Ernst Heinrich Philipp August Haeckel.

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in their inner structure without being able to explain it. We are now
in a position to explain the causes of this, by showing that this
remarkable agreement is the necessary consequence of the inheriting of
common stem-forms; while the striking difference in outward appearance
is a result of adaptation to changes of environment. Heredity and
adaptation alone furnish the true explanation.

But one special part of comparative anatomy is of supreme interest and
of the utmost philosophic importance in this connection. This is the
science of rudimentary or useless organs; I have given it the name of
"dysteleology" in view of its philosophic consequences. Nearly every
organism (apart from the very lowest), and especially every
highly-developed animal or plant, including man, has one or more
organs which are of no use to the body itself, and have no share in
its functions or vital aims. Thus we all have, in various parts of our
frame, muscles which we never use, as, for instance, in the shell of
the ear and adjoining parts. In most of the mammals, especially those
with pointed ears, these internal and external ear-muscles are of
great service in altering the shell of the ear, so as to catch the
waves of sound as much as possible. But in the case of man and other
short-eared mammals these muscles are useless, though they are still
present. Our ancestors having long abandoned the use of them, we
cannot work them at all to-day. In the inner corner of the eye we have
a small crescent-shaped fold of skin; this is the last relic of a
third inner eye-lid, called the nictitating (winking) membrane. This
membrane is highly developed and of great service in some of our
distant relations, such as fishes of the shark type and several other
vertebrates; in us it is shrunken and useless. In the intestines we
have a process that is not only quite useless, but may be very
harmful - the vermiform appendage. This small intestinal appendage is
often the cause of a fatal illness. If a cherry-stone or other hard
body is unfortunately squeezed through its narrow aperture during
digestion, a violent inflammation is set up, and often proves fatal.
This appendix has no use whatever now in our frame; it is a dangerous
relic of an organ that was much larger and was of great service in our
vegetarian ancestors. It is still large and important in many
vegetarian animals, such as apes and rodents.

There are similar rudimentary organs in all parts of our body, and in
all the higher animals. They are among the most interesting phenomena
to which comparative anatomy introduces us; partly because they
furnish one of the clearest proofs of evolution, and partly because
they most strikingly refute the teleology of certain philosophers. The
theory of evolution enables us to give a very simple explanation of
these phenomena.

We have to look on them as organs which have fallen into disuse in the
course of many generations. With the decrease in the use of its
function, the organ itself shrivels up gradually, and finally
disappears. There is no other way of explaining rudimentary organs.
Hence they are also of great interest in philosophy; they show clearly
that the monistic or mechanical view of the organism is the only
correct one, and that the dualistic or teleological conception is
wrong. The ancient legend of the direct creation of man according to a
pre-conceived plan and the empty phrases about "design" in the
organism are completely shattered by them. It would be difficult to
conceive a more thorough refutation of teleology than is furnished by
the fact that all the higher animals have these rudimentary organs.

The theory of evolution finds its broadest inductive foundation in the
natural classification of living things, which arranges all the
various forms in larger and smaller groups, according to their degree
of affinity. These groupings or categories of classification - the
varieties, species, genera, families, orders, classes, etc. - show such
constant features of coordination and subordination that we are bound
to look on them as genealogical, and represent the whole system in the
form of a branching tree. This is the genealogical tree of the
variously related groups; their likeness in form is the expression of
a real affinity. As it is impossible to explain in any other way the
natural tree-like form of the system of organisms, we must regard it
at once as a weighty proof of the truth of evolution. The careful
construction of these genealogical trees is, therefore, not an
amusement, but the chief task of modern classification.

