Ernst Haeckel, who was born in Potsdam, Germany, Feb. 16, 1834,
descends from a long line of lawyers and politicians. To his
father's annoyance, he turned to science, and graduated in
medicine. After a long tour in Italy in 1859, during which he
wavered between art and science, he decided for zoology, and made a
masterly study of a little-known group of sea-animalcules, the
Radiolaria. In 1861 he began to teach zoology at Jena University.
Darwin's "Origin of Species" had just been translated into German,
and he took up the defence of Darwinism against almost the whole of
his colleagues. His first large work on evolution, "General
Morphology," was published in 1866. He has since published
forty-two distinct works. He is not only a master of zoology, but
has a good command of botany and embryology. Haeckel's "Evolution
of Man" (Anthropogenie), is generally accepted as being his most
important production. Published in 1874, at a time when the theory
of natural evolution had few supporters in Germany, the work was
hailed with a storm of controversy, one celebrated critic declaring
that it was a blot on the escutcheon of Germany. From the hands of
English scientists, however, the treatise received a warm welcome.
Darwin said he would probably never have written his "Descent of
Man" had Haeckel published his work earlier.
_I.--The Science of Man_
The natural history of mankind, or anthropology, must always excite the
most lively interest, and no part of the science is more attractive than
that which deals with the question of man's origin. In order to study
this with full profit, we must combine the results of two sciences,
ontogeny (or embryology) and phylogeny (the science of evolution). We do
this because we have now discovered that the forms through which the
embryo passes in its development correspond roughly to the series of
forms in its ancestral development. The correspondence is by no means
complete or precise, since the embryonic life itself has been modified
in the course of time; but the general law is now very widely accepted.
I have called it "the biogenetic law," and will constantly appeal to it
in the course of this study.
It is only in recent times that the two sciences have advanced
sufficiently to reveal the correspondence of the two series of forms.
Aristotle provided a good foundation for embryology, and made some
interesting discoveries, but no progress was made in the science for
2,000 years after him. Then the Reformation brought some liberty of
research, and in the seventeenth century several works were written on
embryology.
For more than a hundred years the science was still hampered by the lack
of good microscopes. It was generally believed that all the organs of
the body existed, packed in a tiny point of space, in the germ. About
the middle of the eighteenth century, Caspar Friedrich Wolff discovered
the true development; but his work was ignored, and it was only fifty
years later that modern embryology began to work on the right line. K.E.
von Baer made it clear that the fertilised ovum divides into a group of
cells, and that the various organs of the body are developed from these
layers of cells, in the way I shall presently describe.
The science of phylogeny, or, as it is popularly called, the evolution
of species, had an equally slow growth. On the ground of the Mosaic
narrative, no less than in view of the actual appearance of the living
world, the great naturalist Linné (1735) set up the dogma of the
unchangeability of species. Even when quite different remains of animals
were discovered by the advancing science of geology, they were forced
into the existing narrow framework of science by Cuvier. Sir Charles
Lyell completely undid the fallacious work of Cuvier, but in the
meantime the zoologists themselves were moving toward the doctrine of
evolution.
Jean Lamarck made the first systematic attempt to expound the theory in
his "Zoological Philosophy" (1809). He suggested that animals modified
their organs by use or disuse, and that the effect of this was
inherited. In the course of time these inherited modifications reached
such a pitch that the organism fell into a new "species." Goethe also
made some remarkable contributions to the science of evolution. But it
was reserved for Charles Darwin to win an enduring place in science for
the theory. "The Origin of Species" (1859) not only sustained it with a
wealth of positive knowledge which Lamarck did not command, but it
provided a more luminous explanation in the doctrine of natural
selection. Huxley (1863) followed with an application of the law to man,
and in 1866 I gave a comprehensive sketch of its application throughout
the whole animal world. In 1874 I published the first edition of the
present work.
The doctrine of evolution is now a vital part of biology, and we might
accept the evolution of man as a special deduction from the general law.
