Ernst Heinrich Philipp August Haeckel.

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fully in Chapter 1.10.

The many new points of view and fresh ideas suggested by my gastraea
theory and Hertwig's coelom theory led to the publication of a number
of writings on the theory of germinal layers. Most of them set out to
oppose it at first, but in the end the majority supported it. Of late
years both theories are accepted in their essential features by nearly
every competent man of science, and light and order have been
introduced into this once dark and contradictory field of research. A
further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of
the need for phylogenetic study and explanation.

Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of
phylogenetic methods. Hundreds of assiduous and able observers are now
engaged in the development of comparative embryology and its
establishment on a basis of evolution, whereas they numbered only a
few dozen not many decades ago. It would take too long to enumerate
even the most important of the countless valuable works which have
enriched embryological literature since that time. References to them
will be found in the latest manuals of embryology of Kolliker,
Balfour, Hertwig, Kollman, Korschelt, and Heider.

Kolliker's Entwickelungsgeschichte des Menschen und der hoherer
Thiere, the first edition of which appeared forty-two years ago, had
the rare merit at that time of gathering into presentable form the
scattered attainments of the science, and expounding them in some sort
of unity on the basis of the cellular theory and the theory of
germinal layers. Unfortunately, the distinguished Wurtzburg anatomist,
to whom comparative anatomy, histology, and ontogeny owe so much, is
opposed to the theory of descent generally and to Darwinism in
particular. All the other manuals I have mentioned take a decided
stand on evolution. Francis Balfour has carefully collected and
presented with discrimination, in his Manual of Comparative Embryology
(1880), the very scattered and extensive literature of the subject; he
has also widened the basis of the gastraea theory by a comparative
description of the rise of the organs from the germinal layers in all
the chief groups of the animal kingdom, and has given a most thorough
empirical support to the principles I have formulated. A comparison of
his work with the excellent Text-book of the Embryology of the
Vertebrates (1890) [translation 1895] of Korschelt and Heider shows
what astonishing progress has been made in the science in the course
of ten years. I would especially recommend the manuals of Julius
Kollmann and Oscar Hertwig to those readers who are stimulated to
further study by these chapters on human embryology. Kollmann's work
is commendable for its clear treatment of the subject and very fine
original illustrations; its author adheres firmly to the biogenetic
law, and uses it throughout with considerable profit. That is not the
case in Oscar Hertwig's recent Text-book of the Embryology of Man and
the Mammals [translations 1892 and 1899] (seventh edition 1902). This
able anatomist has of late often been quoted as an opponent of the
biogenetic law, although he himself had demonstrated its great value
thirty years ago. His recent vacillation is partly due to the timidity
which our "exact" scientists have with regard to hypotheses; though it
is impossible to make any headway in the explanation of facts without
them. However, the purely descriptive part of embryology in Hertwig's
Text-book is very thorough and reliable.

A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century - namely,
"experimental embryology." The great importance which has been
attached to the application of physical experiments to the living
organism for the last hundred years, and the valuable results that it
has given to physiology in the study of the vital phenomena, have led
to its extension to embryology. I was the first to make experiments of
this kind during a stay of four months on the Canary Island,
Lanzerote, in 1866. I there made a thorough investigation of the
almost unknown embryology of the siphonophorae. I cut a number of the
embryos of these animals (which develop freely in the water, and pass
through a very curious transformation), at an early stage, into
several pieces, and found that a fresh organism (more or less
complete, according to the size of the piece) was developed from each
particle. More recently some of my pupils have made similar
experiments with the embryos of vertebrates (especially the frog) and
some of the invertebrates. Wilhelm Roux, in particular, has made
extensive experiments, and based on them a special "mechanical
embryology," which has given rise to a good deal of discussion and
controversy. Roux has published a special journal for these subjects
since 1895, the Archiv fur Entwickelungsmechanik. The contributions to
it are very varied in value. Many of them are valuable papers on the
physiology and pathology of the embryo. Pathological experiments - the
placing of the embryo in abnormal conditions - have yielded many
interesting results; just as the physiology of the normal body has for
a long time derived assistance from the pathology of the diseased
organism. Other of these mechanical-embryological articles return to
the erroneous methods of His, and are only misleading. This must be
said of the many contributions of mechanical embryology which take up
a position of hostility to the theory of descent and its chief
embryological foundation - the biogenetic law. This law, however, when
rightly understood, is not opposed to, but is the best and most solid
support of, a sound mechanical embryology. Impartial reflection and a
due attention to paleontology and comparative anatomy should convince
these one-sided mechanicists that the facts they have discovered - and,
indeed, the whole embryological process - cannot be fully understood
without the theory of descent and the biogenetic law.


