1889) by Alfred Russel Wallace

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For the explanation of these we have no resource but variation and
selection, to the effects of which, acting alternately with regression
or degradation as above explained (p. 328) must be imputed the
development of the countless floral structures we now behold. Even the
primitive flowers, whose initiation may, perhaps, have been caused, or
rendered possible, by the irritation set up by insects' visits, must,
from their very origin, have been modified, in accordance with the
supreme law of utility, by means of variation and survival of the
fittest.]

[Footnote 212: In an essay on "The Duration of Life," forming part of
the translation of Dr. Weismann's papers already referred to, the author
still further extends the sphere of natural selection by showing that
the average duration of life in each species has been determined by it.
A certain length of life is essential in order that the species may
produce offspring sufficient to ensure its continuance under the most
unfavourable conditions; and it is shown that the remarkable
inequalities of longevity in different species and groups may be thus
accounted for. Yet more, the occurrence of death in the higher
organisms, in place of the continued survival of the unicellular
organisms however much they may increase by subdivision, may be traced
to the same great law of utility for the race and survival of the
fittest. The whole essay is of exceeding interest, and will repay a
careful perusal. A similar idea occurred to the present writer about
twenty years back, and was briefly noted down at the time, but
subsequently forgotten.]

[Footnote 213: The outline here given is derived from two articles in
_Nature_, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which
Weismann's papers are summarised and partly translated.]

[Footnote 214: There are many indications that this explanation of the
cause of variation is the true one. Mr. E.B. Poulton suggests one, in
the fact that parthenogenetic reproduction only occurs in isolated
species, not in groups of related species; as this shows that
parthenogenesis cannot lead to the evolution of new forms. Again, in
parthenogenetic females the complete apparatus for fertilisation remains
unreduced; but if these varied as do sexually produced animals, the
organs referred to, being unused, would become rudimentary.

Even more important is the significance of the "polar bodies," as
explained by Weismann in one of his _Essays_; since, if his
interpretation of them be correct, variability is a necessary
consequence of sexual generation.]

[Footnote 215: Darwin's _Animals and Plants_, vol. ii. pp. 23, 24.]

[Footnote 216: In his essay on "Heredity," Dr. Weismann discusses many
other cases of supposed inheritance of acquired characters, and shows
that they can all be explained in other ways. Shortsightedness among
civilised nations, for example, is due partly to the absence of
selection and consequent regression towards a mean, and partly to its
individual production by constant reading.]

[Footnote 217: Weismann explains instinct on similar lines, and gives
many interesting illustrations (see _Essays on Heredity_). He holds
"that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences, but
upon variations of the germ." Many interesting and difficult cases of
instinct are discussed by Darwin in Chapter VIII of the _Origin of
Species_, which should be read in connection with the above remarks.

Since this chapter was written my attention has been directed to Mr.
Francis Galton's _Theory of Heredity_ (already referred to at p. 417)
which was published thirteen years ago as an alternative for Darwin's
theory of pangenesis.

Mr. Galton's theory, although it attracted little attention, appears to
me to be substantially the same as that of Professor Weismann. Galton's
"stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements
in the offspring to be directly formed from the residue of the _stirp_
not used up in the development of the body of the parent--Weismann's
"continuity of the germ-plasm." Galton also draws many of the same
conclusions from his theory. He maintains that characters acquired by
the individual as the result of external influences cannot be inherited,
unless such influences act directly on the reproductive
elements--instancing the possible heredity of alcoholism, because the
alcohol permeates the tissues and may reach the sexual elements. He
discusses the supposed heredity of effects produced by use or disuse,
and explains them much in the same manner as does Weismann. Galton is an
anthropologist, and applies the theory, mainly, to explain the
peculiarities of hereditary transmission in man, many of which
peculiarities he discusses and elucidates. Weismann is a biologist, and
is mostly concerned with the application of the theory to explain
variation and instinct, and to the further development of the theory of
evolution. He has worked it out more thoroughly, and has adduced
embryological evidence in its support; but the views of both writers are
substantially the same, and their theories were arrived at quite
independently. The names of Galton and Weismann should therefore be
associated as discoverers of what may be considered (if finally
established) the most important contribution to the evolution theory
since the appearance of the _Origin of Species_.]