Among the chief phenomena that bear witness to the inductive law of
evolution we have the geographical distribution of the various species
of animals and plants over the surface of the earth, and their
topographical distribution on the summits of mountains and in the
depths of the ocean. The scientific study of these features - the
"science of distribution," or chorology (chora = a place) - has been
pursued with lively interest since the discoveries made by Alexander
von Humboldt. Until Darwin's time the work was confined to the
determination of the facts of the science, and chiefly aimed at
settling the spheres of distribution of the existing large and small
groups of living things. It was impossible at that time to explain the
causes of this remarkable distribution, or the reasons why one group
is found only in one locality and another in a different place, and
why there is this manifold distribution at all. Here, again, the
theory of evolution has given us the solution of the problem. It
furnishes the only possible explanation when it teaches that the
various species and groups of species descend from common stem-forms,
whose ever-branching offspring have gradually spread themselves by
migration over the earth. For each group of species we must admit a
"centre of production," or common home; this is the original habitat
in which the ancestral form was developed, and from which its
descendants spread out in every direction. Several of these
descendants became in their turn the stem-forms for new groups of
species, and these also scattered themselves by active and passive
migration, and so on. As each migrating organism found a different
environment in its new home, and adapted itself to it, it was
modified, and gave rise to new forms.

This very important branch of science that deals with active and
passive migration was founded by Darwin, with the aid of the theory of
evolution; and at the same time he advanced the true explanation of
the remarkable relation or similarity of the living population in any
locality to the fossil forms found in it. Moritz Wagner very ably
developed his idea under the title of "the theory of migration." In my
opinion, this famous traveller has rather over-estimated the value of
his theory of migration when he takes it to be an indispensable
condition of the formation of new species and opposes the theory of
selection. The two theories are not opposed in their main features.
Migration (by which the stem-form of a new species is isolated) is
really only a special case of selection. The striking and interesting
facts of chorology can be explained only by the theory of evolution,
and therefore we must count them among the most important of its
inductive bases.

The same must be said of all the remarkable phenomena which we
perceive in the economy of the living organism. The many and various
relations of plants and animals to each other and to their
environment, which are treated in bionomy (from nomos, law or norm,
and bios, life), the interesting facts of parasitism, domesticity,
care of the young, social habits, etc., can only be explained by the
action of heredity and adaptation. Formerly people saw only the
guidance of a beneficent Providence in these phenomena; to-day we
discover in them admirable proofs of the theory of evolution. It is
impossible to understand them except in the light of this theory and
the struggle for life.

Finally, we must, in my opinion, count among the chief inductive bases
of the theory of evolution the foetal development of the individual
organism, the whole science of embryology or ontogeny. But as the
later chapters will deal with this in detail, I need say nothing
further here. I shall endeavour in the following pages to show, step
by step, how the whole of the embryonic phenomena form a massive chain
of proof for the theory of evolution; for they can be explained in no
other way. In thus appealing to the close causal connection between
ontogenesis and phylogenesis, and taking our stand throughout on the
biogenetic law, we shall be able to prove, stage by stage, from the
facts of embryology, the evolution of man from the lower animals.

The general adoption of the theory of evolution has definitely closed
the controversy as to the nature or definition of the species. The
word has no ABSOLUTE meaning whatever, but is only a group-name, or
category of classification, with a purely relative value. In 1857, it
is true, a famous and gifted, but inaccurate and dogmatic, scientist,
Louis Agassiz, attempted to give an absolute value to these
"categories of classification." He did this in his Essay on
Classification, in which he turns upside down the phenomena of organic
nature, and, instead of tracing them to their natural causes, examines
them through a theological prism. The true species (bona species) was,
he said, an "incarnate idea of the Creator." Unfortunately, this
pretty phrase has no more scientific value than all the other attempts
to save the absolute or intrinsic value of the species.