The embryology of man and the animals, the history of which we have
reviewed in the last two chapters, was mainly a descriptive science
forty years ago. The earlier investigations in this province were
chiefly directed to the discovery, by careful observation, of the
wonderful facts of the embryonic development of the animal body from
the ovum. Forty years ago no one dared attack the question of the
CAUSES of these phenomena. For fully a century, from the year 1759,
when Wolff's solid Theoria generationis appeared, until 1859, when
Darwin published his famous Origin of Species, the real causes of the
embryonic processes were quite unknown. No one thought of seeking the
agencies that effected this marvellous succession of structures. The
task was thought to be so difficult as almost to pass beyond the
limits of human thought. It was reserved for Charles Darwin to
initiate us into the knowledge of these causes. This compels us to
recognise in this great genius, who wrought a complete revolution in
the whole field of biology, a founder at the same time of a new period
in embryology. It is true that Darwin occupied himself very little
with direct embryological research, and even in his chief work he only
touches incidentally on the embryonic phenomena; but by his reform of
the theory of descent and the founding of the theory of selection he
has given us the means of attaining to a real knowledge of the causes
of embryonic formation. That is, in my opinion, the chief feature in
Darwin's incalculable influence on the whole science of evolution.

When we turn our attention to this latest period of embryological
research, we pass into the second division of organic
evolution - stem-evolution, or phylogeny. I have already indicated in
Chapter 1.1 the important and intimate causal connection between these
two sections of the science of evolution - between the evolution of the
individual and that of his ancestors. We have formulated this
connection in the biogenetic law; the shorter evolution, that of the
individual, or ontogenesis, is a rapid and summary repetition, a
condensed recapitulation, of the larger evolution, or that of the
species. In this principle we express all the essential points
relating to the causes of evolution; and we shall seek throughout this
work to confirm this principle and lend it the support of facts. When
we look to its CAUSAL significance, perhaps it would be better to
formulate the biogenetic law thus: "The evolution of the species and
the stem (phylon) shows us, in the physiological functions of heredity
and adaptation, the conditioning causes on which the evolution of the
individual depends"; or, more briefly: "Phylogenesis is the mechanical
cause of ontogenesis."

But before we examine the great achievement by which Darwin revealed
the causes of evolution to us, we must glance at the efforts of
earlier scientists to attain this object. Our historical inquiry into
these will be even shorter than that into the work done in the field
of ontogeny. We have very few names to consider here. At the head of
them we find the great French naturalist, Jean Lamarck, who first
established evolution as a scientific theory in 1809. Even before his
time, however, the chief philosopher, Kant, and the chief poet,
Goethe, of Germany had occupied themselves with the subject. But their
efforts passed almost without recognition in the eighteenth century. A
"philosophy of nature" did not arise until the beginning of the
nineteenth century. In the whole of the time before this no one had
ventured to raise seriously the question of the origin of species,
which is the culminating point of phylogeny. On all sides it was
regarded as an insoluble enigma.

The whole science of the evolution of man and the other animals is
intimately connected with the question of the nature of species, or
with the problem of the origin of the various animals which we group
together under the name of species. Thus the definition of the species
becomes important. It is well known that this definition was given by
Linne, who, in his famous Systema Naturae (1735), was the first to
classify and name the various groups of animals and plants, and drew
up an orderly scheme of the species then known. Since that time
"species" has been the most important and indispensable idea in
descriptive natural history, in zoological and botanical
classification; although there have been endless controversies as to
its real meaning.

What, then, is this "organic species"? Linne himself appealed directly
to the Mosaic narrative; he believed that, as it is stated in Genesis,
one pair of each species of animals and plants was created in the
beginning, and that all the individuals of each species are the
descendants of these created couples. As for the hermaphrodites
(organisms that have male and female organs in one being), he thought
it sufficed to assume the creation of one sole individual, since this
would be fully competent to propagate its species. Further developing
these mystic ideas, Linne went on to borrow from Genesis the account
of the deluge and of Noah's ark as a ground for a science of the
geographical and topographical distribution of organisms. He accepted
the story that all the plants, animals, and men on the earth were
swept away in a universal deluge, except the couples preserved with
Noah in the ark, and ultimately landed on Mount Ararat. This mountain
seemed to Linne particularly suitable for the landing, as it reaches a
height of more than 16,000 feet, and thus provides in its higher zones
the several climates demanded by the various species of animals and
plants: the animals that were accustomed to a cold climate could
remain at the summit; those used to a warm climate could descend to
the foot; and those requiring a temperate climate could remain
half-way down. From this point the re-population of the earth with
animals and plants could proceed.