CHAPTER XV

DARWINISM APPLIED TO MAN

General identity of human and animal structure--Rudiments and
variations showing relation of man to other mammals--The
embryonic development of man and other mammalia--Diseases common
to man and the lower animals--The animals most nearly allied to
man--The brains of man and apes--External differences of man and
apes--Summary of the animal characteristics of man--The
geological antiquity of man--The probable birthplace of man--The
origin of the moral and intellectual nature of man--The argument
from continuity--The origin of the mathematical faculty--The
origin of the musical and artistic faculties--Independent proof
that these faculties have not been developed by natural
selection--The interpretation of the facts--Concluding remarks.

Our review of modern Darwinism might fitly have terminated with the
preceding chapter; but the immense interest that attaches to the origin
of the human race, and the amount of misconception which prevails
regarding the essential teachings of Darwin's theory on this question,
as well as regarding my own special views upon it, induce me to devote a
final chapter to its discussion.

To any one who considers the structure of man's body, even in the most
superficial manner, it must be evident that it is the body of an animal,
differing greatly, it is true, from the bodies of all other animals, but
agreeing with them in all essential features. The bony structure of man
classes him as a vertebrate; the mode of suckling his young classes him
as a mammal; his blood, his muscles, and his nerves, the structure of
his heart with its veins and arteries, his lungs and his whole
respiratory and circulatory systems, all closely correspond to those of
other mammals, and are often almost identical with them. He possesses
the same number of limbs terminating in the same number of digits as
belong fundamentally to the mammalian class. His senses are identical
with theirs, and his organs of sense are the same in number and occupy
the same relative position. Every detail of structure which is common to
the mammalia as a class is found also in man, while he only differs from
them in such ways and degrees as the various species or groups of
mammals differ from each other. If, then, we have good reason to believe
that every existing group of mammalia has descended from some common
ancestral form--as we saw to be so completely demonstrated in the case
of the horse tribe,--and that each family, each order, and even the
whole class must similarly have descended from some much more ancient
and more generalised type, it would be in the highest degree
improbable--so improbable as to be almost inconceivable--that man,
agreeing with them so closely in every detail of his structure, should
have had some quite distinct mode of origin. Let us, then, see what
other evidence bears upon the question, and whether it is sufficient to
convert the probability of his animal origin into a practical certainty.

_Rudiments and Variations as Indicating the Relation of Man to other
Mammals._

All the higher animals present rudiments of organs which, though useless
to them, are useful in some allied group, and are believed to have
descended from a common ancestor in which they were useful. Thus there
are in ruminants rudiments of incisor teeth which, in some species,
never cut through the gums; many lizards have external rudimentary legs;
while many birds, as the Apteryx, have quite rudimentary wings. Now man
possesses similar rudiments, sometimes constantly, sometimes only
occasionally present, which serve intimately to connect his bodily
structure with that of the lower animals. Many animals, for example,
have a special muscle for moving or twitching the skin. In man there are
remnants of this in certain parts of the body, especially in the
forehead, enabling us to raise our eyebrows; but some persons have it in
other parts. A few persons are able to move the whole scalp so as to
throw off any object placed on the head, and this property has been
proved, in one case, to be inherited. In the outer fold of the ear there
is sometimes a projecting point, corresponding in position to the
pointed ear of many animals, and believed to be a rudiment of it. In the
alimentary canal there is a rudiment--the vermiform appendage of the
caecum--which is not only useless, but is sometimes a cause of disease
and death in man; yet in many vegetable feeding animals it is very long,
and even in the orang-utan it is of considerable length and convoluted.
So, man possesses rudimentary bones of a tail concealed beneath the
skin, and, in some rare cases, this forms a minute external tail.