The dogma of the fixity and creation of species lost its last great
champion when Agassiz died in 1873. The opposite theory, that all the
different species descend from common stem-forms, encounters no
serious difficulty to-day. All the endless research into the nature of
the species, and the possibility of several species descending from a
common ancestor, has been closed to-day by the removal of the sharp
limits that had been set up between species and varieties on the one
hand, and species and genera on the other. I gave an analytic proof of
this in my monograph on the sponges (1872), having made a very close
study of variability in this small but highly instructive group, and
shown the impossibility of making any dogmatic distinction of species.
According as the classifier takes his ideas of genus, species, and
variety in a broader or in a narrower sense, he will find in the small
group of the sponges either one genus with three species, or three
genera with 238 species, or 113 genera with 591 species. Moreover, all
these forms are so connected by intermediate forms that we can
convincingly prove the descent of all the sponges from a common
stem-form, the olynthus.

Here, I think, I have given an analytic solution of the problem of the
origin of species, and so met the demand of certain opponents of
evolution for an actual instance of descent from a stem-form. Those
who are not satisfied with the synthetic proofs of the theory of
evolution which are provided by comparative anatomy, embryology,
paleontology, dysteleology, chorology, and classification, may try to
refute the analytic proof given in my treatise on the sponge, the
outcome of five years of assiduous study. I repeat: It is now
impossible to oppose evolution on the ground that we have no
convincing example of the descent of all the species of a group from a
common ancestor. The monograph on the sponges furnishes such a proof,
and, in my opinion, an indisputable proof. Any man of science who will
follow the protracted steps of my inquiry and test my assertions will
find that in the case of the sponges we can follow the actual
evolution of species in a concrete case. And if this is so, if we can
show the origin of all the species from a common form in one single
class, we have the solution of the problem of man's origin, because we
are in a position to prove clearly his descent from the lower animals.

At the same time, we can now reply to the often-repeated assertion,
even heard from scientists of our own day, that the descent of man
from the lower animals, and proximately from the apes, still needs to
be "proved with certainty." These "certain proofs" have been available
for a long time; one has only to open one's eyes to see them. It is a
mistake to seek them in the discovery of intermediate forms between
man and the ape, or the conversion of an ape into a human being by
skilful education. The proofs lie in the great mass of empirical
material we have already collected. They are furnished in the
strongest form by the data of comparative anatomy and embryology,
completed by paleontology. It is not a question now of detecting new
proofs of the evolution of man, but of examining and understanding the
proofs we already have.

I was almost alone thirty-six years ago when I made the first attempt,
in my General Morphology, to put organic science on a mechanical
foundation through Darwin's theory of descent. The association of
ontogeny and phylogeny and the proof of the intimate causal connection
between these two sections of the science of evolution, which I
expounded in my work, met with the most spirited opposition on nearly
all sides. The next ten years were a terrible "struggle for life" for
the new theory. But for the last twenty-five years the tables have
been turned. The phylogenetic method has met with so general a
reception, and found so prolific a use in every branch of biology,
that it seems superfluous to treat any further here of its validity
and results. The proof of it lies in the whole morphological
literature of the last three decades. But no other science has been so
profoundly modified in its leading thoughts by this adoption, and been
forced to yield such far-reaching consequences, as that science which
I am now seeking to establish - monistic anthropogeny.

This statement may seem to be rather audacious, since the very next
branch of biology, anthropology in the stricter sense, makes very
little use of these results of anthropogeny, and sometimes expressly
opposes them.* (*This does not apply to English anthropologists, who
are almost all evolutionists.) This applies especially to the attitude
which has characterised the German Anthropological Society (the
Deutsche Gesellschaft fur Anthropologie) for some thirty years. Its
powerful president, the famous pathologist, Rudolph Virchow, is
chiefly responsible for this. Until his death (September 5th, 1902) he
never ceased to reject the theory of descent as unproven, and to
ridicule its chief consequence - the descent of man from a series of
mammal ancestors - as a fantastic dream. I need only recall his
well-known expression at the Anthropological Congress at Vienna in
1894, that "it would be just as well to say man came from the sheep or
the elephant as from the ape."

