distinguishing characteristics of the primates seem to be trivial in
nature. It is surprising to find how insignificant are the details to
which appeal must be made in order to draw a line between our own division
of mammalia and the others. It is well to review them as they are given in
the standard text-books of comparative anatomy. Primates are eutheria, or
true mammalia possessing a placental attachment of the young within the
parent. The first digits, namely, the "great toe" and the "thumb," are
freely movable and opposable to the others, so that the limbs are
prehensile and clasping structures; usually but not always the animals of
this order are tree-dwellers in correlation with the grasping powers of
the feet and hands. The permanent teeth succeed a shorter series of
so-called "milk teeth," and they are diverse in structure, being incisors,
canines, or "eye teeth," premolars, and molars; the particular numbers of
each kind are almost invariable throughout the order and markedly
different from those of other orders. The number of digits is always five,
and with few exceptions they bear nails instead of claws. The clavicles,
or "collar bones," are well developed in correlation with the prehensile
nature of the fore limbs; a bony ring surrounds the orbit or eye socket.
Finally there are two mammary glands by which the young are suckled. It is
because any other details of difference between man and other forms are
far less marked than the agreements in these respects, that the human
species must be regarded as a primate mammalian vertebrate.
* * * * *
The comparative study of the human organism as a structural type has now
been narrowed down to a review of the various members of the order of
primates. It is the duty of science to arrange these organisms according
to the minor differences beneath the agreements in major qualities, and to
show how they are related in an order of evolution. It will appear, when
this is done, that the supreme place is given to the human species on
account of four and only four characteristics; these are (1) an entirely
erect posture, (2) greater brain development, (3) the power of articulate
speech, and (4) the power of reason. As we are treating the human body as
a subject for comparative structural study, the third and fourth
characters do not concern us here; but it is well to point out that they
depend entirely upon the second, and that they are the functional
concomitants of the improved type of brain belonging to the highest type.
Two characters remain, and in both cases it is significant that
differences in degree only are to be found by even the closest analysis.
The human brain is the same kind of brain that lower primates possess; its
structure is unique in no general respect. And as regards the
first-mentioned character, comparative anatomy shows, in the first place,
that this also is something differing only in degree, and in the second
place, that it is due directly to the development of the brain. For these
reasons a survey of the various members of the order of primates must deal
largely with the progressive elaboration of the brain and the entailed
effects of this enlargement.
The order of primates is subdivided as follows :--
Sub-order 1. _PROSIMII_. Lemurs.
Sub-order 2. _ANTHROPOIDEA_.
Family 1. _Hapalidæ_. The marmosets.
Family 2. _Cebidæ_. The American or tailed monkeys.
Family 3. _Cercopithecidæ_. The baboons.
Family 4. _Simiidæ_. The true apes.
Family 5. _Hominidæ_. The human species. Primates
Each one of these subdivisions is interesting in its own way, either
because its members depart from the typical condition of the whole order
in some respects, or because of some character that foreshadows and leads
to a more developed element of the animals placed in the higher sections.
The lemurs are small animals very much like squirrels in their general
form and in their tree-climbing habits. They live now almost exclusively
on the island of Madagascar, but palæontology shows that they were more
widely spread at an earlier time. Their teeth are exactly like our own,
except that there is one more premolar on each side of each jaw. The
"fingers" and "toes" bear nails like ours, again with an exception in the
case of the second digits of the hind limbs, which bear claws. The details
of structure that set these animals apart from all the rest of the
primates are too small to deserve comment in the present connection.
Passing to the true anthropoids, or man-like primates and man himself, the
first forms encountered are the little marmosets, which are like the
lemurs in some ways, but in other respects they resemble the familiar
tailed monkeys. They are peculiar in having three premolars and two molars
on either side of both upper and lower jaws, and also in the fact that the
"thumb" is not opposable to the other fingers, while all the digits except
the "great toes" bear claws instead of manlike nails. The proportion of
brain-case and face does not differ much from that in the lemurs and even
lower forms like cats, for the brain has not increased greatly in total
mass, though the cerebrum is more convoluted than in the lower forms.