The variability of every part of man's structure is very great, and many
of these variations tend to approximate towards the structure of other
animals. The courses of the arteries are eminently variable, so that for
surgical purposes it has been necessary to determine the probable
proportion of each variation. The muscles are so variable that in fifty
cases the muscles of the foot were found to be not strictly alike in any
two, and in some the deviations were considerable; while in thirty-six
subjects Mr. J. Wood observed no fewer than 558 muscular variations. The
same author states that in a single male subject there were no fewer
than seven muscular variations, all of which plainly represented muscles
proper to various kinds of apes. The muscles of the hands and
arms--parts which are so eminently characteristic of man--are extremely
liable to vary, so as to resemble the corresponding muscles of the lower
animals. That such variations are due to reversion to a former state of
existence Mr. Darwin thinks highly probable, and he adds: "It is quite
incredible that a man should, through mere accident, abnormally resemble
certain apes in no less than seven of his muscles, if there had been no
genetic connection between them. On the other hand, if man is descended
from some ape-like creature, no valid reason can be assigned why certain
muscles should not suddenly reappear after an interval of many thousand
generations, in the same manner as, with horses, asses, and mules, dark
coloured stripes suddenly reappear on the legs and shoulders, after an
interval of hundreds, or more probably of thousands of
generations."[218]

_The Embryonic Development of Man and other Mammalia._

The progressive development of any vertebrate from the ovum or minute
embryonic egg affords one of the most marvellous chapters in Natural
History. We see the contents of the ovum undergoing numerous definite
changes, its interior dividing and subdividing till it consists of a
mass of cells, then a groove appears marking out the median line or
vertebral column of the future animal, and thereafter are slowly
developed the various essential organs of the body. After describing in
some detail what takes place in the case of the ovum of the dog,
Professor Huxley continues: "The history of the development of any other
vertebrate animal, lizard, snake, frog, or fish tells the same story.
There is always to begin with, an egg having the same essential
structure as that of the dog; the yelk of that egg undergoes division or
segmentation, as it is called, the ultimate products of that
segmentation constitute the building materials for the body of the young
animal; and this is built up round a primitive groove, in the floor of
which a notochord is developed. Furthermore, there is a period in which
the young of all these animals resemble one another, not merely in
outward form, but in all essentials of structure, so closely, that the
differences between them are inconsiderable, while in their subsequent
course they diverge more and more widely from one another. And it is a
general law that the more closely any animals resemble one another in
adult structure, the larger and the more intimately do their embryos
resemble one another; so that, for example, the embryos of a snake and
of a lizard remain like one another longer than do those of a snake and
a bird; and the embryos of a dog and of a cat remain like one another
for a far longer period than do those of a dog and a bird, or of a dog
and an opossum, or even than those of a dog and a monkey."[219]

We thus see that the study of development affords a test of affinity in
animals that are externally very much unlike each other; and we
naturally ask how this applies to man. Is he developed in a different
way from other mammals, as we should certainly expect if he has had a
distinct and altogether different origin? "The reply," says Professor
Huxley, "is not doubtful for a moment. Without question, the mode of
origin and the early stages of the development of man are identical with
those of the animals immediately below him in the scale." And again he
tells us: "It is very long before the body of the young human being can
be readily discriminated from that of the young puppy; but at a
tolerably early period the two become distinguishable by the different
forms of their adjuncts, the yelk-sac and the allantois;" and after
describing these differences he continues: "But exactly in those
respects in which the developing man differs from the dog, he resembles
the ape.... So that it is only quite in the latter stages of development
that the young human being presents marked differences from the young
ape, while the latter departs as much from the dog in its development as
the man does. Startling as this last assertion may appear to be, it is
demonstrably true, and it alone appears to me sufficient to place beyond
all doubt the structural unity of man with the rest of the animal world,
and more particularly and closely with the apes."[220]

A few of the curious details in which man passes through stages common
to the lower animals may be mentioned. At one stage the os coccyx
projects like a true tail, extending considerably beyond the rudimentary
legs. In the seventh month the convolutions of the brain resemble those
of an adult baboon. The great toe, so characteristic of man, forming the
fulcrum which most assists him in standing erect, in an early stage of
the embryo is much shorter than the other toes, and instead of being
parallel with them, projects at an angle from the side of the foot, thus
corresponding with its permanent condition in the quadrumana. Numerous
other examples might be quoted, all illustrating the same general law.

