progenitor
(or representative of the progenitors) of the horse-types. In size
it must have been something like the rabbit or the hyrax. Still early
in the Eocene, however, we find the remains of a small animal (Eohippus),
about the size of a fox, which is described as "undoubtedly horse-like."
It had only three toes on its hind feet, and four on its front feet;
though it had also a splint-bone, representing the shrunken and discarded
fifth toe, on its fore feet. Another form of the same period (Protorohippus)
shows the central of the three toes on the hind foot much enlarged,
and the lateral toes shrinking. The teeth, and the bones and joints
of the limbs, are also developing in the direction of the horse.
In the succeeding geological period, the Oligocene, we find several
horse-types in which the adaptation of the limbs to running on the
firm grassy plains and of the teeth to eating the grass continues.
Mesohippus has lost the fourth toe of the fore foot, which is now
reduced to a splintbone, and the lateral toes of its hind foot are
shrinking. In the Miocene period there is a great development of the
horse-like mammals. We have the remains of more than forty species,
some continuing the main line of development on the firm and growing
prairies of the Miocene, some branching into the softer meadows or
the forests, and giving rise to types which will not outlive the Tertiary.
They have three toes on each foot, and have generally lost even the
rudimentary trace of the fourth toe. In most of them, moreover, the
lateral toes--except in the marsh-dwelling species, with spreading
feet--scarcely touch the ground, while the central toe is developing
a strong hoof. The leg-bones are longer, and have a new type of joint;
the muscles are concentrated near the body. The front teeth are now
chopping incisors, and the grinding teeth approach those of the modern
horse in the distribution of the enamel, dentine, and cement. They
are now about the size of a donkey, and must have had a distinctly
horsy appearance, with their long necks and heads and tapering limbs.
One of them, Merychippus, was probably in the direct line of the evolution
of the horse. From Hipparion some of the authorities believe that
the zebras may have been developed. Miohippus, Protohippus, and Hypohippus,
varying in size from that of a sheep to that of a donkey, are other
branches of this spreading family.
In the Pliocene period the evolution of the main stem culminates in
the appearance of the horse, and the collateral branches are destroyed.
Pliohippus is a further intermediate form. It has only one toe on
each foot, with two large splint bones, but its hoof is less round
than that of the horse, and it differs in the shape of the skull and
the length of the teeth. The true horse (Equus) at length appears,
in Europe and America, before the close of the Tertiary period. As
is well known, it still has the rudimentary traces of its second and
fourth toes in the shape of splint bones, and these bones are not
only more definitely toe-shaped in the foal before birth, but are
occasionally developed and give us a three-toed horse.
From these successive remains we can confidently picture the evolution,
during two or three million years, of one of our most familiar mammals.
It must not, of course, be supposed that these fossil remains all
represent "ancestors of the horse." In some cases they may
very well do so; in others, as we saw, they represent sidebranches
of the family which have become extinct. But even such successive
forms as the Eohippus, Mesohippus, Miohippus, and Pliohippus must
not be arranged in a direct line as the pedigree of the horse. The
family became most extensive in the Miocene, and we must regard the
casual fossil specimens we have discovered as illustrations of the
various phases in the development of the horse from the primitive
Ungulate. When we recollect what we saw in an earlier chapter about
the evolution of grassy plains and the successive rises of the land
during the Tertiary period, and when we reflect on the simultaneous
advance of the carnivores, we can without difficulty realise this
evolution of our familiar companion from a hyrax-like little animal
of two million years ago.
We have not in many cases so rich a collection of intermediate forms
as in the case of the horse, but our fossil mammals are numerous enough
to suggest a similar development of all the mammals of to-day. The
primitive family which gave birth to the horse also gave us, as we
saw, the tapir and the rhinoceros. We find ancestral tapirs in Europe
and America during the Tertiary period, but the later cold has driven
them to the warm swamps of Brazil and Malaysia. The rhinoceros has
had a long and interesting history. From the primitive Hyrochinus
of the Eocene, in which it is dimly foreshadowed, we pass to a large
and varied family in the later periods. In the Oligocene it spreads
into three great branches, adapted, respectively, to life on the elevated
lands, the lowlands, and the water. The upland type (Hyracodon) was
a light-limbed running animal, well illustrating the close relation
to the horse. The aquatic representative (Metamynodon) was a stumpy
and bulky animal. The intermediate lowland type was probably the ancestor
of the modern animal. All three forms were yet hornless. In the Miocene
the lowland type (Leptaceratherium, Aceratherium, etc.) develops vigorously,
while the other branches die. The European types now have two horns,
and in one of the American species (Diceratherium) we see a commencement
of the horny growths from the skull. We shall see later that the rhinoceros
continued in Europe even during the severe conditions of the glacial
period, in a branch that developed a woolly coat.
There were also in the early Tertiary several sidebranches of the
horse-tapir-rhinoceros family. The Palaeotheres were more or less
between the horse and the tapir in structure; the Anoplotheres between
the tapir and the ruminant. A third doomed branch, the Titanotheres,
flourished vigorously for a time, and begot some strange and monstrous
forms (Brontops, Titanops, etc.). In the larger specimens the body
was about fourteen feet long, and stood ten feet from the ground.
The long, low skull had a pair of horns over the snout. They perished
like the equally powerful but equally sluggish and stupid Deinocerata.
The Tertiary was an age of brain rather than of brawn. As compared
with their early Tertiary representatives' some of our modern mammals
have increased seven or eight-fold in brain-capacity.
While the horses and tapirs and rhinoceroses were being gradually
evolved from the primitive types, the Artiodactyl branch of the Ungulates--the
pigs, deer, oxen, etc.--were also developing. We must dismiss them
briefly. We saw that the primitive herbivores divided early in the
Eocene into the "odd-toed" and "even-toed" varieties;
the name refers, it will be remembered, not to the number of toes,
but to the axis of stress. The Artiodactyl group must have quickly
branched in turn, as we find very primitive hogs and camels before
the end of the Eocene. The first hog-like creature (Homacodon) was
much smaller than the hog of to-day, and had strong canine teeth,
but in the Oligocene the family gave rise to a large and numerous
race, the Elotheres. These "giant-pigs," as they have been
called, with two toes on each foot, flourished vigorously for a time
in Europe and America, but were extinguished in the Miocene, when
the true pigs made their appearance. Another doomed race of the time
is represented by the Hyopotamus, an animal between the pig and the
hippopotamus; and the Oreodontids, between the hog and the deer, were
another unsuccessful branch of the early race. The hippopotamus itself
was widespread in Europe, and a familiar form in the rivers of Britain,
in the latter part of the Tertiary.
The camel seems to be traceable to a group of primitive North American
Ungulates (Paebrotherium, etc.) in the later Eocene period. The Paebrotherium,
a small animal about two feet long, is followed by Pliauchenia, which
points toward the llamas and vicunas, and Procamelus, which clearly
foreshadows the true camel. In the Pliocene the one branch went southward,
to develop into the llamas and vicunas, and the other branch crossed
to Asia, to develop into the camels. Since that time they have had
no descendants in North America.
The primitive giraffe appears suddenly in the later Tertiary deposits
of Europe and Asia. The evidence points to an invasion from Africa,
and, as the region of development is unknown and unexplored, the evolution
of the giraffe remains a matter of speculation. Chevrotains flourished
in Europe and North America in the Oligocene, and are still very primitive
in structure, combining features of the hog and the ruminants. Primitive
deer and oxen begin in the Miocene, and seem to have an earlier representative
in certain American animals (Protoceras), of which the male has a
pair of blunt outgrowths between the ears. The first true deer are
hornless (like the primitive muskdeer of Asia to-day), but by the
middle of the Miocene the males have small two-pronged antlers, and
as the period proceeds three or four more prongs are added. It is
some confirmation of the evolutionary embryonic law that we find the
antlers developing in this way in the individual stag to-day. A very
curious race of ruminants in the later Tertiary was a large antelope
(Sivatherium) with four horns. It had not only the dimensions, but
apparently some of the characters, of an elephant.