It was impossible to have any scientific notion of the method of
evolution in Linne's time, as one of the chief sources of information,
paleontology, was still wholly unknown. This science of the fossil
remains of extinct animals and plants is very closely bound up with
the whole question of evolution. It is impossible to explain the
origin of living organisms without appealing to it. But this science
did not rise until a much later date. The real founder of scientific
paleontology was Georges Cuvier, the most distinguished zoologist who,
after Linne, worked at the classification of the animal world, and
effected a complete revolution in systematic zoology at the beginning
of the nineteenth century. In regard to the nature of the species he
associated himself with Linne and the Mosaic story of creation, though
this was more difficult for him with his acquaintance with fossil
remains. He clearly showed that a number of quite different animal
populations have lived on the earth; and he claimed that we must
distinguish a number of stages in the history of our planet, each of
which was characterised by a special population of animals and plants.
These successive populations were, he said, quite independent of each
other, and therefore the supernatural creative act, which was demanded
as the origin of the animals and plants by the dominant creed, must
have been repeated several times. In this way a whole series of
different creative periods must have succeeded each other; and in
connection with these he had to assume that stupendous revolutions or
cataclysms - something like the legendary deluge - must have taken place
repeatedly. Cuvier was all the more interested in these catastrophes
or cataclysms as geology was just beginning to assert itself, and
great progress was being made in our knowledge of the structure and
formation of the earth's crust. The various strata of the crust were
being carefully examined, especially by the famous geologist Werner
and his school, and the fossils found in them were being classified;
and these researches also seemed to point to a variety of creative
periods. In each period the earth's crust, composed of the various
strata, seemed to be differently constituted, just like the population
of animals and plants that then lived on it. Cuvier combined this
notion with the results of his own paleontological and zoological
research; and in his effort to get a consistent view of the whole
process of the earth's history he came to form the theory which is
known as "the catastrophic theory," or the theory of terrestrial
revolutions. According to this theory, there have been a series of
mighty cataclysms on the earth, and these have suddenly destroyed the
whole animal and plant population then living on it; after each
cataclysm there was a fresh creation of living things throughout the
earth. As this creation could not be explained by natural laws, it was
necessary to appeal to an intervention on the part of the Creator.
This catastrophic theory, which Cuvier described in a special work,
was soon generally accepted, and retained its position in biology for
half a century.

However, Cuvier's theory was completely overthrown sixty years ago by
the geologists, led by Charles Lyell, the most distinguished worker in
this field of science. Lyell proved in his famous Principles of
Geology (1830) that the theory was false, in so far as it concerned
the crust of the earth; that it was totally unnecessary to bring in
supernatural agencies or general catastrophes in order to explain the
structure and formation of the mountains; and that we can explain them
by the familiar agencies which are at work to-day in altering and
reconstructing the surface of the earth. These causes are - the action
of the atmosphere and water in its various forms (snow, ice, fog,
rain, the wear of the river, and the stormy ocean), and the volcanic
action which is exerted by the molten central mass. Lyell convincingly
proved that these natural causes are quite adequate to explain every
feature in the build and formation of the crust. Hence Cuvier's theory
of cataclysms was very soon driven out of the province of geology,
though it remained for another thirty years in undisputed authority in
biology. All the zoologists and botanists who gave any thought to the
question of the origin of organisms adhered to Cuvier's erroneous idea
of revolutions and new creations.

In order to illustrate the complete stagnancy of biology from 1830 to
1859 on the question of the origin of the various species of animals
and plants, I may say, from my own experience, that during the whole
of my university studies I never heard a single word said about this
most important problem of the science. I was fortunate enough at that
time (1852 to 1857) to have the most distinguished masters for every
branch of biological science. Not one of them ever mentioned this
question of the origin of species. Not a word was ever said about the
earlier efforts to understand the formation of living things, nor
about Lamarck's Philosophie Zoologique which had made a fresh attack
on the problem in 1809. Hence it is easy to understand the enormous
opposition that Darwin encountered when he took up the question for
the first time. His views seemed to float in the air, without a single
previous effort to support them. The whole question of the formation
of living things was considered by biologists, until 1859, as
pertaining to the province of religion and transcendentalism; even in
speculative philosophy, in which the question had been approached from
various sides, no one had ventured to give it serious treatment. This
was due to the dualistic system of Immanuel Kant, who taught a natural
system of evolution as far as the inorganic world was concerned; but,
on the whole, adopted a supernaturalist system as regards the origin
of living things. He even went so far as to say: "It is quite certain
that we cannot even satisfactorily understand, much less explain, the
nature of an organism and its internal forces on purely mechanical
principles; it is so certain, indeed, that we may confidently say: 'It
is absurd for a man to imagine even that some day a Newton will arise
who will explain the origin of a single blade of grass by natural laws
not controlled by design' - such a hope is entirely forbidden us." In
these words Kant definitely adopts the dualistic and teleological
point of view for biological science.