_Diseases Common to Man and the Lower Animals._

Though the fact is so well known, it is certainly one of profound
significance that many animal diseases can be communicated to man, since
it shows similarity, if not identity, in the minute structure of the
tissues, the nature of the blood, the nerves, and the brain. Such
diseases as hydrophobia, variola, the glanders, cholera, herpes, etc.,
can be transmitted from animals to man or the reverse; while monkeys are
liable to many of the same non-contagious diseases as we are. Rengger,
who carefully observed the common monkey (Cebus Azarae) in Paraguay,
found it liable to catarrh, with the usual symptoms, terminating
sometimes in consumption. These monkeys also suffered from apoplexy,
inflammation of the bowels, and cataract in the eye. Medicines produced
the same effect upon them as upon us. Many kinds of monkeys have a
strong taste for tea, coffee, spirits, and even tobacco. These facts
show the similarity of the nerves of taste in monkeys and in ourselves,
and that their whole nervous system is affected in a similar way. Even
the parasites, both external and internal, that affect man are not
altogether peculiar to him, but belong to the same families or genera as
those which infest animals, and in one case, scabies, even the same
species.[221] These curious facts seem quite inconsistent with the idea
that man's bodily structure and nature are altogether distinct from
those of animals, and have had a different origin; while the facts are
just what we should expect if he has been produced by descent with
modification from some common ancestor.

_The Animals most nearly Allied to Man._

By universal consent we see in the monkey tribe a caricature of
humanity. Their faces, their hands, their actions and expressions
present ludicrous resemblances to our own. But there is one group of
this great tribe in which this resemblance is greatest, and they have
hence been called the anthropoid or man-like apes. These are few in
number, and inhabit only the equatorial regions of Africa and Asia,
countries where the climate is most uniform, the forests densest, and
the supply of fruit abundant throughout the year. These animals are now
comparatively well known, consisting of the orang-utan of Borneo and
Sumatra, the chimpanzee and the gorilla of West Africa, and the group of
gibbons or long-armed apes, consisting of many species and inhabiting
South-Eastern Asia and the larger Malay Islands. These last are far
less like man than the other three, one or other of which has at various
times been claimed to be the most man-like of the apes and our nearest
relations in the animal kingdom. The question of the degree of
resemblance of these animals to ourselves is one of great interest,
leading, as it does, to some important conclusions as to our origin and
geological antiquity, and we will therefore briefly consider it.

If we compare the skeletons of the orang or chimpanzee with that of man,
we find them to be a kind of distorted copy, every bone corresponding
(with very few exceptions), but altered somewhat in size, proportions,
and position. So great is this resemblance that it led Professor Owen to
remark: "I cannot shut my eyes to the significance of that all-pervading
similitude of structure--every tooth, every bone, strictly
homologous--which makes the determination of the difference between
_Homo_ and _Pithecus_ the anatomist's difficulty."

The actual differences in the skeletons of these apes and that of
man--that is, differences dependent on the presence or absence of
certain bones, and not on their form or position--have been enumerated
by Mr. Mivart as follows:--(1) In the breast-bone consisting of but two
bones, man agrees with the gibbons; the chimpanzee and gorilla having
this part consisting of seven bones in a single series, while in the
orang they are arranged in a double series of ten bones. (2) The normal
number of the ribs in the orang and some gibbons is twelve pairs, as in
man, while in the chimpanzee and gorilla there are thirteen pairs. (3)
The orang and the gibbons also agree with man in having five lumbar
vertebrae, while in the gorilla and the chimpanzee there are but four,
and sometimes only three. (4) The gorilla and chimpanzee agree with man
in having eight small bones in the wrist, while the orang and the
gibbons, as well as all other monkeys, have nine.[222]

The differences in the form, size, and attachments of the various bones,
muscles, and other organs of these apes and man are very numerous and
exceedingly complex, sometimes one species, sometimes another agreeing
most nearly with ourselves, thus presenting a tangled web of affinities
which it is very difficult to unravel. Estimated by the skeleton alone,
the chimpanzee and gorilla seem nearer to man than the orang, which last
is also inferior as presenting certain aberrations in the muscles. In
the form of the ear the gorilla is more human than any other ape, while
in the tongue the orang is the more man-like. In the stomach and liver
the gibbons approach nearest to man, then come the orang and chimpanzee,
while the gorilla has a degraded liver more resembling that of the lower
monkeys and baboons.