The elephant itself, the last type of the Ungulates, has a clearer
line of developments. A chance discovery of fossils in the Fayum district
in Egypt led Dr. C. W. Andrews to make a special exploration, and
on the remains which he found he has constructed a remarkable story
of the evolution of the elephant.* It is clear that the elephant was
developed in Africa, and a sufficiently complete series of remains
has been found to give a good idea of the origin of its most distinctive
features. In the Eocene period there lived in the Egyptian region
an animal, something like the tapir in size and appearance, which
had its second incisors developed into small tusks and--to judge from
the nasal opening in the skull--a somewhat prolonged snout. This animal
(Moeritherium) only differed from the ordinary primitive Ungulate
in these incipient elephantine features. In the later Eocene a larger
and more advanced animal, the Palaeomastodon, makes its appearance.
Its tusks are larger (five or six inches long), its molars more elephantine,
the air-cells at the back of the head more developed. It would look
like a small elephant, except that it had a long snout, instead of
a flexible trunk, and a projecting lower jaw on which the snout rested.
*See this short account, "Guide to the Elephants
in the British Museum," 1908.
Up to the beginning of the Miocene, Africa was, as we saw, cut off
from Europe and Asia by the sea which stretched from Spain to India.
Then the land rose, and the elephant passed by the new tracts into
the north. Its next representative, Tetrabelodon, is found in Asia
and Europe, as well as North Africa. The frame is as large as that
of a medium-sized elephant, and the increase of the air-cells at the
back of the skull shows that an increased weight has to be sustained
by the muscles of the neck. The nostrils are shifted further back.
The tusks are from twenty to thirty inches long, and round, and only
differ from those of the elephant in curving slightly downward, The
chin projects as far as the tusks. The neck is shorter and thicker,
and, as the animal increases in height, we can understand that the
long snout--possibly prehensile at its lower end--is necessary for
the animal to reach the ground. But the snout still lies on the projecting
lower jaw, and is not a trunk. Passing over the many collateral branches,
which diverge in various directions, we next kind that the chin is
shortening (in Tetrabelodon longirostris), and, through a long series
of discovered intermediate forms, we trace the evolution of the elephant
from the mastodon. The long supporting skin disappears, and the enormous
snout becomes a flexible trunk. Southern Asia seems to have been the
province of this final transformation, and we have remains of some
of these primitive elephants with tusks nine and a half feet long.
A later species, which wandered over Central and Southern Europe before
the close of the Tertiary, stood fifteen feet high at the shoulder,
while the mammoth, which superseded it in the days of early man, had
at times tusks more than ten feet in length.
It is interesting to reflect that this light on the evolution of one
of our most specialised mammals is due to the chance opening of the
soil in an obscure African region. It suggests to us that as geological
exploration is extended, many similar discoveries may be made. The
slenderness of the geological record is a defect that the future may
considerably modify.
From this summary review of the evolution of the Ungulates we must
now pass to an even briefer account of the evolution of the Carnivores.
The evidence is less abundant, but the characters of the Carnivores
consist so obviously of adaptations to their habits and diet that
we have little difficulty in imagining their evolution. Their early
Eocene ancestors, the Creodonts, gave rise in the Eocene to forms
which we may regard as the forerunners of the cat-family and dog-family,
to which most of our familiar Carnivores belong. Patriofelis, the
"patriarchal cat," about five or six feet in length (without
the tail), curiously combines the features of the cat and the seal-family.
Cyonodon has a wolf-like appearance, and Amphicyon rather suggests
the fox. Primitive weasels, civets, and hyaenas appear also in the
Eocene. The various branches of the Carnivore family are already roughly
represented, but it is an age of close relationships and generalised
characters.
In the Miocene we find the various groups diverging still further
from each other and from the extinct stocks. Definite wolves and foxes
abound in America, and the bear, civet, and hyaena are represented
in Europe, together with vague otter-like forms. The dog-family seems
to have developed chiefly in North America. As in the case of the
Ungulates, we find many strange side-branches which flourished for
a time, but are unknown to-day. Machoerodus, usually known as "the
sabre-toothed tiger," though not a tiger, was one of the most
formidable of these transitory races. Its upper canine teeth (the
"sabres") were several inches in length, and it had enormously
distensible jaws to make them effective. The great development of
such animals, with large numbers of hyaenas, civets, wolves, bears,
and other Carnivores, in the middle and later Tertiary was probably
the most effective agency in the evolution of the horse and deer and
the extinction of the more sluggish races. The aquatic branch of the
Carnivores (seals, walruses, etc.) is little represented in the Tertiary
record. We saw, however, that the most primitive representatives of
the elephant-stock had also some characters of the seal, and it is
thought that the two had a common origin.
The Moeritherium was a marsh-animal, and may very well have been cousin
to the branch of the family which pushed on to the seas, and developed
its fore limbs into paddles.
The Rodents are represented in primitive form early in the Eocene
period. The teeth are just beginning to show the characteristic modification
for gnawing. A large branch of the family, the Tillodonts, attained
some importance a little later. They are described as combining the
head and claws of a bear with the teeth of a rodent and the general
characters of an ungulate. In the Oligocene we find primitive squirrels,
beavers, rabbits, and mice. The Insectivores also developed some of
the present types at an early date, and have since proved so unprogressive
that some regard them as the stock from which all the placental mammals
have arisen.
The Cetacea (whales, porpoises, etc.) are already represented in the
Eocene by a primitive whale-like animal (Zeuglodon) of unknown origin.
Some specimens of it are seventy feet in length. It has large teeth,
sometimes six inches long, and is clearly a terrestrial mammal that
has returned to the waters. Some forms even of the modern whale develop
rudimentary teeth, and in all forms the bony structure of the fore
limbs and degenerate relic of a pelvis and back limbs plainly tell
of the terrestrial origin. Dolphins appear in the Miocene.
Finally, the Edentates (sloths, anteaters, and armadilloes) are represented
in a very primitive form in the early Eocene. They are then barely
distinguishable from the Condylarthra and Creodonta, and seem only
recently to have issued from a common ancestor with those groups.
In the course of the Tertiary we find them--especially in South America,
which was cut off from the North and its invading Carnivores during
the Eocene and Miocene--developed into large sloths, armadilloes,
and anteaters. The reconnection with North America in the Pliocene
allowed the northern animals to descend, but gigantic sloths (Megatherium)
and armadilloes (Glyptodon) flourished long afterwards in South America.
The Megatherium attained a length of eighteen feet in one specimen
discovered, and the Glyptodon often had a dorsal shield (like that
of the armadillo) from six to eight feet long, and, in addition, a
stoutly armoured tail several feet long.
The richness and rapidity of the mammalian development in the Tertiary,
of which this condensed survey will convey some impression, make it
impossible to do more here than glance over the vast field and indicate
the better-known connections. It will be seen that evolution not only
introduces a lucid order and arrangement into our thousands of species
of living and fossil mammals, but throws an admirable light on the
higher animal world of our time. The various orders into which the
zoologist puts our mammals are seen to be the branches of a living
tree, approaching more and more closely to each other in early Tertiary
times, in spite of the imperfectness of the geological record. We
at last trace these diverging lines to a few very primitive, generalised,
patriarchal groups, which in turn approach each other very closely
in structure, and plainly suggest a common Cretaceous ancestor. Whether
that common ancestor was an Edentate, an Insectivore, or Creodont,
or something more primitive than them all, is disputed. But the divergence
of nearly all the lines of our mammal world from those patriarchal
types is admirably clear. In the mutual struggle of carnivore and
herbivore, in adaptation to a hundred different environments (the
water, the land, and the air, the tree, the open plain, the underground,
the marsh, etc.) and forms of diet, we find the descendants of these
patriarchal animals gradually developing their distinctive characters.
Then we find the destructive agencies of living and inorganic nature
blotting out type after type, and the living things that spread over
the land in the later Tertiary are found to be broadly identical with
the living things of to-day. The last great selection, the northern
Ice-Age, will give the last touches of modernisation.
