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it was no light task to trace the identity of this process of
gastrulation in all the animals. It has been done, however; and with
this introduction the reader will be able to follow the proof. The
conclusion is important. If all animals pass through the curious
gastrula stage, it must be because they all had a common ancestor of
that nature. To this conjectural ancestor (it lived before the period
of fossilisation begins) Haeckel gives the name of the Gastraea, and
in the second volume we shall see a number of living animals of this
type ("gastraeads").

The line of argument is the same in the next chapter. After laborious
and careful research (though this stage is not generally admitted in
the same sense as the previous one), a fourth common stage was
discovered, and given the name of the Coelomula. The blastula had one
layer of cells, the blastoderm (derma = skin): the gastrula two
layers, the ectoderm ("outer skin") and entoderm ("inner skin"). Now a
third layer (mesoderm = middle skin) is formed, by the growth inwards
of two pouches or folds of the skin. The pouches blend together, and
form a single cavity (the body cavity, or coelom), and its two walls
are two fresh "germinal layers." Again, the identity of the process
has to be proved in all the higher classes of animals, and when this
is done we have another ancestral stage, the Coelomaea.

The remaining task is to build up the complex frame of the higher
animals - always showing the identity of the process (on which the
evolutionary argument depends) in enormously different conditions of
embryonic life - out of the four "germinal layers." Chapter 1.9
prepares us for the work by giving us a very clear account of the
essential structure of the back-boned (vertebrate) animal, and the
probable common ancestor of all the vertebrates (a small fish of the
lancelet type). Chapters 1.11 to 1.14 then carry out the construction
step by step. The work is now simpler, in the sense that we leave all
the invertebrate animals out of account; but there are so many organs
to be fashioned out of the four simple layers that the reader must
proceed carefully. In the second volume each of these organs will be
dealt with separately, and the parallel will be worked out between its
embryonic and its phylogenetic (evolutionary) development. The general
reader may wait for this for a full understanding. But in the meantime
the wonderful story of the construction of all our organs in the
course of a few weeks (the human frame is perfectly formed, though
less than two inches in length, by the twelfth week) from so simple a
material is full of interest. It would be useless to attempt to
summarise the process. The four chapters are themselves but a summary
of it, and the eighty fine illustrations of the process will make it
sufficiently clear. The last chapter carries the story on to the point
where man at last parts company with the anthropoid ape, and gives a
full account of the membranes or wrappers that enfold him in the womb,
and the connection with the mother.

In conclusion, I would urge the reader to consult, at his free library
perhaps, the complete edition of this work, when he has read the
present abbreviated edition. Much of the text has had to be condensed
in order to bring out the work at our popular price, and the beautiful
plates of the complete edition have had to be omitted. The reader will
find it an immense assistance if he can consult the library edition.

JOSEPH MCCABE.

Cricklewood, March, 1906.

***


HAECKEL'S CLASSIFICATION OF THE ANIMAL WORLD.

UNICELLULAR ANIMALS (PROTOZOA).

1. Unnucleated.

Bacteria.
Protamoebae.

Monera.

2. Nucleated.

2A. Rhizopoda.

Amoebina.
Radiolaria.

2B. Infusoria.

Flagellata.
Ciliata.

3. Cell-Colonies.

Catallacta.
Blastaeada.

MULTICELLULAR ANIMALS (METAZOA).

1. COELENTERIA, COELENTERATA, OR ZOOPHYTES.
Animals without body-cavity, blood or anus.

1A. Gastraeads.

Gastremaria.
Cyemaria.

1B. Sponges.

Protospongiae.
Metaspongiae.

1C. Cnidaria (Stinging Animals).

Hydrozoa.
Polyps.
Medusae.

1D. Platodes (Flat-Worms).

Platodaria.
Turbellaria.
Trematoda.
Cestoda.

2. COELOMARIA OR BILATERALS.
Animals with body-cavity and anus, and generally blood.

2A. Vermalia (Worm-Like).

Rotatoria.
Strongylaria.
Prosopygia.
Frontonia.

2B. Molluscs.