Three great groups of evidence impose that law on us. The first group
consists of the facts of palæontology, or the fossil record of past
animal life. Imperfect as the record is, it shows us a broad divergence
of successively changing types from a simple common root, and in some
cases exhibits the complete transition from one type to another. The
next document is the evidence of comparative anatomy. This science
groups the forms of living animals in such a way that we seem to have
the same gradual divergence of types from simple common ancestors. In
particular, it discovers certain rudimentary organs in the higher
animals, which can only be understood as the shrunken relics of organs
that were once useful to a remote ancestor. Thus, man has still the
rudiment of the third eyelid of his shark-ancestor. The third document
is the evidence of embryology, which shows us the higher organism
substantially reproducing, in its embryonic development, the long
series of ancestral forms.
_II.--Man's Embryonic Development_
The first stage in the development of any animal is the tiny speck of
plasm, hardly visible to the naked eye, which we call the ovum, or
egg-cell. It is a single cell, recalling the earliest single-celled
ancestor of all animals. In its immature form it is not unlike certain
microscopic animalcules known as _amoeboe_. In its mature form it is
about 1/125th of an inch in diameter.
When the male germ has blended with the female in the ovum, the new cell
slowly divides into two, with a very complicated division of the
material composing its nucleus. The two cells divide into four, the four
into eight, and so on until we have a round cluster of cells, something
like a blackberry in shape.
This _morula_, as I have called it, reproduces the next stage in the
development of life. As all animals pass through it, our biogenetic law
forces us to see in it an ancestral stage; and in point of fact we have
animals of this type living in Nature to-day. The round cluster becomes
filled with fluid, and we have a hollow sphere of cells, which I call
the _blastula_. The corresponding early ancestor I name the _Blastæa_,
and again we find examples of it, like the _Volvox_ of the ponds, in
Nature to-day.
The next step is very important. The hollow sphere closes in on itself,
as when an india rubber ball is pressed into the form of a cup. We have
then a vase-shaped body with two layers of cells, an inner and an outer,
and an opening. The inner layer we call the entoderm, the outer the
ectoderm; and the "primitive mouth" is known as the blastopore. In the
higher animals a good deal of food-yolk is stored up in the germ, and so
the vase-shaped structure has been flattened and altered. It has,
however, been shown that all embryos pass through this stage
(gastrulation), and we again infer the existence of a common ancestor of
that type--the _Gastræa_. The lowest group of many-celled animals--the
corals, jelly-fishes, and anemones--are essentially of that structure.
The embryo now consists of two layers of cells, the "germ-layers," an
inner and outer. As the higher embryo develops, a third layer of cells
now pushes between the two. We may say, broadly, that from this middle
layer are developed most of the animal organs of the body; from the
internal germ-layer is developed the lining of the alimentary canal and
its dependent glands; from the outer layer are formed the skin and the
nervous system--which developed originally in the skin.
The embryo of man and all the other higher animals now develops a
cavity, a pair of pouches, by the folding of the layer at the primitive
mouth. Sir E. Ray Lankester, and Professor Balfour, and other students,
traced this formation through the whole embryonic world, and we are
therefore again obliged to see in it a reminiscence of an ancestral
form--a primitive worm-like animal, of a type we shall see later. The
next step is the formation of the first trace of what will ultimately be
the backbone. It consists at first of a membraneous tube, formed by the
folding of the inner layer along the axis of the embryo-body. Later this
tube will become cartilage, and in the higher animals the cartilage will
give place to bone.
The other organs of the body now gradually form from the germ-layers,
principally by the folding of the layers into tubes. A light area
appears on the surface of the germ. A streak or groove forms along its
axis, and becomes the nerve-cord running along the back. Cube-shaped
structures make their appearance on either side of it; these prove to be
the rudiments of the vertebræ--or separate bones of the backbone--and
gradually close round the cord. The heart is at first merely a
spindle-shaped enlargement of the main ventral blood-vessel. The nose is
at first only a pair of depressions in the skin above the mouth.