CHAPTER XVIII. THE EVOLUTION OF MAN
We have reserved for a closer inquiry that order of the placental
mammals to which we ourselves belong, and on which zoologists have
bestowed the very proper and distinguishing name of the Primates.
Since the days of Darwin there has been some tendency to resent the
term "lower animals," which man applies to his poorer relations.
But, though there is no such thing as an absolute standard by which
we may judge the "higher" or "lower" status of
animals or plants, the extraordinary power which man has by his brain
development attained over both animate and inanimate nature fully
justifies the phrase. The Primate order is, therefore, of supreme
interest as the family that gave birth to man, and it is important
to discover the agencies which impelled some primitive member of it
to enter upon the path which led to this summit of organic nature.
The order includes the femurs, a large and primitive family with ape-like
features--the Germans call them "half-apes"--the monkeys,
the man-like apes, and man. This classification according to structure
corresponds with the successive appearance of the various families
in the geological record. The femurs appear in the Eocene; the monkeys,
and afterwards the apes, in the Miocene, the first semi-human forms
in the Pleistocene, though they must have been developed before this.
It is hardly necessary to say that science does not regard man as
a descendant of the known anthropoid apes, or these as descended from
the monkeys. They are successive types or phases of development, diverging
early from each other. Just as the succeeding horse-types of the record
are not necessarily related to each other in a direct line, yet illustrate
the evolution of a type which culminates in the horse, so the spreading
and branching members of the Primate group illustrate the evolution
of a type of organism which culminates in man. The particular relationship
of the various families, living and dead, will need careful study.
That there is a general blood-relationship, and that man is much more
closely related to the anthropoid apes than to any of the lower Primates,
is no longer a matter of controversy. In Rudolph Virchow there died,
a few years ago, the last authoritative man of science to express
any doubt about it. There are, however, non-scientific writers who,
by repeating the ambiguous phrase that it is "only a theory,"
convey the impression to inexpert readers that it is still more or
less an open question. We will therefore indicate a few of the lines
of evidence which have overcome the last hesitations of scientific
men, and closed the discussion as to the fact.
The very close analogy of structure between man and the ape at once
suggests that they had a common ancestor. There are cases in which
two widely removed animals may develop a similar organ independently,
but there is assuredly no possibility of their being alike in all
organs, unless by common inheritance. Yet the essential identity of
structure in man and the ape is only confirmed by every advance of
science, and would of itself prove the common parentage. Such minor
differences as there are between man and the higher ape--in the development
of the cerebrum, the number of the teeth or ribs, the distribution
of the hair, and so on--are quite explicable when we reflect that
the two groups must have diverged from each other more than a million
years ago
Examining the structure of man more closely, we find this strong suggestion
of relationship greatly confirmed. It is now well known that the human
body contains a number of vestigial "organs"--organs of
no actual use, and only intelligible as vestiges of organs that were
once useful. Whatever view we take of the origin of man, each organ
in his frame must have a meaning; and, as these organs are vestigial
and useless even in the lowest tribes of men, who represent primitive
man, they must be vestiges of organs that were of use in a remote
pre-human ancestor. The one fact that the ape has the same vestigial
organs as man would, on a scientific standard of evidence, prove the
common descent of the two. But these interesting organs themselves
point back far earlier than a mixed ape-human ancestor in many cases.
The shell of cartilage which covers the entrance to the ear--the gristly
appendage which is popularly called the ear--is one of the clearest
and most easily recognised of these organs. The "ear" of
a horse or a cat is an upright mobile shell for catching the waves
of sound. The human ear has the appearance of being the shrunken relic
of such an organ, and, when we remove the skin, and find seven generally
useless muscles attached to it, obviously intended to pull the shell
in all directions (as in the horse), there can be no doubt that the
external ear is a discarded organ, a useless legacy from an earlier
ancestor. In cases where it has been cut off it was found that the
sense of hearing was scarcely, if at all, affected. Now we know that
it is similarly useless in all tribes of men, and must therefore come
from a pre-human ancestor. It is also vestigial in the higher apes,
and it is only when we descend to the lower monkeys and femurs that
we see it approaching its primitive useful form. One may almost say
that it is a reminiscence of the far-off period when, probably in
the early Tertiary, the ancestors of the Primates took to the trees.
The animals living on the plain needed acute senses to detect the
approach of their prey or their enemies; the tree-dweller found less
demand on his sense of hearing, the "speaking-trumpet" was
discarded, and the development of the internal ear proceeded on the
higher line of the perception of musical sounds.
We might take a very large number of parts of the actual human body,
and discover that they are similar historical or archaeological monuments
surviving in a modern system, but we have space only for a few of
the more conspicuous.
The hair on the body is a vestigial organ, of actual use to no race
of men, an evident relic of the thick warm coat of an earlier ancestor.
It in turn recalls the dwellers in the primeval forest. In most cases--not
all, because the wearing of clothes for ages has modified this feature--it
will be found that the hairs on the arm tend upward from the wrist
to the elbow, and downward from the shoulder to the elbow. This very
peculiar feature becomes intelligible when we find that some of the
apes also have it, and that it has a certain use in their case. They
put their hands over their heads as they sit in the trees during ram,
and in that position the sloping hair acts somewhat like the thatched
roof of a cottage.
Again, it will be found that in the natural position of standing we
are not perfectly flat-footed, but tend to press much more on the
outer than on the inner edge of the foot. This tendency, surviving
after ages of living on the level ground, is a lingering effect of
the far-off arboreal days.
A more curious reminiscence is seen in the fact that the very young
infant, flabby and powerless as it is in most of its muscles, is so
strong in the muscles of the hand and arm that it can hang on to a
stick by its hands, and sustain the whole weight of its body, for
several minutes. Finally, our vestigial tail--for we have a tail comparable
to that of the higher apes--must be mentioned. In embryonic development
the tail is much longer than the legs, and some children are born
with a real tail, which they move as the puppy does, according to
their emotional condition. Other features of the body point back to
an even earlier stage. The vermiform appendage--in which some recent
medical writers have vainly endeavoured to find a utility-- is the
shrunken remainder of a large and normal intestine of a remote ancestor.
This interpretation of it would stand even if it were found to have
a certain use in the human body. Vestigial organs are sometimes pressed
into a secondary use when their original function has been lost. The
danger of this appendage in the human body to-day is due to the fact
that it is a blind alley leading off the alimentary canal, and has
a very narrow opening. In the ape the opening is larger, and, significantly
enough, it is still larger in the human foetus. When we examine some
of the lower mammals we discover the meaning of it. It is in them
an additional storage chamber in the alimentary system. It is believed
that a change to a more digestible diet has made this additional chamber
superfluous in the Primates, and the system is slowly suppressing
it.
Other reminiscences of this earlier phase are found in the many vestigial
muscles which are found in the body to-day. The head of the quadruped
hangs forward, and is held by powerful muscles and ligaments in the
neck. We still have the shrunken remainder of this arrangement. Other
vestigial muscles are found in the forehead, the scalp, the nose--many
people can twitch the nostrils and the scalp--and under the skin in
many parts of the body. These are enfeebled remnants of the muscular
coat by which the quadruped twitches its skin, and drives insects
away. A less obvious feature is found by the anatomist in certain
blood-vessels of the trunk. As the blood flows vertically in a biped
and horizontally in a quadruped, the arrangement of the valves in
the blood-vessels should be different in the two cases; but it is
the same in us as in the quadruped. Another trace of the quadruped
ancestor is found in the baby. It walks "on all fours" so
long, not merely from weakness of the limbs, but because it has the
spine of a quadruped.
A much more interesting fact, but one less easy to interpret, is that
the human male has, like the male ape, organs for suckling the young.
That there are real milk-glands, usually vestigial, underneath the
teats in the breast of the boy or the man is proved by the many known
cases in which men have suckled the young. Several friends of the
present writer have seen this done in India and Ceylon by male "wet-nurses."