Cochlides.
Conchades.
Teuthodes.

2C. Articulates.

Annelida.
Crustacea.
Tracheata.

2D. Echinoderms.

Monorchonia.
Pentorchonia.

2E. Tunicates.

Copelata.
Ascidiae.
Thalidiae.

2F. Vertebrates.

2F.1. Acrania-Lancelet (Without Skull).

2F.2. Craniota (With Skull).

2F.2A. Cyclostomes. ("Round-Mouthed").

2F.2B. Fishes.

Selachii.
Ganoids.
Teleosts.
Dipneusts.

2F.2C. Amphibia.

2F.2D. Reptiles.

2F.2E. Birds.

2F.2F. Mammal.

Monotremes.

Marsupials.

Placentals: -
Rodents.
Edentates.
Ungulates.
Cetacea.
Sirenia.
Insectivora.
Cheiroptera.
Carnassia.
Primates.

(This classification is given for the purpose of explaining Haeckel's
use of terms in this volume. The general reader should bear in mind
that it differs very considerably from more recent schemes of
classification. He should compare the scheme framed by Professor E.
Ray Lankester.)

***


THE EVOLUTION OF MAN.


CHAPTER 1.1. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION.

The field of natural phenomena into which I would introduce my readers
in the following chapters has a quite peculiar place in the broad
realm of scientific inquiry. There is no object of investigation that
touches man more closely, and the knowledge of which should be more
acceptable to him, than his own frame. But among all the various
branches of the natural history of mankind, or anthropology, the story
of his development by natural means must excite the most lively
interest. It gives us the key of the great world-riddles at which the
human mind has been working for thousands of years. The problem of the
nature of man, or the question of man's place in nature, and the
cognate inquiries as to the past, the earliest history, the present
situation, and the future of humanity - all these most important
questions are directly and intimately connected with that branch of
study which we call the science of the evolution of man, or, in one
word, "Anthropogeny" (the genesis of man). Yet it is an astonishing
fact that the science of the evolution of man does not even yet form
part of the scheme of general education. In fact, educated people even
in our day are for the most part quite ignorant of the important
truths and remarkable phenomena which anthropogeny teaches us.

As an illustration of this curious state of things, it may be pointed
out that most of what are considered to be "educated" people do not
know that every human being is developed from an egg, or ovum, and
that this egg is one simple cell, like any other plant or animal egg.
They are equally ignorant that in the course of the development of
this tiny, round egg-cell there is first formed a body that is totally
different from the human frame, and has not the remotest resemblance
to it. Most of them have never seen such a human embryo in the earlier
period of its development, and do not know that it is quite
indistinguishable from other animal embryos. At first the embryo is no
more than a round cluster of cells, then it becomes a simple hollow
sphere, the wall of which is composed of a layer of cells. Later it
approaches very closely, at one period, to the anatomic structure of
the lancelet, afterwards to that of a fish, and again to the typical
build of the amphibia and mammals. As it continues to develop, a form
appears which is like those we find at the lowest stage of mammal-life
(such as the duck-bills), then a form that resembles the marsupials,
and only at a late stage a form that has a resemblance to the ape;
until at last the definite human form emerges and closes the series of
transformations. These suggestive facts are, as I said, still almost
unknown to the general public - so completely unknown that, if one
casually mentions them, they are called in question or denied outright
as fairy-tales. Everybody knows that the butterfly emerges from the
pupa, and the pupa from a quite different thing called a larva, and
the larva from the butterfly's egg. But few besides medical men are
aware that MAN, in the course of his individual formation, passes
through a series of transformations which are not less surprising and
wonderful than the familiar metamorphoses of the butterfly.

The mere description of these remarkable changes through which man
passes during his embryonic life should arouse considerable interest.
But the mind will experience a far keener satisfaction when we trace
these curious facts to their causes, and when we learn to behold in
them natural phenomena which are of the highest importance throughout
the whole field of human knowledge. They throw light first of all on
the "natural history of creation," then on psychology, or "the science
of the soul," and through this on the whole of philosophy. And as the
general results of every branch of inquiry are summed up in
philosophy, all the sciences come in turn to be touched and influenced
more or less by the study of the evolution of man.