When the human embryo is only a quarter of an inch in length, it has
gill-clefts and gill-arches in the throat like a fish, and no limbs. The
heart--as yet with only the simple two-chambered structure of a fish's
heart--is up in the throat--as in the fish--and the principal arteries
run to the gill-slits. These structures never have any utility in man or
the other land-animals, though the embryo always has them for a time.
They point clearly to a fish ancestor.
Later, they break up, the limbs sprout out like blunt fins at the sides,
and the long tail begins to decrease. By the twelfth week the human
frame is perfectly formed, though less than two inches long. Last of
all, it retains its resemblance to the ape. In the embryonic apparatus,
too, man closely resembles the higher ape.
_III.--Our Ancestral Tree_
The series of forms which we thus trace in man's embryonic development
corresponds to the ancestral series which we would assign to man on the
evidence of palæontology and comparative anatomy. At one time, the
tracing of this ancestral series encountered a very serious check. When
we examined the groups of living animals, we found none that illustrated
or explained the passage from the non-backboned--invertebrate--to the
backboned--vertebrate--animals. This gap was filled some years ago by
the discovery of the lancelet--_Amphioxus_--and the young of the
sea-squirt--_Ascidia_. The lancelet has a slender rod of cartilage along
its back, and corresponds very closely with the ideal I have sketched of
our primitive backboned ancestor. It may be an offshoot from the same
group. The sea-squirt further illustrates the origin of the backbone,
since it has a similar rod of cartilage in its youth, and loses it, by
degeneration, in its maturity.
In this way the chief difficulty was overcome, and it was possible to
sketch the probable series of our ancestors. It must be well understood
that not only is the whole series conjectural, but no living animal must
be regarded as an ancestral form. The parental types have long been
extinct, and we may, at the most, use very conservative living types to
illustrate their nature, just as, in the matter of languages, German is
not the parent, but the cousin of Anglo-Saxon, or Greek of Latin. The
original parental languages are lost. But a language like Sanscrit
survives to give us a good idea of the type.
The law of evolution is based on such a mass of evidence that we may
justly draw deductions from it, where the direct evidence is incomplete.
This is especially necessary in the early part of our ancestral tree,
because the fossil record quite fails us. For millions of years the
early soft-bodied animals left no trace in the primitive mud, which time
has hardened into rocks, and we are restricted to the evidence of
embryology and of comparative zoology. This suffices to give us a
general idea of the line of development.
In nature to-day, one of the lowest animal forms is a tiny speck of
living plasm called the _amoeba_. We have still more elementary forms,
such as the minute particles which make up the bluish film on damp
rocks, but they are of a vegetal character, or below it. They give us
some idea of the very earliest forms of life; minute living particles,
with no organs, down to the ten-thousandth part of an inch in diameter.
The amoeba represents the lowest animal, and, as we saw, the ovum in
many cases resembles an amoeba. We therefore take some such one-celled
creature as our first animal ancestor. Taking food in at all parts of
its surface, having no permanent organs of locomotion, and reproducing
by merely splitting into two, it exhibits the lowest level of animal
life.
The next step in development would be the clustering together of these
primitive microbes as they divided. This is actually the stage that
comes next in the development of the germ, and it is the next stage
upward in the existing animal world. We assume that these clusters of
microbes--or cells, as we will now call them--bent inward, as we saw the
embryo do, and became two-layered, cup-shaped organisms, with a hollow
interior (primitive stomach) and an aperture (primitive mouth). The
inner cells now do the work of digestion alone; the outer cells effect
locomotion, by means of lashes like oars, and are sensitive. This is, in
the main, the structure of the next great group of animals, the hydra,
coral, meduca, and anemone. They have remained at this level, though
they have developed, special organs for stinging their prey and bringing
the food into their mouths.
Both zoology and the appearance of the embryo point to a worm-like
animal as the next stage. Constant swimming in the water would give the
animal a definite head, with special groups of nerve-cells, a definite
tail, and a two-sided or evenly-balanced body.