As there is no tribe of men or species of ape in which the male suckles
the young normally, we seem to be thrown back once more upon an earlier
ancestor. The difficulty is that we know of no mammal of which both
parents suckle the young, and some authorities think that the breasts
have been transferred to the male by a kind of embryonic muddle. That
is difficult to believe, as no other feature has ever been similarly
transferred to the opposite sex. In any case the male breasts are
vestigial organs. Another peculiarity of the mammary system is that
sometimes three, four, or five pairs of breasts appear in a woman
(and several have been known even in a man). This is, apparently,
an occasional reminiscence of an early mammal ancestor which had large
litters of young and several pairs of breasts.
But there are features of the human body which recall an ancestor
even earlier than the quadruped. The most conspicuous of these is
the little fleshy pad at the inner corner of each eye. It is a common
feature in mammals, and is always useless. When, however, we look
lower down in the animal scale we find that fishes and reptiles (and
birds) have a third eyelid, which is drawn across the eye from this
corner. There is little room to doubt that the little fleshy vestige
in the mammal's eye is the shrunken remainder of the lateral eyelid
of a remote fish-ancestor.
A similar reminiscence is found in the pineal body, a small and useless
object, about the size and shape of a hazel-nut, in the centre of
the brain. When we examine the reptile we find a third eye in the
top of the head. The skin has closed over it, but the skull is still,
in many cases, perforated as it is for the eyes in front. I have seen
it standing out like a ball on the head of a dead crocodile, and in
the living tuatara--the very primitive New Zealand lizard--it still
has a retina and optic nerve. As the only animal in nature to-day
with an eye in this position (the Pyrosome, a little marine animal
of the sea-squirt family) is not in the line of reptile and mammal
ancestry, it is difficult to locate the third eye definitely. But
when we find the skin closing over it in the amphibian and reptile,
then the bone, and then see it gradually atrophying and being buried
under the growing brain, we must refer it to some early fish-ancestor.
This ancestor, we may recall, is also reflected for a time in the
gill-slits and arches, with their corresponding fish-like heart and
blood-vessels, during man's embryonic development, as we saw in a
former chapter.
These are only a few of the more conspicuous instances of vestigial
structures in man. Metchnikoff describes about a hundred of them.
Even if there were no remains of primitive man pointing in the direction
of a common ancestry with the ape, no lower types of men in existence
with the same tendency, no apes found in nature to-day with a structure
so strikingly similar to that of man, and no fossil records telling
of the divergence of forms from primitive groups in past time, we
should be forced to postulate the evolution of man in order to explain
his actual features. The vestigial structures must be interpreted
as we interpret the buttons on the back of a man's coat. They are
useless reminiscences of an age in which they were useful. When their
witness to the past is supported by so many converging lines of evidence
it becomes irresistible. I will add only one further testimony which
has been brought into court in recent years.
The blood consists of cells, or minute disk-shaped corpuscles, floating
in a watery fluid, or serum. It was found a few years ago, in the
course of certain experiments in mixing the blood of animals, that
the serum of one animal's blood sometimes destroyed the cells of the
other animal's blood, and at other times did not. When the experiments
were multiplied, it was found that the amount of destructive action
exercised by one specimen of blood upon another depended on the nearness
or remoteness of relationship between the animals. If the two are
closely related, there is no disturbance when their blood is mixed;
when they are not closely related, the serum of one destroys the cells
of the other, and the intensity of the action is in proportion to
their remoteness from each other. Another and more elaborate form
of the experiment was devised, and the law was confirmed. On both
tests it was found by experiment that the blood of man and of the
anthropoid ape behaved in such a way as to prove that they were closely
related. The blood of the monkey showed a less close relationship--a
little more remote in the New World than in the Old World monkeys;
and the blood of the femur showed a faint and distant relationship.
The FACT of the evolution of man and the apes from a common ancestor
is, therefore, outside the range of controversy in science; we are
concerned only to retrace the stages of that evolution, and the agencies
which controlled it. Here, unfortunately, the geological record gives
us little aid. Tree-dwelling animals are amongst the least likely
to be buried in deposits which may preserve their bones for ages.
The distribution of femur and ape remains shows that the order of
the Primates has been widespread and numerous since the middle of
the Tertiary Era, yet singularly few remains of the various families
have been preserved.
Hence the origin of the Primates is obscure. They are first foreshadowed
in certain femur-like forms of the Eocene period, which are said in
some cases (Adapis) to combine the characters of pachyderms and femurs,
and in others (Anaptomorphus) to unite the features of Insectivores
and femurs. Perhaps the more common opinion is that they were evolved
from a branch of the Insectivores, but the evidence is too slender
to justify an opinion. It was an age when the primitive placental
mammals were just beginning to diverge from each other, and had still
many features in common. For the present all we can say is that in
the earliest spread of the patriarchal mammal race one branch adopted
arboreal life, and evolved in the direction of the femurs and the
apes. The generally arboreal character of the Primates justifies this
conclusion.
In the Miocene period we find a great expansion of the monkeys. These
in turn enter the scene quite suddenly, and the authorities are reduced
to uncertain and contradictory conjectures as to their origin. Some
think that they develop not from the femurs, but along an independent
line from the Insectivores, or other ancestors of the Primates. We
will not linger over these early monkeys, nor engage upon the hopeless
task of tracing their gradual ramification into the numerous families
of the present age. It is clear only that they soon divided into two
main streams, one of which spread into the monkeys of America and
the other into the monkeys of the Old World. There are important anatomical
differences between the two. The monkeys remained in Central and Southern
Europe until near the end of the Tertiary. Gradually we perceive that
the advancing cold is driving them further south, and the monkeys
of Gibraltar to-day are the diminished remnant of the great family
that had previously wandered as far as Britain and France.
A third wave, also spreading in the Miocene, equally obscure in its
connection with the preceding, introduces the man-like apes to the
geologist. Primitive gibbons (Pliopithecus and Pliobylobates), primitive
chimpanzees (Palaeopithecus), and other early anthropoid apes (Oreopithecus,
Dryopithecus, etc.), lived in the trees of Southern Europe in the
second part of the Tertiary Era. They are clearly disconnected individuals
of a large and flourishing family, but from the half-dozen specimens
we have yet discovered no conclusion can be drawn, except that the
family is already branching into the types of anthropoid apes which
are familiar to us.
Of man himself we have no certain and indisputable trace in the Tertiary
Era. Some remains found in Java of an ape-man (Pithecanthropus), which
we will study later, are now generally believed, after a special investigation
on the spot, to belong to the Pleistocene period. Yet no authority
on the subject doubts that the human species was evolved in the Tertiary
Era, and very many, if not most, of the authorities believe that we
have definite proof of his presence. The early story of mankind is
gathered, not so much from the few fragments of human remains we have,
but from the stone implements which were shaped by his primitive intelligence
and remain, almost imperishable, in the soil over which he wandered.
The more primitive man was, the more ambiguous would be the traces
of his shaping of these stone implements, and the earliest specimens
are bound to be a matter of controversy. It is claimed by many distinguished
authorities that flints slightly touched by the hand of man, or at
least used as implements by man, are found in abundance in England,
France, and Germany, and belong to the Pliocene period. Continental
authorities even refer some of them to the Miocene and the last part
of the Oligocene.
The question whether an implement-using animal, which nearly all would
agree to regard as in some degree human, wandered over what is now
the South of England (Kent, Essex, Dorsetshire, etc.) as many hundred
thousand years ago as this claim would imply, is certainly one of
great interest. But there would be little use in discussing here the
question of the "Eoliths," as these disputed implements
are called. A very keen controversy is still being conducted in regard
to them, and some of the highest authorities in England, France, and
Germany deny that they show any trace of human workmanship or usage.
Although they have the support of such high authorities as Sir J.
Prestwich, Sir E. Ray Lankester, Lord Avebury, Dr. Keane, Dr. Blackmore,
Professor Schwartz, etc., they are one of those controverted testimonies
on which it would be ill-advised to rely in such a work as this.