But when I say that I propose to present here the most important
features of these phenomena and trace them to their causes, I take the
term, and I interpret my task, in a very much wider sense than is
usual. The lectures which have been delivered on this subject in the
universities during the last half-century are almost exclusively
adapted to medical men. Certainly, the medical man has the greatest
interest in studying the origin of the human body, with which he is
daily occupied. But I must not give here this special description of
the embryonic processes such as it has hitherto been given, as most of
my readers have not studied anatomy, and are not likely to be
entrusted with the care of the adult organism. I must content myself
with giving some parts of the subject only in general outline, and
must not enter upon all the marvellous, but very intricate and not
easily described, details that are found in the story of the
development of the human frame. To understand these fully a knowledge
of anatomy is needed. I will endeavour to be as plain as possible in
dealing with this branch of science. Indeed, a sufficient general idea
of the course of the embryonic development of man can be obtained
without going too closely into the anatomic details. I trust we may be
able to arouse the same interest in this delicate field of inquiry as
has been excited already in other branches of science; though we shall
meet more obstacles here than elsewhere.

The story of the evolution of man, as it has hitherto been expounded
to medical students, has usually been confined to embryology - more
correctly, ontogeny - or the science of the development of the
individual human organism. But this is really only the first part of
our task, the first half of the story of the evolution of man in that
wider sense in which we understand it here. We must add as the second
half - as another and not less important and interesting branch of the
science of the evolution of the human stem - phylogeny: this may be
described as the science of the evolution of the various animal forms
from which the human organism has been developed in the course of
countless ages. Everybody now knows of the great scientific activity
that was occasioned by the publication of Darwin's Origin of Species
in 1859. The chief direct consequence of this publication was to
provoke a fresh inquiry into the origin of the human race, and this
has proved beyond question our gradual evolution from the lower
species. We give the name of "Phylogeny" to the science which
describes this ascent of man from the lower ranks of the animal world.
The chief source that it draws upon for facts is "Ontogeny," or
embryology, the science of the development of the individual organism.
Moreover, it derives a good deal of support from paleontology, or the
science of fossil remains, and even more from comparative anatomy, or
morphology.

These two branches of our science - on the one side ontogeny or
embryology, and on the other phylogeny, or the science of
race-evolution - are most vitally connected. The one cannot be
understood without the other. It is only when the two branches fully
co-operate and supplement each other that "Biogeny" (or the science of
the genesis of life in the widest sense) attains to the rank of a
philosophic science. The connection between them is not external and
superficial, but profound, intrinsic, and causal. This is a discovery
made by recent research, and it is most clearly and correctly
expressed in the comprehensive law which I have called "the
fundamental law of organic evolution," or "the fundamental law of
biogeny." This general law, to which we shall find ourselves
constantly recurring, and on the recognition of which depends one's
whole insight into the story of evolution, may be briefly expressed in
the phrase: "The history of the foetus is a recapitulation of the
history of the race"; or, in other words, "Ontogeny is a
recapitulation of phylogeny." It may be more fully stated as follows:
The series of forms through which the individual organism passes
during its development from the ovum to the complete bodily structure
is a brief, condensed repetition of the long series of forms which the
animal ancestors of the said organism, or the ancestral forms of the
species, have passed through from the earliest period of organic life
down to the present day.

The causal character of the relation which connects embryology with
stem-history is due to the action of heredity and adaptation. When we
have rightly understood these, and recognised their great importance
in the formation of organisms, we can go a step further and say:
Phylogenesis is the mechanical cause of ontogenesis.* (* The term
"genesis," which occurs throughout, means, of course, "birth" or
origin. From this we get: Biogeny = the origin of life (bios);
Anthropogeny = the origin of man (anthropos); Ontogeny = the origin of
the individual (on); Phylogeny = the origin of the species (phulon);
and so on. In each case the term may refer to the process itself, or
to the science describing the process. - Translator.) In other words,
the development of the stem, or race, is, in accordance with the laws
of heredity and adaptation, the cause of all the changes which appear
in a condensed form in the evolution of the foetus.