Virchow's assistant, the secretary of the German Anthropological
Society, Professor Johannes Ranke of Munich, has also indefatigably
opposed transformism: he has succeeded in writing a work in two
volumes (Der Mensch), in which all the facts relating to his
organisation are explained in a sense hostile to evolution. This work
has had a wide circulation, owing to its admirable illustrations and
its able treatment of the most interesting facts of anatomy and
physiology - exclusive of the sexual organs! But, as it has done a
great deal to spread erroneous views among the general public, I have
included a criticism of it in my History of Creation, as well as met
Virchow's attacks on anthropogeny.

Neither Virchow, nor Ranke, nor any other "exact" anthropologist, has
attempted to give any other natural explanation of the origin of man.
They have either set completely aside this "question of questions" as
a transcendental problem, or they have appealed to religion for its
solution. We have to show that this rejection of the rational
explanation is totally without justification. The fund of knowledge
which has accumulated in the progress of biology in the nineteenth
century is quite adequate to furnish a rational explanation, and to
establish the theory of the evolution of man on the solid facts of his


In order to understand clearly the course of human embryology, we must
select the more important of its wonderful and manifold processes for
fuller explanation, and then proceed from these to the innumerable
features of less importance. The most important feature in this sense,
and the best starting-point for ontogenetic study, is the fact that
man is developed from an ovum, and that this ovum is a simple cell.
The human ovum does not materially differ in form and composition from
that of the other mammals, whereas there is a distinct difference
between the fertilised ovum of the mammal and that of any other

(FIGURE 1.1. The human ovum, magnified 100 times. The globular mass of
yelk (b) is enclosed by a transparent membrane (the ovolemma or zona
pellucida [a]), and contains a noncentral nucleus (the germinal
vesicle, c). Cf. Figure 1.14.)

This fact is so important that few should be unaware of its extreme
significance; yet it was quite unknown in the first quarter of the
nineteenth century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to
that time the larger vesicles, in which the real and much smaller ovum
is contained, had been wrongly regarded as ova. The important
circumstance that this mammal ovum is a simple cell, like the ovum of
other animals, could not, of course, be recognised until the cell
theory was established. This was not done, by Schleiden for the plant
and Schwann for the animal, until 1838. As we have seen, this cell
theory is of the greatest service in explaining the human frame and
its embryonic development. Hence we must say a few words about the
actual condition of the theory and the significance of the views it
has suggested.

In order properly to appreciate the cellular theory, the most
important element in our science, it is necessary to understand in the
first place that the cell is a UNIFIED ORGANISM, a self-contained
living being. When we anatomically dissect the fully-formed animal or
plant into its various organs, and then examine the finer structure of
these organs with the microscope, we are surprised to find that all
these different parts are ultimately made up of the same structural
element or unit. This common unit of structure is the cell. It does
not matter whether we thus dissect a leaf, flower, or fruit, or a
bone, muscle, gland, or bit of skin, etc.; we find in every case the
same ultimate constituent, which has been called the cell since
Schleiden's discovery. There are many opinions as to its real nature,
but the essential point in our view of the cell is to look upon it as
a self-contained or independent living unit. It is, in the words of
Brucke, "an elementary organism." We may define it most precisely as
the ultimate organic unit, and, as the cells are the sole active
principles in every vital function, we may call them the "plastids,"
or "formative elements." This unity is found in both the anatomic
structure and the physiological function. In the case of the protists,
the entire organism usually consists of a single independent cell
throughout life. But in the tissue-forming animals and plants, which
are the great majority, the organism begins its career as a simple
cell, and then grows into a cell-community, or, more correctly, an
organised cell-state. Our own body is not really the simple unity that
it is generally supposed to be. On the contrary, it is a very
elaborate social system of countless microscopic organisms, a colony
or commonwealth, made up of innumerable independent units, or very
different tissue-cells.