We must say, then, that we have no undisputed traces of man in the
Tertiary Era. The Tertiary implements which have been at various times
claimed in France, Italy, and Portugal are equally disputed; the remains
which were some years ago claimed as Tertiary in the United States
are generally disallowed; and the recent claims from South America
are under discussion. Yet it is the general feeling of anthropologists
that man was evolved in the Tertiary Era. On the one hand, the anthropoid
apes were highly developed by the Miocene period, and it would be
almost incredible that the future human stock should linger hundreds
of thousands of years behind them. On the other hand, when we find
the first traces of man in the Pleistocene, this development has already
proceeded so far that its earlier phase evidently goes back into the
Tertiary. Let us pass beyond the Tertiary Era for a moment, and examine
the earliest and most primitive remains we have of human or semi-human
beings.
The first appearance of man in the chronicle of terrestrial life is
a matter of great importance and interest. Even the least scientific
of readers stands, so to say, on tiptoe to catch a first glimpse of
the earliest known representative of our race, and half a century
of discussion of evolution has engendered a very wide interest in
the early history of man.*
* A personal experience may not be without interest
in this connection. Among the many inquiries directed to me in regard
to evolution I received, in one month, a letter from a negro in British
Guiana and an extremely sensible query from an inmate of an English
asylum for the insane! The problem that beset the latter of the two
was whether the Lemuranda preceded the Lemurogona in Eocene times.
He had found a contradiction in the statements of two scientific writers.
Fortunately, although these patriarchal bones are very scanty--two
teeth, a thigh-bone, and the skull-cap--we are now in a position to
form some idea of the nature of their living owner. They have been
subjected to so searching a scrutiny and discussion since they were
found in Java in 1891 and 1892 that there is now a general agreement
as to their nature. At first some of the experts thought that they
were the remains of an abnormally low man, and others that they belonged
to an abnormally high ape. The majority held from the start that they
belonged to a member of a race almost midway between the highest family
of apes and the lowest known tribe of men, and therefore fully merited
the name of "Ape-Man" (Pithecanthropus). This is now the
general view of anthropologists.
The Ape-Man of Java was in every respect entitled to that name. The
teeth suggest a lower part of the face in which the teeth and lips
projected more than in the most ape-like types of Central Africa.
The skull-cap has very heavy ridges over the eyes and a low receding
forehead, far less human than in any previously known prehistoric
skull. The thigh-bone is very much heavier than any known human femur
of the same length, and so appreciably curved that the owner was evidently
in a condition of transition from the semi-quadrupedal crouch of the
ape to the erect attitude of man. The Ape-Man, in other words, was
a heavy, squat, powerful, bestial-looking animal; of small stature,
but above the pygmy standard; erect in posture, but with clear traces
of the proneness of his ancestor; far removed from the highest ape
in brainpower, but almost equally far removed from the lowest savage
that is known to us. We shall see later that there is some recent
criticism, by weighty authorities, of the earlier statements in regard
to the brain of primitive man. This does not apply to the Ape-Man
of Java. The average cranial capacity (the amount of brain-matter
the skull may contain) of the chimpanzees, the highest apes, is about
600 cubic centimetres. The average cranial capacity of the lowest
races of men, of moderate stature, is about 1200. And the cranial
capacity of Ape-Man was about 900
It is immaterial whether or no these bones belong to the same individual.
If they do not, we have remains of two or three individuals of the
same intermediate species. Nor does it matter whether or no this early
race is a direct ancestor of the later races of men, or an extinct
offshoot from the advancing human stock. It is, in either case, an
illustration of the intermediate phase between the ape and man The
more important tasks are to trace the relationship of this early human
stock to the apes, and to discover the causes of its superior evolution.
The first question has a predominantly technical interest, and the
authorities are not agreed in replying to it. We saw that, on the
blood-test, man showed a very close relationship to the anthropoid
apes, a less close affinity to the Old World monkeys, a more remote
affinity to the American monkeys, and a very faint and distant affinity
to the femurs. A comparison of their structures suggests the same
conclusion. It is, therefore, generally believed that the anthropoid
apes and man had a common ancestor in the early Miocene or Oligocene,
that this group was closely related to the ancestral group of the
Old World monkeys, and that all originally sprang from a primitive
and generalised femur-group. In other words, a branch of the earliest
femur-like forms diverges, before the specific femur-characters are
fixed, in the direction of the monkey; in this still vague and patriarchal
group a branch diverges, before the monkey-features are fixed, in
the direction of the anthropoids; and this group in turn spreads into
a number of types, some of which are the extinct apes of the Miocene,
four become the gorilla, chimpanzee, orang, and gibbon of to-day,
and one is the group that will become man. To put it still more precisely,
if we found a whole series of remains of man's ancestors during the
Tertiary, we should probably class them, broadly, as femur-remains
in the Eocene, monkey-remains in the Oligocene, and ape-remains in
the Miocene. In that sense only man "descends from a monkey."
The far more important question is: How did this one particular group
of anthropoid animals of the Miocene come to surpass all its cousins,
and all the rest of the mammals, in brain-development? Let us first
rid the question of its supposed elements of mystery and make of it
a simple problem. Some imagine that a sudden and mysterious rise in
intelligence lifted the progenitor of man above its fellows. The facts
very quickly dispel this illusion. We may at least assume that the
ancestor of man was on a level with the anthropoid ape in the Miocene
period, and we know from their skulls that the apes were as advanced
then as they are now. But from the early Miocene to the Pleistocene
is a stretch of about a million years on the very lowest estimate.
In other words, man occupied about a million years in travelling from
the level of the chimpanzee to a level below that of the crudest savage
ever discovered. If we set aside the Java man, as a possible survivor
of an earlier phase, we should still have to say that, much more than
a million years after his departure from the chimpanzee level, man
had merely advanced far enough to chip stone implements; because we
find no other trace whatever of intelligence than this until near
the close of the Palaeolithic period. If there is any mystery, it
is in the slowness of man's development.
Let us further recollect that it is a common occurrence in the calendar
of life for a particular organ to be especially developed in one member
of a particular group more than in the others. The trunk of the elephant,
the neck of the giraffe, the limbs of the horse or deer, the canines
of the satire-toothed tiger, the wings of the bat, the colouring of
the tiger, the horns of the deer, are so many examples in the mammal
world alone. The brain is a useful organ like any other, and it is
easy to conceive that the circumstances of one group may select it
just as the environment of another group may lead to the selection
of speed, weapons, or colouring. In fact, as we saw, there was so
great and general an evolution of brain in the Tertiary Era that our
modern mammals quite commonly have many times the brain of their Tertiary
ancestors. Can we suggest any reasons why brain should be especially
developed in the apes, and more particularly still in the ancestors
of man?
The Primate group generally is a race of tree-climbers. The appearance
of fruit on early Tertiary trees and the multiplication of carnivores
explain this. The Primate is, except in a few robust cases, a particularly
defenceless animal. When its earliest ancestors came in contact with
fruit and nut-bearing trees, they developed climbing power and other
means of defence and offense were sacrificed. Keenness of scent and
range of hearing would now be of less moment, but sight would be stimulated,
especially when soft-footed climbing carnivores came on the scene.
There is, however, a much deeper significance in the adoption of climbing,
and we must borrow a page from the modern physiology of the brain
to understand it.
The stress laid in the modern education of young children on the use
of the hands is not merely due to a feeling that they should handle
objects as well as read about them. It is partly due to the belief
of many distinguished physiologists that the training of the hands
has a direct stimulating effect on the thought- centres in the brain.
The centre in the cerebrum which controls the use of the hands is
on the fringe of the region which seems to be concerned in mental
operations. For reasons which will appear presently, we may add that
the centres for controlling the muscles of the face and head are in
the same region. Any finer training or the use of the hands will develop
the centre for the fore limbs, and, on the principles, may react on
the more important region of the cortex. Hence in turning the fore
foot into a hand, for climbing and grasping purposes, the primitive
Primate entered upon the path of brain-development. Even the earliest
Primates show large brains in comparison with the small brains of
their contemporaries.