_The Brains of Man and Apes._

We come now to that part of his organisation in which man is so much
higher than all the lower animals--the brain; and here, Mr. Mivart
informs us, the orang stands highest in rank. The height of the orang's
cerebrum in front is greater in proportion than in either the chimpanzee
or the gorilla. "On comparing the brain of man with the brains of the
orang, chimpanzee, and baboon, we find a successive decrease in the
frontal lobe, and a successive and very great increase in the relative
size of the occipital lobe. Concomitantly with this increase and
decrease, certain folds of brain substance, called 'bridging
convolutions,' which in man are conspicuously interposed between the
parietal and occipital lobes, seem as utterly to disappear in the
chimpanzee, as they do in the baboon. In the orang, however, though much
reduced, they are still to be distinguished.... The actual and absolute
mass of the brain is, however, slightly greater in the chimpanzee than
in the orang, as is the relative vertical extent of the middle part of
the cerebrum, although, as already stated, the frontal portion is higher
in the orang; while, according to M. Gratiolet, the gorilla is not only
inferior to the orang in cerebral development, but even to his smaller
African congener, the chimpanzee."[223]

On the whole, then, we find that no one of the great apes can be
positively asserted to be nearest to man in structure. Each of them
approaches him in certain characteristics, while in others it is widely
removed, giving the idea, so consonant with the theory of evolution as
developed by Darwin, that all are derived from a common ancestor, from
which the existing anthropoid apes as well as man have diverged. When,
however, we turn from the details of anatomy to peculiarities of
external form and motions, we find that, in a variety of characters, all
these apes resemble each other and differ from man, so that we may
fairly say that, while they have diverged somewhat from each other, they
have diverged much more widely from ourselves. Let us briefly enumerate
some of these differences.

_External Differences of Man and Apes._

All apes have large canine teeth, while in man these are no longer than
the adjacent incisors or premolars, the whole forming a perfectly even
series. In apes the arms are proportionately much longer than in man,
while the thighs are much shorter. No ape stands really erect, a posture
which is natural in man. The thumb is proportionately larger in man, and
more perfectly opposable than in that of any ape. The foot of man
differs largely from that of all apes, in the horizontal sole, the
projecting heel, the short toes, and the powerful great toe firmly
attached parallel to the other toes; all perfectly adapted for
maintaining the erect posture, and for free motion without any aid from
the arms or hands. In apes the foot is formed almost exactly like our
hand, with a large thumb-like great toe quite free from the other toes,
and so articulated as to be opposable to them; forming with the long
finger-like toes a perfect grasping hand. The sole cannot be placed
horizontally on the ground; but when standing on a level surface the
animal rests on the outer edge of the foot with the finger and
thumb-like toes partly closed, while the hands are placed on the ground
resting on the knuckles. The illustration on the next page (Fig. 37)
shows, fairly well, the peculiarities of the hands and feet of the
chimpanzee, and their marked differences, both in form and use, from
those of man.

The four limbs, with the peculiarly formed feet and hands, are those of
arboreal animals which only occasionally and awkwardly move on level
ground. The arms are used in progression equally with the feet, and the
hands are only adapted for uses similar to those of our hands when the
animal is at rest, and then but clumsily. Lastly, the apes are all hairy
animals, like the majority of other mammals, man alone having a smooth
and almost naked skin. These numerous and striking differences, even
more than those of the skeleton and internal anatomy, point to an
enormously remote epoch when the race that was ultimately to develop
into man diverged from that other stock which continued the animal type
and ultimately produced the existing varieties of anthropoid apes.