We mean that those animals would be fittest to live, and multiply most,
which developed this organisation. Sense-organs would now appear in the
head, in the form of simple depressions, lined with sensitive cells, as
they do in the embryo; and a clump of nerve-cells within would represent
the primitive brain. In the vast and varied worm-group we find
illustrations of nearly every step in this process of evolution.
The highest type of worm-like creature, the acorn-headed
worm--_Balanoglossus_--takes us an important step further. It has
gill-openings for breathing, and a cord of cartilage down its back. We
saw that the human embryo has a gill-apparatus, and that, comparing the
lancelet and the sea-squirt, the backbone must have begun as a string of
cartilage-cells. We are now on firmer ground, for there is no doubt that
all the higher land-animals come from a fish ancestor. The shark, one of
the most primitive of fishes in organisation, probably best suggests
this ancestor to us. In fact, in the embryonic development of the human
face there is a clear suggestion of the shark.
Up to this period the story of evolution had run its course in the sea.
The area of dry land was now increasing, and certain of the primitive
fishes adapted themselves to living on land. They walked on their fins,
and used their floating-bladders--large air-bladders in the fish, for
rising in the water--to breathe air. We not only have fishes of this
type in Australia to-day, but we have the fossil remains of similar
fishes in the Old Red Sandstone rocks. From mud-fish the amphibian would
naturally develop, as it did in the coal-forest period. Walking on the
fins would strengthen the main stem, the broad paddle would become
useless, and we should get in time the bony five-toed limb. We have many
of these giant salamander forms in the rocks.
The reptile now evolved from the amphibian, and a vast reptile
population spread over the earth. From one of these early reptiles the
birds were evolved. Geology furnishes the missing link between the bird
and the reptile in the _Archæopteryx_, a bird with teeth, claws on its
wings, and a reptilian tail. From another primitive reptile the
important group of the mammals was evolved. We find what seem to be the
transitional types in the rocks of South Africa. The scales gave way to
tufts of hair, the heart evolved a fourth chamber, and thus supplied
purer blood (warm blood), the brain profited by the richer food, and the
mother began to suckle the young. We have still a primitive mammal of
this type in the duck-mole, or duck-billed platypus (_Ornithorhyncus_)
of Australia. There are grounds for thinking that the next stage was an
opossum-like animal, and this led on to the lowest ape-like being, the
lemur. Judging from the fossil remains, the black lemur of Madagascar
best suggests this ancestor.
The apes of the Old and New Worlds now diverged from this level, and
some branch of the former gave rise to the man-like apes and man. In
bodily structure and embryonic development the large apes come very
close to man, and two recent discoveries have put their
blood-relationship beyond question. One is that experiments in the
transfusion of blood show that the blood of the man-like ape and man
have the same action on the blood of lower animals. The other is that we
have discovered, in Java, several bones of a being which stands just
midway between the highest living ape and lowest living race of men.
This ape-man (_Pithecanthropus_) represents the last of our animal and
first of our human ancestors.
_IV.--Evolution of Separate Organs_
So far, we have seen how the human body as a whole develops through a
long series of extinct ancestors. We may now take the various systems of
organs one by one, and, if we are careful to consult embryology as well
as zoology, we can trace the manner of their development. It is, in
accordance with our biogenetic law, the same in the embryo, as a rule,
as in the story of past evolution.
We take first the nervous system. In the lowest animals, as in the early
stages of the embryo, there are no nerve-cells. In the embryo the
nerve-cells develop from the outer, or skin layer, of cells. This,
though strange as regards the human nervous system, is a correct
preservation of the primitive seat of the nerves. It was the surface of
the animal that needed to be sensitive in the primitive organism. Later,
when definite connecting nerves were formed, only special points in the
surface, protected by coverings which did not interfere with the
sensitiveness, needed to be exposed, and the nerves transmitted the
impressions to the central brain.
This development is found in the animal world to-day. In such animals as
the hydra we find the first crude beginning of unorganised nerve-cells.