It is a familiar fact in the animal world that when a certain group
enters upon a particular path of evolution, some members of the group
advance only a little way along it, some go farther, and some outstrip
all the others. The development of social life among the bees will
illustrate this. Hence we need not be puzzled by the fact that the
lemurs have remained at one mental level, the monkeys at another,
and the apes at a third. It is the common experience of life; and
it is especially clear among the various races of men. A group becomes
fitted to its environment, and, as long as its surroundings do not
change, it does not advance. A related group, in a different environment,
receives a particular stimulation, and advances. If, moreover, a group
remains unstimulated for ages, it may become so rigid in its type
that it loses the capacity to advance. It is generally believed that
the lowest races of men, and even some of the higher races like the
Australian aboriginals, are in this condition. We may expect this
"unteachability" in a far more stubborn degree in the anthropoid
apes, which have been adapted to an unchanging environment for a million
years.
All that we need further suppose is--and it is one of the commonest
episodes in terrestrial life--that one branch of the Miocene anthropoids,
which were spread over a large part of the earth, received some stimulus
to change which its cousins did not experience. It is sometimes suggested
that social life was the great advantage which led to the superior
development of mind in man. But such evidence as there is would lead
us to suppose that primitive man was solitary, not social. The anthropoid
apes are not social, but live in families, and are very unprogressive.
On the other hand, the earliest remains of prehistoric man give no
indication of social life. Fire-places, workshops, caves, etc., enter
the story in a later phase. Some authorities on prehistoric man hold
very strongly that during the greater part of the Old Stone Age (two-thirds,
at least, of the human period) man wandered only in the company of
his mate and children.*
* The point will be more fully discussed later. This account of prehistoric
life is well seen in Mortillet's Prehistorique (1900). The lowest
races also have no tribal life, and Professor Westermarck is of opinion
that early man was not social.
We seem to have the most plausible explanation of the divergence of
man from his anthropoid cousins in the fact that he left the trees
of his and their ancestors. This theory has the advantage of being
a fact--for the Ape-Man race of Java has already left the trees--and
providing a strong ground for brain-advance. A dozen reasons might
be imagined for his quitting the trees--migration, for instance, to
a region in which food was more abundant, and carnivores less formidable,
on the ground-level--but we will be content with the fact that he
did. Such a change would lead to a more consistent adoption of the
upright attitude, which is partly found in the anthropoid apes, especially
the gibbons. The fore limb would be no longer a support of the body;
the hand would be used more for grasping; and the hand-centre in the
brain would be proportionately stimulated. The adoption of the erect
attitude would further lead to a special development of the muscles
of the head and face, the centre for which is in the same important
region in the cortex. There would also be a direct stimulation of
the brain, as, having neither weapons nor speed, the animal would
rely all the more on sight and mind. If we further suppose that this
primitive being extended the range of his hunting, from insects and
small or dead birds to small land-animals, the stimulation would be
all the greater. In a word, the very fact of a change from the trees
to the ground suggests a line of brain-development which may plausibly
be conceived, in the course of a million years, to evolve an Ape-Man
out of a man-like ape. And we are not introducing any imaginary factor
in this view of human origins.
The problem of the evolution of man is often approached in a frame
of mind not far removed from that of the educated, but inexpert, European
who stands before the lowly figure of the chimpanzee, and wonders
by what miracle the gulf between it and himself was bridged. That
is to lay a superfluous strain on the imagination. The proper term
of comparison is the lowest type of human being known to us, since
the higher types of living men have confessedly evolved from the lower.
But even the lowest type of existing or recent savage is not the lowest
level of humanity. Whether or no the Tasmanian or the Yahgan is a
primitive remnant of the Old Stone Age, we have a far lower depth
in the Java race. What we have first to do is to explain the advance
to that level, in the course of many hundreds of thousands of years:
a period fully a hundred times as long as the whole history of civilisation.
Time itself is no factor in evolution, but in this case it is a significant
condition. It means that, on this view of the evolution of man, we
are merely assuming that an advance in brain-development took place
between the Miocene and the Pleistocene, not similar to, but immeasurably
less than, the advance which we know to have been made in the last
fifty thousand years. In point of fact, the most mysterious feature
of the evolution of man was its slowness. We shall see that, to meet
the facts, we must suppose man to have made little or no progress
during most of this vast period, and then to have received some new
stimulation to develop. What it was we have now to inquire.
CHAPTER XIX. MAN AND THE GREAT ICE-AGE
In discussing the development of plants and animals during the Tertiary
Era we have already perceived the shadow of the approaching Ice-Age.
We found that in the course of the Tertiary the types which were more
sensitive to cold gradually receded southward, and before its close
Europe, Asia, and North America presented a distinctly temperate aspect.
This is but the penumbra of the eclipse. When we pass the limits of
the Tertiary Era, and enter the Quaternary, the refrigeration steadily
proceeds, and, from temperate, the aspect of much of Europe and North
America becomes arctic. From six to eight million square miles of
the northern hemisphere are buried under fields of snow and ice, and
even in the southern regions smaller glacial sheets spread from the
foot of the higher ranges of mountains.
It is unnecessary to-day to explain at any length the evidences by
which geologists trace this enormous glaciation of the northern hemisphere.
There are a few works still in circulation in which popular writers,
relying on the obstinacy of a few older geologists, speak lightly
of the "nightmare" of the Ice-Age. But the age has gone
by in which it could seriously be suggested that the boulders strewn
along the east of Scotland--fragments of rock whose home we must seek
in Scandinavia--were brought by the vikings as ballast for their ships.
Even the more serious controversy, whether the scratches and the boulders
which we find on the face of Northern Europe and America were due
to floating or land ice, is virtually settled. Several decades of
research have detected the unmistakable signs of glacial action over
this vast area of the northern hemisphere. Most of Europe north of
the Thames and the Danube, nearly all Canada and a very large part
of the United States, and a somewhat less expanse of Northern Asia,
bear to this day the deep scars of the thick, moving ice-sheets. Exposed
rock-surfaces are ground and scratched, beds of pebbles are twisted
and contorted hollows are scooped out, and moraines--the rubbish-heaps
of the glaciers--are found on every side. There is now not the least
doubt that, where the great Deinosaurs had floundered in semi-tropical
swamps, where the figs and magnolias had later flourished, where the
most industrious and prosperous hives of men are found to-day, there
was, in the Pleistocene period, a country to which no parallel can
be found outside the polar circles to-day.
The great revolution begins with the gathering of snows on the mountains.
The Alps and Pyrenees had now, we saw, reached their full stature,
and the gathering snows on their summits began to glide down toward
the plains in rivers of ice. The Apennines (and even the mountains
of Corsica), the Balkans, Carpathians, Caucasus, and Ural Mountains,
shone in similar mantles of ice and snow. The mountains of Wales,
the north of England, Scotland, and Scandinavia had even heavier burdens,
and, as the period advanced, their sluggish streams of ice poured
slowly over the plains. The trees struggled against the increasing
cold in the narrowing tracts of green; the animals died, migrated
to the south, or put on arctic coats. At length the ice-sheets of
Scandinavia met the spreading sheets from Scotland and Wales, and
crept over Russia and Germany, and an almost continuous mantle, from
which only a few large areas of arctic vegetation peeped out, was
thrown over the greater part of Europe. Ten thousand feet thick where
it left the hills of Norway and Sweden, several thousand feet thick
even in Scotland, the ice-sheet that resulted from the fusion of the
glaciers gradually thinned as it went south, and ended in an irregular
fringe across Central Europe. The continent at that time stretched
westward beyond the Hebrides and some two hundred miles beyond Ireland.
The ice-front followed this curve, casting icebergs into the Atlantic,
then probably advanced up what is now the Bristol Channel, and ran
across England and Europe, in a broken line, from Bristol to Poland.
South of this line there were smaller ice-fields round the higher
mountains, north of it almost the whole country presented the appearance
that we find in Greenland to-day.
In North America the glaciation was even more extensive. About four
million square miles of the present temperate zone were buried under
ice and snow. From Greenland, Labrador, and the higher Canadian mountains
the glaciers poured south, until, in the east, the mass of ice penetrated
as far as the valley of the Mississippi. The great lakes of North
America are permanent memorials of its Ice-Age, and over more than
half the country we trace the imprint and the relics of the sheet.