The true monkeys, or Cebidæ, are more interesting, and at the same time
they are much more familiar to every one, as they are the commonest
anthropoids of the menagerie and circus. Their wonderful agility and
sureness in climbing about is partly due to the perfect grasping power of
the lower limb. To all intents and purposes the foot is a hand; the first
toe is shorter than the others, and its free motion is unrestricted as in
the thumb of the hand. These animals usually possess a long tail which
they can use as a prehensile organ, curling it about the branch of a tree
with hand-like ease and grasp. When they run on all fours, they plant the
palms and soles flat upon the ground. The feature of primary importance in
a comparative sense is the advanced structure of the skull. These
anthropoids are much more intelligent than the lower forms, which is a
correlate of their larger and more convoluted brains. The increase in the
total bulk of the brain has wrought considerable change, not only in the
head, but also in the relation of head to the trunk. The cranium, or
brain-case of bone, is relatively larger than the "face," and it bulges
upward so as to lie no longer behind the latter as it does in the lower
mammalia. In consequence of this cranial enlargement, the face and eyes
are swung downward, as it were, so that the line of vision is not straight
ahead, but depressed below the horizontal. In order to look to the front
and to the immediate foreground to which it is progressing or to where its
food or enemies may be, the monkey must bend back its head; if it is
still, it finds greater ease in the upright sitting posture which it
assumes readily and naturally.
The next division, called the Cercopithecidæ, includes the baboons of the
Old World. These animals also run upon all fours, and their feet are
handlike as before, but the tail is much reduced. The general appearance
of the head is doglike, and the brain-case arches little more than it does
in the monkeys, but the face projects forward as a long muzzle, with
terminal nostrils close together. In some respects the baboons stand
somewhat away from the line leading from the lower to higher anthropoids;
in other characters they approach the latter, for in the teeth especially
they are identical with the apes and with the human species.
The Simiidæ, or true apes, possess an overwhelming importance, far beyond
that of the baboons and monkeys. There are only four principal kinds now
existing, namely, the gibbon, orang-outang, chimpanzee, and the gorilla,
of which the first is much less familiar than the others. The known
species of gibbons occur in Indo-China and the Malay Peninsula. The
typical animal stands about three feet high; its overarching braincase,
enlarged in conformity with the much greater brain development, has pushed
the eyes and face still further around underneath, so that if the animal
walks upon all fours the eyes look almost straight into the ground.
Therefore it must bend back its head at an extremely uncomfortable angle
if it is to remain upon all four feet, but it prefers to raise itself up
into the human sitting posture, or, when it walks, it stands erect upon
its hind limbs. Hence we who are accustomed to think of ourselves as the
only erect animals must revise our opinion, for we find in the gibbon an
organism that is nearly, if not quite, as advanced in this respect as we
are. One peculiar difference may be pointed out,--the walking gibbon
stretches out its great long arms to the sides in order to preserve its
balance. The animal seems awkward to us, perhaps, but it is possible that
the human method of balancing the body by vigorously swinging the arms
might seem quite as awkward to a gibbon as its grotesque posture does to
us.
The orang-outang comes next in this series. It inhabits the islands of
Borneo and Sumatra, where we find two distinct species. It is a reddish
colored animal standing about four feet four inches high, with rather long
hair. It is bulky, slow and deliberate in action, and when it walks in a
semi-erect position it rests its knuckles upon the ground, swinging its
long arms as crutch-like supports. Like the gibbon, it does not walk upon
all four feet in the way that the monkeys and baboons do, and we find in
the still further development of the brain and the higher arch of the
cranium the reasons for its semi-erectness. It cannot remain with its
hands and feet upon the ground and bend back its head so as to direct its
vision forward.
The chimpanzee of intertropical Africa brings us to a still less
monkey-like and more manlike stage. This creature attains the height of
five feet, which is more than that of some of the lower races of man. It
possesses large ears and heavy overarching brows; its thumb and great toe
are more like those of man, though its foot is still practically a hand.
Its lower limb curves like those of the other apes, and its soles are
turned toward one another; in brief, it is naturally bow-legged, a
character that adapts it for a tree-climbing life. This animal also is
nearly, though not quite, erect. It shows a most marked advance in the
matter of the brain, for the cerebrum is richly folded or convoluted, and
with this higher degree of physical complexity is correlated its superior
intelligence; it is well known that chimpanzees can be taught to wear
clothing and to use a cup and spoon and bowl like a human child. Indeed,
in mental respects, the chimpanzee surpasses all of the other mammalia,
with the sole exception of man. An eminent psychologist has stated that it
is about the equal, in mental ability, of a nine months' old human infant.
The last form among the apes, the gorilla, is one that brings us to a
realization of our own human physical degeneracy. The animal lives in West
Equatorial Africa, and it is a veritable giant in bulk, though its height
may not exceed five feet six inches. The heavy ridges over the eyes, the
upturned nostrils and triangular nose, place it near to the orang-outang,
but it is superior to that form in its relatively greater brain-box, and
in the fact that its heavy lower jaws do not protrude so greatly. It, too,
is semi-erect, so that the line of the vertebral axis makes an angle with
the plane of the ground of about seventy degrees. Its anterior limbs, or
arms, are again very long and bulky; and like the chimpanzee, it rests its
knuckles upon the ground in walking.