The chain of manifold animal forms which represent the ancestry of
each higher organism, or even of man, according to the theory of
descent, always form a connected whole. We may designate this
uninterrupted series of forms with the letters of the alphabet: A, B,
C, D, E, etc., to Z. In apparent contradiction to what I have said,
the story of the development of the individual, or the ontogeny of
most organisms, only offers to the observer a part of these forms; so
that the defective series of embryonic forms would run: A, B, D, F, H,
K, M, etc.; or, in other cases, B, D, H, L, M, N, etc. Here, then, as
a rule, several of the evolutionary forms of the original series have
fallen out. Moreover, we often find - to continue with our illustration
from the alphabet - one or other of the original letters of the
ancestral series represented by corresponding letters from a different
alphabet. Thus, instead of the Roman B and D, we often have the Greek
Beta and Delta. In this case the text of the biogenetic law has been
corrupted, just as it had been abbreviated in the preceding case. But,
in spite of all this, the series of ancestral forms remains the same,
and we are in a position to discover its original complexion.

In reality, there is always a certain parallel between the two
evolutionary series. But it is obscured from the fact that in the
embryonic succession much is wanting that certainly existed in the
earlier ancestral succession. If the parallel of the two series were
complete, and if this great fundamental law affirming the causal
connection between ontogeny and phylogeny in the proper sense of the
word were directly demonstrable, we should only have to determine, by
means of the microscope and the dissecting knife, the series of forms
through which the fertilised ovum passes in its development; we should
then have before us a complete picture of the remarkable series of
forms which our animal ancestors have successively assumed from the
dawn of organic life down to the appearance of man. But such a
repetition of the ancestral history by the individual in its embryonic
life is very rarely complete. We do not often find our full alphabet.
In most cases the correspondence is very imperfect, being greatly
distorted and falsified by causes which we will consider later. We are
thus, for the most part, unable to determine in detail, from the study
of its embryology, all the different shapes which an organism's
ancestors have assumed; we usually - and especially in the case of the
human foetus - encounter many gaps. It is true that we can fill up most
of these gaps satisfactorily with the help of comparative anatomy, but
we cannot do so from direct embryological observation. Hence it is
important that we find a large number of lower animal forms to be
still represented in the course of man's embryonic development. In
these cases we may draw our conclusions with the utmost security as to
the nature of the ancestral form from the features of the form which
the embryo momentarily assumes.

To give a few examples, we can infer from the fact that the human ovum
is a simple cell that the first ancestor of our species was a tiny
unicellular being, something like the amoeba. In the same way, we
know, from the fact that the human foetus consists, at the first, of
two simple cell-layers (the gastrula), that the gastraea, a form with
two such layers, was certainly in the line of our ancestry. A later
human embryonic form (the chordula) points just as clearly to a
worm-like ancestor (the prochordonia), the nearest living relation of
which is found among the actual ascidiae. To this succeeds a most
important embryonic stage (acrania), in which our headless foetus
presents, in the main, the structure of the lancelet. But we can only
indirectly and approximately, with the aid of comparative anatomy and
ontogeny, conjecture what lower forms enter into the chain of our
ancestry between the gastraea and the chordula, and between this and
the lancelet. In the course of the historical development many
intermediate structures have gradually fallen out, which must
certainly have been represented in our ancestry. But, in spite of
these many, and sometimes very appreciable, gaps, there is no
contradiction between the two successions. In fact, it is the chief
purpose of this work to prove the real harmony and the original
parallelism of the two. I hope to show, on a substantial basis of
facts, that we can draw most important conclusions as to our
genealogical tree from the actual and easily-demonstrable series of
embryonic changes. We shall then be in a position to form a general
idea of the wealth of animal forms which have figured in the direct
line of our ancestry in the lengthy history of organic life.