South America, Australia, Tasmania, and New Zealand had their glaciated
areas. North Asia was largely glaciated, but the range of the ice-sheet
is not yet determined in that continent.
This summary statement will convey some idea of the extraordinary
phase through which the earth passed in the early part of the present
geological era. But it must be added that a singular circumstance
prolonged the glacial regime in the northern hemisphere. Modern geologists
speak rather of a series of successive ice-sheets than of one definite
Ice-Age. Some, indeed, speak of a series of Ice-Ages, but we need
not discuss the verbal question. It is now beyond question that the
ice-sheet advanced and retreated several times during the Glacial
Epoch. The American and some English geologists distinguished six
ice-sheets, with five intermediate periods of more temperate climate.
The German and many English and French geologists distinguish four
sheets and three interglacial epochs. The exact number does not concern
us, but the repeated spread of the ice is a point of some importance.
The various sheets differed considerably in extent. The wide range
of the ice which I have described represents the greatest extension
of the glaciation, and probably corresponds to the second or third
of the six advances in Dr. Geikie's (and the American) classification.
Before we consider the biological effect of this great of refrigeration
of the globe, we must endeavour to understand the occurrence itself.
Here we enter a world of controversy, but a few suggestions at least
may be gathered from the large literature of the subject, which dispel
much of the mystery of the Great Ice-Age.
It was at one time customary to look out beyond the earth itself for
the ultimate causes of this glaciation. Imagine the sheet of ice,
which now spreads widely round the North Pole, shifted to another
position on the surface of the planet, and you have a simple explanation
of the occurrence. In other words, if we suppose that the axis of
the earth does not consistently point in one direction-- that the
great ball does not always present the same average angle in relation
to the sun--the poles will not always be where they are at present,
and the Pleistocene Ice-Age may represent a time when the north pole
was in the latitude of North Europe and North America. This opinion
had to be abandoned. We have no trace whatever of such a constant
shifting of the polar regions as it supposes, and, especially, we
have no trace that the warm zone correspondingly shifted in the Pleistocene.
A much more elaborate theory was advanced by Dr. Croll, and is still
entertained by many. The path of the earth round the sun is not circular,
but elliptical, and there are times when the gravitational pull of
the other planets increases the eccentricity of the orbit. It was
assumed that there are periods of great length, separated from each
other by still longer periods, when this eccentricity of the orbit
is greatly exaggerated. The effect would be to prolong the winter
and shorten the summer of each hemisphere in turn. The total amount
of heat received would not alter, but there would be a long winter
with less heat per hour, and a short summer with more heat. The short
summer would not suffice to melt the enormous winter accumulations
of ice and snow, and an ice-age would result. To this theory, again,
it is objected that we do not find the regular succession of ice-ages
in the story of the earth which the theory demands, and that there
is no evidence of an alternation of the ice between the northern and
southern hemispheres.
More recent writers have appealed to the sun itself, and supposed
that some prolonged veiling of its photosphere greatly reduced the
amount of heat emitted by it. More recently still it has been suggested
that an accumulation of cosmic or meteoric dust in our atmosphere,
or between us and the sun, had, for a prolonged period, the effect
of a colossal "fire-screen." Neither of these suppositions
would explain the localisation of the ice. In any case we need not
have recourse to purely speculative accidents in the world beyond
until it is clear that there were no changes in the earth itself which
afford some explanation.
This is by no means clear. Some writers appeal to changes in the ocean
currents. It is certain that a change in the course of the cold and
warm currents of the ocean to-day might cause very extensive changes
of climate, but there seems to be some confusion of ideas in suggesting
that this might have had an equal, or even greater, influence in former
times. Our ocean currents differ so much in temperature because the
earth is now divided into very pronounced zones of climate. These
zones did not exist before the Pliocene period, and it is not at all
clear that any redistribution of currents in earlier times could have
had such remarkable consequences. The same difficulty applies to wind-currents.
On the other hand, we have already, in discussing the Permian glaciation,
discovered two agencies which are very effective in lowering the temperature
of the earth. One is the rise of the land; the other is the thinning
of the atmosphere. These are closely related agencies, and we found
them acting in conjunction to bring about the Permian Ice-Age. Do
we find them at work in the Pleistocene?
It is not disputed that there was a very considerable upheaval of
the land, especially in Europe and North America, at the end of the
Tertiary Era. Every mountain chain advanced, and our Alps, Pyrenees,
Himalaya, etc., attained, for the first time, their present, or an
even greater elevation. The most critical geologists admit that Europe,
as a whole, rose 4000 feet above its earlier level. Such an elevation
would be bound to involve a great lowering of the temperature. The
geniality of the Oligocene period was due, like that of the earlier
warm periods, to the low-lying land and very extensive water-surface.
These conditions were revolutionised before the end of the Tertiary.
Great mountains towered into the snow-line, and vast areas were elevated
which had formerly been sea or swamp.
This rise of the land involved a great decrease in the proportion
of moisture in the atmosphere. The sea surface was enormously lessened,
and the mountains would now condense the moisture into snow or cloud
to a vastly greater extent than had ever been known before There would
also be a more active circulation of the atmosphere, the moist warm
winds rushing upward towards the colder elevations and parting with
their vapour. As the proportion of moisture in the atmosphere lessened
the surface-heat would escape more freely into space, the general
temperature would fall, and the evaporation--or production of moisture
would be checked, while the condensation would continue. The prolonging
of such conditions during a geological period can be understood to
have caused the accumulation of fields of snow and ice in the higher
regions. It seems further probable that these conditions would lead
to a very considerable formation of fog and cloud, and under this
protecting canopy the glaciers would creep further down toward the
plains.
We have then to consider the possibility of a reduction of the quantity
of carbon-dioxide in the atmosphere The inexpert reader probably has
a very exaggerated idea of the fall in temperature that would be required
to give Europe an Ice-Age. If our average temperature fell about 5-8
degrees C. below the average temperature of our time it would suffice;
and it is further calculated that if the quantity of carbon-dioxide
in our atmosphere were reduced by half, we should have this required
fall in temperature. So great a reduction would not be necessary in
view of the other refrigerating agencies. Now it is quite certain
that the proportion of carbon-dioxide was greatly reduced in the Pleistocene.
The forests of the Tertiary Era would steadily reduce it, but the
extensive upheaval of the land at its close would be even more important.
The newly exposed surfaces would absorb great quantities of carbon.
The ocean, also, as it became colder, would absorb larger and larger
quantities of carbon-dioxide. Thus the Pleistocene atmosphere, gradually
relieved of its vapours and carbon-dioxide, would no longer retain
the heat at the surface. We may add that the growth of reflective
surfaces--ice, snow, cloud, etc.--would further lessen the amount
of heat received from the sun.
Here, then, we have a series of closely related causes and effects
which would go far toward explaining, if they do not wholly suffice
to explain, the general fall of the earth's temperature. The basic
cause is the upheaval of the land--a fact which is beyond controversy,
the other agencies are very plain and recognisable consequences of
the upheaval. There are, however, many geologists who do not think
this explanation adequate.
It is pointed out, in the first place, that the glaciation seems to
have come long after the elevation. The difficulty does not seem to
be insurmountable. The reduction of the atmospheric vapour would be
a gradual process, beginning with the later part of the elevation
and culminating long afterwards. The reduction of the carbon-dioxide
would be even more gradual. It is impossible to say how long it would
take these processes to reach a very effective stage, but it is equally
impossible to show that the interval between the upheaval and the
glaciation is greater than the theory demands.
It is also said that we cannot on these principles understand the
repeated advance and retreat of the ice-sheet.