In reality, the term "cell," which existed long before the cell theory
was formulated, is not happily chosen. Schleiden, who first brought it
into scientific use in the sense of the cell theory, gave this name to
the elementary organisms because, when you find them in the dissected
plant, they generally have the appearance of chambers, like the cells
in a bee-hive, with firm walls and a fluid or pulpy content. But some
cells, especially young ones, are entirely without the enveloping
membrane, or stiff wall. Hence we now generally describe the cell as a
living, viscous particle of protoplasm, enclosing a firmer nucleus in
its albuminoid body. There may be an enclosing membrane, as there
actually is in the case of most of the plants; but it may be wholly
lacking, as is the case with most of the animals. There is no membrane
at all in the first stage. The young cells are usually round, but they
vary much in shape later on. Illustrations of this will be found in
the cells of the various parts of the body shown in Figures 1.3 to

Hence the essential point in the modern idea of the cell is that it is
made up of two different active constituents - an inner and an outer
part. The smaller and inner part is the nucleus (or caryon or
cytoblastus, Figure 1.1 c and Figure 1.2 k). The outer and larger
part, which encloses the other, is the body of the cell (celleus,
cytos, or cytosoma). The soft living substance of which the two are
composed has a peculiar chemical composition, and belongs to the group
of the albuminoid plasma-substances ("formative matter"), or
protoplasm. The essential and indispensable element of the nucleus is
called nuclein (or caryoplasm); that of the cell body is called
plastin (or cytoplasm). In the most rudimentary cases both substances
seem to be quite simple and homogeneous, without any visible
structure. But, as a rule, when we examine them under a high power of
the microscope, we find a certain structure in the protoplasm. The
chief and most common form of this is the fibrous or net-like "thready
structure" (Frommann) and the frothy "honeycomb structure" (Butschli).

(FIGURE 1.2. Stem-cell of one of the echinoderms (cytula, or "first
segmentation-cell" = fertilised ovum), after Hertwig. k is the nucleus
or caryon.)

The shape or outer form of the cell is infinitely varied, in
accordance with its endless power of adapting itself to the most
diverse activities or environments. In its simplest form the cell is
globular (Figure 1.2). This normal round form is especially found in
cells of the simplest construction, and those that are developed in a
free fluid without any external pressure. In such cases the nucleus
also is not infrequently round, and located in the centre of the
cell-body (Figure 1.2 k). In other cases, the cells have no definite
shape; they are constantly changing their form owing to their
automatic movements. This is the case with the amoebae (Figures 1.15
and 1.16) and the amoeboid travelling cells (Figure 1.11), and also
with very young ova (Figure 1.13). However, as a rule, the cell
assumes a definite form in the course of its career. In the tissues of
the multicellular organism, in which a number of similar cells are
bound together in virtue of certain laws of heredity, the shape is
determined partly by the form of their connection and partly by their
special functions. Thus, for instance, we find in the mucous lining of
our tongue very thin and delicate flat cells of roundish shape (Figure
1.3). In the outer skin we find similar, but harder, covering cells,
joined together by saw-like edges (Figure 1.4). In the liver and other
glands there are thicker and softer cells, linked together in rows
(Figure 1.5).

The last-named tissues (Figures 1.3 to 1.5) belong to the simplest and
most primitive type, the group of the "covering-tissues," or
epithelia. In these "primary tissues" (to which the germinal layers
belong) simple cells of the same kind are arranged in layers. The
arrangement and shape are more complicated in the "secondary tissues,"
which are gradually developed out of the primary, as in the tissues of
the muscles, nerves, bones, etc. In the bones, for instance, which
belong to the group of supporting or connecting organs, the cells
(Figure 1.6) are star-shaped, and are joined together by numbers of
net-like interlacing processes; so, also, in the tissues of the teeth
(Figure 1.7), and in other forms of supporting-tissue, in which a soft
or hard substance (intercellular matter, or base) is inserted between
the cells.

(FIGURE 1.3. Three epithelial cells from the mucous lining of the

FIGURE 1.4. Five spiny or grooved cells, with edges joined, from the
outer skin (epidermis): one of them (b) is isolated.

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