Nevertheless, Kant deserted this point of view at times, particularly
in several remarkable passages which I have dealt with at length in my
Natural History of Creation (chapter 5), where he expresses himself in
the opposite, or monistic, sense. In fact, these passages would
justify one, as I showed, in claiming his support for the theory of
evolution. However, these monistic passages are only stray gleams of
light; as a rule, Kant adheres in biology to the obscure dualistic
ideas, according to which the forces at work in inorganic nature are
quite different from those of the organic world. This dualistic system
prevails in academic philosophy to-day - most of our philosophers still
regarding these two provinces as totally distinct. They put, on the
one side, the inorganic or "lifeless" world, in which there are at
work only mechanical laws, acting necessarily and without design; and,
on the other, the province of organic nature, in which none of the
phenomena can be properly understood, either as regards their inner
nature or their origin, except in the light of preconceived design,
carried out by final or purposive causes.

The prevalence of this unfortunate dualistic prejudice prevented the
problem of the origin of species, and the connected question of the
origin of man, from being regarded by the bulk of people as a
scientific question at all until 1859. Nevertheless, a few
distinguished students, free from the current prejudice, began, at the
commencement of the nineteenth century, to make a serious attack on
the problem. The merit of this attaches particularly to what is known
as "the older school of natural philosophy," which has been so much
misrepresented, and which included Jean Lamarck, Buffon, Geoffroy St.
Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold
Treviranus, Schelling, and Lorentz Oken in Germany [and Erasmus Darwin
in England].

The gifted natural philosopher who treated this difficult question
with the greatest sagacity and comprehensiveness was Jean Lamarck. He
was born at Bazentin, in Picardy, on August 1st, 1744; he was the son
of a clergyman, and was destined for the Church. But he turned to seek
glory in the army, and eventually devoted himself to science.

His Philosophie Zoologique was the first scientific attempt to sketch
the real course of the origin of species, the first "natural history
of creation" of plants, animals, and men. But, as in the case of
Wolff's book, this remarkably able work had no influence whatever;
neither one nor the other could obtain any recognition from their
prejudiced contemporaries. No man of science was stimulated to take an
interest in the work, and to develop the germs it contained of the
most important biological truths. The most distinguished botanists and
zoologists entirely rejected it, and did not even deign to reply to
it. Cuvier, who lived and worked in the same city, has not thought fit
to devote a single syllable to this great achievement in his memoir on
progress in the sciences, in which the pettiest observations found a
place. In short, Lamarck's Philosophie Zoologique shared the fate of
Wolff's theory of development, and was for half a century ignored and
neglected. The German scientists, especially Oken and Goethe, who were
occupied with similar speculations at the same time, seem to have
known nothing about Lamarck's work. If they had known it, they would
have been greatly helped by it, and might have carried the theory of
evolution much farther than they found it possible to do.

To give an idea of the great importance of the Philosophie Zoologique,
I will briefly explain Lamarck's leading thought. He held that there
was no essential difference between living and lifeless beings. Nature
is one united and connected system of phenomena; and the forces which
fashion the lifeless bodies are the only ones at work in the kingdom
of living things. We have, therefore, to use the same method of
investigation and explanation in both provinces. Life is only a
physical phenomenon. All the plants and animals, with man at their
head, are to be explained, in structure and life, by mechanical or
efficient causes, without any appeal to final causes, just as in the
case of minerals and other inorganic bodies. This applies equally to
the origin of the various species. We must not assume any original
creation, or repeated creations (as in Cuvier's theory), to explain
this, but a natural, continuous, and necessary evolution. The whole
evolutionary process has been uninterrupted. All the different kinds
of animals and plants which we see to-day, or that have ever lived,
have descended in a natural way from earlier and different species;
all come from one common stock, or from a few common ancestors. These
remote ancestors must have been quite simple organisms of the lowest
type, arising by spontaneous generation from inorganic matter. The
succeeding species have been constantly modified by adaptation to
their varying environment (especially by use and habit), and have
transmitted their modifications to their successors by heredity.

Lamarck was the first to formulate as a scientific theory the natural
origin of living things, including man, and to push the theory to its
extreme conclusions - the rise of the earliest organisms by spontaneous
generation (or abiogenesis) and the descent of man from the nearest
related mammal, the ape. He sought to explain this last point, which
is of especial interest to us here, by the same agencies which he
found at work in the natural origin of the plant and animal species.
He considered use and habit (adaptation) on the one hand, and heredity
on the other, to be the chief of these agencies. The most important
modifications of the organs of plants and animals are due, in his
opinion, to the function of these very organs, or to the use or disuse
of them. To give a few examples, the woodpecker and the humming-bird
have got their peculiarly long tongues from the habit of extracting
their food with their tongues from deep and narrow folds or canals;
the frog has developed the web between his toes by his own swimming;

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