In the jelly-fish we find nerve-cells clustered into definite sensitive
organs. In the lower worms we have the beginning of organs of smell and
vision. They are at first merely blind, sensitive pits in the skin, as
in the embryo. The ear has a peculiar origin. Up to the fish level there
is no power of hearing. There is merely a little stone rolling in a
sensitive bed, to warn the animal of its movement from side to side. In
the higher animals this evolves into the ear.
The glands of the skin (sweat, fat, tears, etc.) appear at first as
blunt, simple ingrowths. The hair first appears in tufts, representing
the scales, from underneath which they were probably evolved. The thin
coat of hair on the human body to-day is an ancestral inheritance. This
is well shown by the direction of the hairs on the arm. As on the ape's
arm, both on the upper and lower arm, they grow toward the elbow. The
ape finds this useful in rain, using his arms like a thatched roof, and
on our arm this can only be a reminiscence of the habits of an ape
ancestor.
We have seen how the spinal cord first appears as a tube in the axis of
the back, and the cartilaginous column closes round it. All bone appears
first as membrane, then cartilage, and finally ossifies. This is the
order both in past evolution and in present embryonic development. The
brain is at first a bulbous expansion of the spinal nerve-cord. It is at
first simple, but gradually, both in the scale of nature and in the
embryo, divides into five parts. One of these parts, the cerebrum, is
mainly connected with mental life. We find it increasing in size, in
proportion to the animal's intelligence, until in man it comes to cover
the whole of the brain. When we remove it from the head of the mammal,
without killing the animal, we find all mental life suspended, and the
whole vitality used in vegetative functions.
In the evolution of the bony system we find the same correspondence of
embryology and evolution. The main column is at first a rod of
cartilage. In time the separate cubes appear which are to form the
vertebræ of the flexible column. The skull develops in the same way.
Just as the brain is a specially modified part of the nerve-rod, the
skull is only a modified part of the vertebral column. The bones that
compose it are modified vertebræ, as Goethe long ago suspected. The
skull of the shark gives us a hint of the way in which the modification
took place, and the formation of the skull in the embryo confirms it.
That adult man is devoid of that prolongation of the vertebral column
which we call a tail is not a distinctive peculiarity. The higher apes
are equally without it. We find, however, that the human embryo has a
long tail, much longer than the legs, when they are developing. At
times, moreover, children are born with tails--perfect tails, with
nerves and muscles, which they move briskly under emotion, and these
have to be amputated. The development of the limb from the fin offers no
serious difficulty to the osteologist. All the higher animals descend
from a five-toed ancestor. The whale has taken again to the water, and
reconverted its limb into a paddle. The bones of the front feet still
remain under the flesh. Animals of the horse type have had the central
toe strengthened, for running purposes, at the expense of the rest. The
serpent has lost its limbs from disuse, but in the python a rudimentary
limb-bone is still preserved.
The alimentary system, blood-vessel system, and reproductive system
have been evolved gradually in the same way. The stomach is at first the
whole cavity in the animal. Later it becomes a straight, simple tube,
strengthened by a gullet in front. The liver is an outgrowth from this
tube; the stomach proper is a bulbous expansion of its central part,
later provided with a valve. The kidneys are at first simple channels in
the skin for drainage, then closed tubes, which branch out more and
more, and then gather into our compact kidneys. We thus see that the
building up of the human body from a single cell is a substantial
epitome of the long story of evolution, which occupied many millions of
years. We find man bearing in his body to-day traces of organs which
were useful to a remote ancestor, but of no advantage, and often a
source of mischief to himself. We learn that the origin of man, instead
of being placed a few thousand years ago, must be traced back to the
point where, hundreds of thousands of years ago, he diverged from his
ape-cousins, though he retains to-day the plainest traces of that
relationship. Body and mind--for the development of mind follows with
the utmost precision on the development of brain--he is the culmination
of a long process of development. His spirit is a form of energy
inseparably bound up with the substance of his body. His evolution has
been controlled by the same "eternal, iron laws" as the development of
any other body--the laws of heredity and adaptation.