In this evolutionary appreciation of the facts of embryology we must,
of course, take particular care to distinguish sharply and clearly
between the primitive, palingenetic (or ancestral) evolutionary
processes and those due to cenogenesis.* (* Palingenesis = new birth,
or re-incarnation (palin = again, genesis or genea = development);
hence its application to the phenomena which are recapitulated by
heredity from earlier ancestral forms. Cenogenesis = foreign or
negligible development (kenos and genea); hence, those phenomena which
come later in the story of life to disturb the inherited structure, by
a fresh adaptation to environment. - Translator.) By palingenetic
processes, or embryonic recapitulations, we understand all those
phenomena in the development of the individual which are transmitted
from one generation to another by heredity, and which, on that
account, allow us to draw direct inferences as to corresponding
structures in the development of the species. On the other hand, we
give the name of cenogenetic processes, or embryonic variations, to
all those phenomena in the foetal development that cannot be traced to
inheritance from earlier species, but are due to the adaptation of the
foetus, or the infant-form, to certain conditions of its embryonic
development. These cenogenetic phenomena are foreign or later
additions; they allow us to draw no direct inference whatever as to
corresponding processes in our ancestral history, but rather hinder us
from doing so.

This careful discrimination between the primary or palingenetic
processes and the secondary or cenogenetic is of great importance for
the purposes of the scientific history of a species, which has to draw
conclusions from the available facts of embryology, comparative
anatomy, and paleontology, as to the processes in the formation of the
species in the remote past. It is of the same importance to the
student of evolution as the careful distinction between genuine and
spurious texts in the works of an ancient writer, or the purging of
the real text from interpolations and alterations, is for the student
of philology. It is true that this distinction has not yet been fully
appreciated by many scientists. For my part, I regard it as the first
condition for forming any just idea of the evolutionary process, and I
believe that we must, in accordance with it, divide embryology into
two sections - palingenesis, or the science of recapitulated forms; and
cenogenesis, or the science of supervening structures.

To give at once a few examples from the science of man's origin in
illustration of this important distinction, I may instance the
following processes in the embryology of man, and of all the higher
vertebrates, as palingenetic: the formation of the two primary
germinal layers and of the primitive gut, the undivided structure of
the dorsal nerve-tube, the appearance of a simple axial rod between
the medullary tube and the gut, the temporary formation of the
gill-clefts and arches, the primitive kidneys, and so on.* (* All
these, and the following structures, will be fully described in later
chapters. - Translator.) All these, and many other important
structures, have clearly been transmitted by a steady heredity from
the early ancestors of the mammal, and are, therefore, direct
indications of the presence of similar structures in the history of
the stem. On the other hand, this is certainly not the case with the
following embryonic forms, which we must describe as cenogenetic
processes: the formation of the yelk-sac, the allantois, the placenta,
the amnion, the serolemma, and the chorion - or, generally speaking,
the various foetal membranes and the corresponding changes in the
blood vessels. Further instances are: the dual structure of the heart
cavity, the temporary division of the plates of the primitive
vertebrae and lateral plates, the secondary closing of the ventral and
intestinal walls, the formation of the navel, and so on. All these and
many other phenomena are certainly not traceable to similar structures
in any earlier and completely-developed ancestral form, but have
arisen simply by adaptation to the peculiar conditions of embryonic
life (within the foetal membranes). In view of these facts, we may now
give the following more precise expression to our chief law of
biogeny: The evolution of the foetus (or ontogenesis) is a condensed
and abbreviated recapitulation of the evolution of the stem (or
phylogenesis); and this recapitulation is the more complete in
proportion as the original development (or palingenesis) is preserved
by a constant heredity; on the other hand, it becomes less complete in
proportion as a varying adaptation to new conditions increases the
disturbing factors in the development (or cenogenesis).

The cenogenetic alterations or distortions of the original
palingenetic course of development take the form, as a rule, of a
gradual displacement of the phenomena, which is slowly effected by
adaptation to the changed conditions of embryonic existence during the
course of thousands of years. This displacement may take place as
regards either the position or the time of a phenomenon.

The great importance and strict regularity of the time-variations in



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