It is a short step further to the human organism, whose brain has become
larger and more complex, with a corresponding advance in the functional
powers of reason and the like that owe their existence to the improved
structural basis. After what has been said earlier regarding the relation
between the erect attitude in walking and the increased size of the
cranial part of the skull as compared with the face, it will not be
difficult to see how inevitably the former is the result of the latter.
Should we get upon the ground upon our hands and knees in the position of
a tailed monkey, the eyes look straight into the ground, for the bulging
cranium has pushed out over the jaws and face so that they lie _under_ the
brain-case instead of in front. A person in this position can bend back
the head so as to look ahead, but the strain is too great for comfort.
Rising to the knees, and lifting the hands from the ground, a feeling of
ease at once succeeds that of tension. In the course of evolution
accomplished primarily by the increase of the higher portions of the
brain, the erect position has been assumed gradually and naturally, and to
maintain it has necessitated many other changes in skeleton and muscles;
for example, the pelvis has broadened to support the intestines, which
bear downwards instead of upon the abdominal walls; a double curve has
arisen in the axis of the vertebral column, giving an easier balance to
the upper part of the body and the head. Countless structures of the human
frame testify to an originally four-footed position and to a rotation of
the longer axis through an angle of ninety degrees, as evolution has
produced the human type.
The conclusion that the human brain has made mankind is thus established
as one of fundamental importance. Proceeding further, we learn that this
organ proves to be essentially the same as the brain of lower primates; it
does not gain its greater size and efficiency by the origination of wholly
new and unique parts, but solely by the further elaboration of the ones
present in lower forms. In a word, it is only a difference in _degree_ and
not in essential _kind_ that separates man from the apes and other
primates. Human nature is animal nature, and human structure is animal
structure, for nowhere can final and absolute differences be found. This
does not mean that no differences appear, for it would be absurd to
contend that man and the apes are identical in every respect; but it does
mean that the resemblances are fundamental and comprehensive, and any
details of dissimilarity are in the degree of complexity only. The supreme
place in nature attained by man is therefore due to progressive evolution
in the nervous system. The other systems have degenerated to a greater or
less degree, but such regressive changes are more than compensated for by
the superior control exerted by the improved brain. In purely physical and
mechanical respects, the human body is a degenerate as compared with a
gorilla; the arm of the latter is more powerful than the lower limb of the
former, while the gorilla's chest is more than twice as broad as the
human, and more than four times as capacious. It is not through superior
physique, but by superior ability to direct the activities of his body,
that man excels in the struggle for existence with the lower animals.
* * * * *
Moreover, the human body is a veritable museum of rare and interesting
relics of antiquity. This characterization is justified by those vestigial
and rudimentary structures that represent organs of value to human
relatives among the lower animals, though they play a less active part at
the present time in human economy. There is scarcely a single system that
does not exhibit many or fewer of these rudimentary structures, but only a
few need be specified. As compared with those of the apes, the human
wisdom teeth are degenerate; in the gorilla they are cut at the same time
as the other molars; and in the lower human races they come through the
gums in early youth, while in the more advanced Caucasic races they are
cut only in later life or not at all. The reduced vermiform appendix of
man, a source of much ill health, is another structure that is a
counterpart of a relatively larger and useful part of the digestive tract
in the lower primates and other animals. Furthermore, the human tail is a
reality, not a fiction. Now and then an individual is born with a tail
that may reach a length in later life of eight or ten inches; such
structures are, of course, abnormal. But in every normal human being there
is a series of little bones at the lower end of the vertebral column,
constituting the coccyx, and this is just where the abbreviated tail of
the ape and the still longer prehensile tail of the monkey arises from the
body. Unless the coccyx is a tail, what can it be? And if it does not
represent a reduced counterpart of the tails of other mammals, what does
it represent?
Many of the vestigial structures of man appear more clearly in infancy and
in embryonic development. The human embryo possesses a complete coat of
hair, called the lanugo, which usually disappears before birth. This hair
cannot be regarded as any less significant than the coat of hair which the
infant whale possesses; it means a completely haired ancestor. The
elements of this coat are arranged precisely as they are in the apes; upon
the arm, for example, they point from shoulder to elbow and from wrist to
elbow. Unless the anterior limb of the hairy human ancestor was held in
the position of the climbing ape's, this arrangement would be
disadvantageous, for the hair as a rain-shedding thatch would be effective
only upon the upper arm, while the hairs upon the forearm would catch the
rain. In a word, this vestigial coat indicates in the clearest possible
manner that the ancestor of the human species was not only hairy, but also
arboreal in its mode of life.