This objection, again, seems to fail. It is an established fact that
the land sank very considerably during the Ice-Age, and has risen
again since the ice disappeared. We find that the crust in places
sank so low that an arctic ocean bathed the slopes of some of the
Welsh mountains; and American geologists say that their land has risen
in places from 2000 to 3000 feet (Chamberlin) since the burden of
ice was lifted from it. Here we have the possibility of an explanation
of the advances and retreats of the glaciers. The refrigerating agencies
would proceed until an enormous burden of ice was laid on the land
of the northern hemisphere. The land apparently sank under the burden,
the ice and snow melted at the lower level and there was a temperate
interglacial period. But the land, relieved of its burden, rose once
more, the exposed surface absorbed further quantities of carbon, and
a fresh period of refrigeration opened. This oscillation might continue
until the two sets of opposing forces were adjusted, and the crust
reached a condition of comparative stability.
Finally, and this is the more serious difficulty, it is said that
we cannot in this way explain the localisation of the glacial sheets.
Why should Europe and North America in particular suffer so markedly
from a general thinning of the atmosphere? The simplest answer is
to suggest that they especially shared the rise of the land. Geology
is not in a position either to prove or disprove this, and it remains
only a speculative interpretation of the fact We know at least that
there was a great uprise of land in Europe and North America in the
Pliocene and Pleistocene and may leave the precise determination of
the point to a later age. At the same time other local causes are
not excluded. There may have been a large extension of the area of
atmospheric depression which we have in the region of Greenland to-day.
When we turn to the question of chronology we have the same acute
difference of opinion as we have found in regard to all questions
of geological time. It used to be urged, on astronomical grounds,
that the Ice-Age began about 240,000 years ago, and ended about 60,000
years ago, but the astronomical theory is, as I said, generally abandoned.
Geologists, on the other hand, find it difficult to give even approximate
figures. Reviewing the various methods of calculation, Professor Chamberlin
concludes that the time of the first spread of the ice-sheet is quite
unknown, the second and greatest extension of the glaciation may have
been between 300,000 and a million years ago, and the last ice-extension
from 20,000 to 60,000 years ago; but he himself attaches "very
little value" to the figures. The chief ice-age was some hundreds
of thousands of years ago, that is all we can say with any confidence.
In dismissing the question of climate, however, we should note that
a very serious problem remains unsolved. As far as present evidence
goes we seem to be free to hold that the ice-ages which have at long
intervals invaded the chronicle of the earth were due to rises of
the land. Upheaval is the one constant and clearly recognisable feature
associated with, or preceding, ice-ages. We saw this in the case of
the Cambrian, Permian, Eocene, and Pleistocene periods of cold, and
may add that there are traces of a rise of mountains before the glaciation
of which we find traces in the middle of the Archaean Era. There are
problems still to be solved in connection with each of these very
important ages, but in the rise of the land and consequent thinning
of the atmosphere we seem to have a general clue to their occurrence.
Apart from these special periods of cold, however, we have seen that
there has been, in recent geological times, a progressive cooling
of the earth, which we have not explained. Winter seems now to be
a permanent feature of the earth's life, and polar caps are another
recent, and apparently permanent, acquisition. I find no plausible
reason assigned for this.
The suggestion that the disk of the sun is appreciably smaller since
Tertiary days is absurd; and the idea that the earth has only recently
ceased to allow its internal heat to leak through the crust is hardly
more plausible. The cause remains to be discovered.
We turn now to consider the effect of the great Ice-Age, and the relation
of man to it. The Permian revolution, to which the Pleistocene Ice-Age
comes nearest in importance, wrought such devastation that the overwhelming
majority of living things perished. Do we find a similar destruction
of life, and selection of higher types, after the Pleistocene perturbation?
In particular, had it any appreciable effect upon the human species?
A full description of the effect of the great Ice-Age would occupy
a volume. The modern landscape in Europe and North America was very
largely carved and modelled by the ice-sheet and the floods that ensued
upon its melting. Hills were rounded, valleys carved, lakes formed,
gravels and soils distributed, as we find them to-day. In its vegetal
aspect, also, as we saw, the modern landscape was determined by the
Pleistocene revolution. A great scythe slowly passed over the land.
When the ice and snow had ended, and the trees and flowers, crowded
in the southern area, slowly spread once more over the virgin soil,
it was only the temperate species that could pass the zone guarded
by the Alps and the Pyrenees. On the Alps themselves the Pleistocene
population still lingers, their successful adaptation to the cold
now preventing them from descending to the plains.
The animal world in turn was winnowed by the Pleistocene episode.
The hippopotamus, crocodile, turtle, flamingo, and other warm-loving
animals were banished to the warm zone. The mammoth and the rhinoceros
met the cold by developing woolly coats, but the disappearance of
the ice, which had tempted them to this departure, seems to have ended
their fitness. Other animals which became adapted to the cold--arctic
bears, foxes, seals, etc.--have retreated north with the ice, as the
sheet melted. For hundreds of thousands of years Europe and North
America, with their alternating glacial and interglacial periods,
witnessed extraordinary changes and minglings of their animal population.
At one time the reindeer, the mammoth, and the glutton penetrate down
to the Mediterranean, in the next phase the elephant and hippopotamus
again advance nearly to Central Europe. It is impossible here to attempt
to unravel these successive changes and migrations. Great numbers
of species were destroyed, and at length, when the climatic condition
of the earth reached a state of comparative stability, the surviving
animals settled in the geographical regions in which we find them
to-day.
The only question into which we may enter with any fullness is that
of the relation of human development to this grave perturbation of
the condition of the globe. The problem is sometimes wrongly conceived.
The chief point to be determined is not whether man did or did not
precede the Ice-Age. As it is the general belief that he was evolved
in the Tertiary, it is clear that he existed in some part of the earth
before the Ice-Age. Whether he had already penetrated as far north
as Britain and Belgium is an interesting point, but not one of great
importance. We may, therefore, refrain from discussing at any length
those disputed crude stone implements (Eoliths) which, in the opinion
of many, prove his presence in northern regions before the close of
the Tertiary. We may also now disregard the remains of the Java Ape-Man.
There are authorities, such as Deniker, who hold that even the latest
research shows these remains to be Pliocene, but it is disputed. The
Java race may be a surviving remnant of an earlier phase of human
evolution.
The most interesting subject for inquiry is the fortune of our human
and prehuman forerunners during the Pliocene and Pleistocene periods.
It may seem that if we set aside the disputable evidence of the Eoliths
and the Java remains we can say nothing whatever on this subject.
In reality a fact of very great interest can be established. It can
be shown that the progress made during this enormous lapse of time--at
least a million years--was remarkably slow. Instead of supposing that
some extraordinary evolution took place in that conveniently obscure
past, to which we can find no parallel within known times, it is precisely
the reverse. The advance that has taken place within the historical
period is far greater, comparatively to the span of time, than that
which took place in the past.
To make this interesting fact clearer we must attempt to measure the
progress made in the Pliocene and Pleistocene. We may assume that
the precursor of man had arrived at the anthropoid-ape level by the
middle of the Miocene period. He is not at all likely to have been
behind the anthropoid apes, and we saw that they were well developed
in the mid-Tertiary. Now we have a good knowledge of man as he was
in the later stage of the Ice-Age--at least a million years later--and
may thus institute a useful comparison and form some idea of the advance
made.
In the later stages of the Pleistocene a race of men lived in Europe
of whom we have a number of skulls and skeletons, besides vast numbers
of stone implements. It is usually known as the Neanderthal race,
as the first skeleton was found, in 1856, at Neanderthal, near Dusseldorf.
Further skeletons were found at Spy, in Belgium, and Krapina, in Croatia.
A skull formerly found at Gibraltar is now assigned to the same race.
In the last five years a jaw of the same (or an earlier) age has been
found at Mauer, near Heidelberg, and several skeletons have been found
in France (La Vezere and Chapelle-aux-Saints). From these, and a few
earlier fragments, we have a confident knowledge of the features of
this early human race.
The highest appreciation of the Neanderthal man--a somewhat flattering
appreciation, as we shall see--is that he had reached the level of
the Australian black of to-day. The massive frontal ridges over his
eyes, the very low, retreating forehead, the throwing of the mass
of the brain toward the back of the head, the outthrust of the teeth
and jaws, and the complete absence (in some cases) or very slight
development of the chin, combine to
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