Every human infant is bow-legged at birth, and the natural position of its
curved limbs is like that of the gorilla's, for the soles of the feet are
turned toward one another. Again, the so-called great toe is at first
shorter than the others, and for a time it retains the power of free
movement that indicates a handlike character of the lower limb in the
ancestor. Many savage human races, however, whose feet remain unshod, make
use of the primitive grasping power of the foot which the higher races
lose completely. An Australian and Polynesian can pick up small objects
with the foot very much as we may with the hand.
Among the wonderful reminiscent characters displayed by the human infant
is the firm clasping power of the hand, which it possesses for a time
after birth and which enables it to hang suspended for several minutes
from a stick placed in its grasp. The muscles which enable the infant to
do this gradually dwindle, so that the two-year-old child can hang
suspended for only a few seconds. This grasping muscle is a heritage from
the ape, where there is an obvious necessity for the newborn individual to
have a firm hold upon the hairy coat of its tree-climbing mother. When the
newborn child hangs in this way, it bends its curved lower limbs so that
the soles of the feet are turned toward one another, thus increasing its
resemblance to the ape.
Let us realize that these curious relics found in so many places in the
framework of man are not unique, and that they are reduced counterparts of
larger and more valuable structures in the ape. Unless evolution is true,
they have absolutely no sensible reasons for existence. Science prefers
the evolutionary explanation of their occurrence because this explanation
is more in harmony with the facts known about other organisms, and it is
more reasonable than any other.
* * * * *
When we dealt with the general doctrine of natural transformation, it
appeared that the evidence of embryology was in many respects more cogent
and conclusive than that derived from the comparative study of animal
structures. In the case of man, as before, no one could demand any surer
or more convincing proof that an organic mechanism with one structure can
change into an organic mechanism with a different structure, than the
obvious facts of development. The embryo, which is not an infant or an
adult, becomes an infant which must work its way onward by the gradual
accumulation of slight changes here and there and everywhere in its
anatomy, until it becomes mature. Each and every one of us has actually
undergone the process of organic change in becoming what we are, and we
cannot deny the reality of such a process without challenging the evidence
of our senses.
When the full import of this history is realized, and when we look further
into the nature of these preliminary conditions through which the human
organism passes in development, we are forcibly impressed by other facts
than the one to which I have directed your attention, for not only do we
find natural transformation, as in the other mammals, but the embryonic
stages are marvelously similar to the earlier conditions in other mammals.
Not very long before birth the human embryo is strikingly similar to the
embryo of the ape; still earlier, it presents an appearance very like that
of the embryos of other mammals lower in the scale, like the cat and the
rabbit,--forms which comparative anatomy independently holds to be more
remote relatives of the human species. Indeed, as we trace back the still
earlier history, more and more characters are found which are the common
properties of wider and wider arrays of organisms, for at one time the
embryo exhibits gill-slits in the sides of its throat which in all
essential respects are just like those of the embryos of birds and
reptiles and amphibia, as well as of other embryo mammals and these
gill-slits are furthermore like those of the fishes which use them
throughout life. All the other organic systems exhibit everywhere the
common characteristics in which the embryos of the so-called higher animals
agree with one another and with the adult forms among lower creatures; the
human embryo possesses a fishlike heart and brain and primitive backbone,
fishlike muscles and alimentary tract. Can we reasonably regard these
resemblances as indications of anything else but a community of ancestry
of the forms that exhibit them?
Yet a still more wonderful fact is revealed by the study of the very
earliest stages of individual development. The human embryo begins its
very existence as a single cell,--nothing more and nothing less; in
general structure the human egg, like the eggs of all other many-celled
organisms, is just one of the unitary building blocks of the entire
organic world. And yet the egg may ultimately become the adult man. Does
this mean that man and all the other higher forms have evolved from
protozoa in the course of long ages? Science asks if it can mean anything
else. When the comparative anatomist bids us look upon the wide and varied
series of adult animals lower than man as his relatives, because they
display similar structural plans beneath their minor differences, it may
be difficult at first to obey him. But in the brief time necessary for the
human egg to develop into an adult, the entire range is compassed from the
single cell to the highest adult we know. There are no breaks in the
series of embryonic stages like those between the diverse adult animals of
the comparative array. I do not think we could ask nature for more
complete proof that human beings have evolved from one-cell ancestors as
simple as modern protozoa beyond the obvious facts of human transformation
during development. They at least are real and not the logical deductions
of reason; yet their very reality and familiarity render us blind to the
deeper meaning revealed to us only when science places the facts in
intelligible order.
* * * * *