Now that we have traversed the wonderful region of human embryology
and are familiar with the principal parts of it, it will be well to
look back on the way we have come, and forward to the further path to
truth to which it has led us. We started from the simplest facts of
ontogeny, or the development of the individual--from observations that
we can repeat and verify by microscopic and anatomic study at any
moment. The first and most important of these facts is that every man,
like every other animal, begins his existence as a simple cell. This
round ovum has the same characteristic form and origin as the ovum of
any other mammal. From it is developed in the same manner in all the
Placentals, by repeated cleavage, a multicellular blastula. This is
converted into a gastrula, and this in turn into a blastocystis (or
embryonic vesicle). The two strata of cells that compose its wall are
the primary germinal layers, the skin-layer (ectoderm), and gut-layer
(entoderm). This two-layered embryonic form is the ontogenetic
reproduction of the extremely important phylogenetic stem-form of all
the Metazoa, which we have called the Gastraea. As the human embryo
passes through the gastrula-form like that of all the other Metazoa,
we can trace its phylogenetic origin to the Gastraea.
As we continued to follow the embryonic development of the two-layered
structure, we saw that first a third, or middle layer (mesoderm),
appears between the two primary layers; when this divides into two, we
have the four secondary germinal layers. These have just the same
composition and genetic significance in man as in all the other
Vertebrates. From the skin-sense layer are developed the epidermis,
the central nervous system, and the chief part of the sense-organs.
The skin-fibre layer forms the corium and the motor organs--the
skeleton and the muscular system. From the gut-fibre layer are
developed the vascular system, the muscular wall of the gut, and the
sexual glands. Finally, the gut-gland layer only forms the epithelium,
or the inner cellular stratum of the mucous membrane of the alimentary
canal and glands (lungs, liver, etc.).
The manner in which these different systems of organs arise from the
secondary germinal layers is essentially the same from the start in
man as in all the other Vertebrates. We saw, in studying the embryonic
development of each organ, that the human embryo follows the special
lines of differentiation and construction that are only found
otherwise in the Vertebrates. Within the limits of this vast stem we
have followed, step by step, the development both of the body as a
whole and of its various parts. This higher development follows in the
human embryo the form that is peculiar to the mammals. Finally, we saw
that, even within the limits of this class, the various phylogenetic
stages that we distinguish in a natural classification of the mammals
correspond to the ontogenetic stages that the human embryo passes
through in the course of its evolution. We were thus in a position to
determine precisely the position of man in this class, and so to
establish his relationship to the different orders of mammals.
The line of argument we followed in this explanation of the
ontogenetic facts was simply a consistent application of the
biogenetic law. In this we have throughout taken strict account of the
distinction between palingenetic and cenogenetic phenomena.
Palingenesis (or "synoptic development") alone enables us to draw
conclusions from the observed embryonic form to the stem-form
preserved by heredity. Such inference becomes more or less precarious
when there has been cenogenesis, or disturbance of development, owing
to fresh adaptations. We cannot understand embryonic development
unless we appreciate this very important distinction. Here we stand at
the very limit that separates the older and the new science or
philosophy of nature. The whole of the results of recent morphological
research compel us irresistibly to recognise the biogenetic law and
its far-reaching consequences. These are, it is true, irreconcilable
with the legends and doctrines of former days, that have been
impressed on us by religious education. But without the biogenetic
law, without the distinction between palingenesis and cenogenesis, and
without the theory of evolution on which we base it, it is quite
impossible to understand the facts of organic development; without
them we cannot cast the faintest gleam of explanation over this
marvellous field of phenomena. But when we recognise the causal
correlation of ontogeny and phylogeny expressed in this law, the
wonderful facts of embryology are susceptible of a very simple
explanation; they are found to be the necessary mechanical effects of
the evolution of the stem, determined by the laws of heredity and
adaptation. The correlative action of these laws under the universal
influence of the struggle for existence, or--as we may say in a word,
with Darwin--"natural selection," is entirely adequate to explain the
whole process of embryology in the light of phylogeny. It is the chief
merit of Darwin that he explained by his theory of selection the
correlation of the laws of heredity and adaptation that Lamarck had
recognised, and pointed out the true way to reach a causal
interpretation of evolution.
The phenomenon that it is most imperative to recognise in this
connection is the inheritance of functional variations. Jean Lamarck
was the first to appreciate its fundamental importance in 1809, and we
may therefore justly give the name of Lamarckism to the theory of
descent he based on it. Hence the radical opponents of the latter have
very properly directed their attacks chiefly against the former. One
of the most distinguished and most narrow-minded of these opponents,
Wilhelm His, affirms very positively that "characteristics acquired in
the life of the individual are not inherited."
The inheritance of acquired characters is denied, not only by thorough
opponents of evolution, but even by scientists who admit it and have
contributed a good deal to its establishment, especially Weismann,
Galton, Ray Lankester, etc. Since 1884 the chief opponent has been
August Weismann, who has rendered the greatest service in the
development of Darwin's theory of selection. In his work on The
Continuity of the Germ-plasm, and in his recent excellent Lectures on
the Theory of Descent (1902), he has with great success advanced the
opinion that "only those characters can be transmitted to subsequent
generations that were contained in rudimentary form in the embryo."
However, this germ-plasm theory, with its attempt to explain heredity,
is merely a "provisional molecular hypothesis"; it is one of those
metaphysical speculations that attribute the evolutionary phenomena
exclusively to internal causes, and regard the influence of the
environment as insignificant. Herbert Spencer, Theodor Eimer, Lester
Ward, Hering, and Zehnder have pointed out the untenable consequences
of this position. I have given my view of it in the tenth edition of
the History of Creation (pages 192 and 203). I hold, with Lamarck and
Darwin, that the hereditary transmission of acquired characters is one
of the most important phenomena in biology, and is proved by thousands
of morphological and physiological experiences. It is an indispensable
foundation of the theory of evolution.
Of the many and weighty arguments for the truth of this conception of
evolution I will for the moment merely point to the invaluable
evidence of dysteleology, the science of rudimentary organs. We cannot
insist too often or too strongly on the great morphological
significance of these remarkable organs, which are completely useless
from the physiological point of view. We find some of these useless
parts, inherited from our lower vertebrate ancestors, in every system
of organs in man and the higher Vertebrates. Thus we find at once on
the skin a scanty and rudimentary coat of hair, only fully developed
on the head, under the shoulders, and at a few other parts of the
body. The short hairs on the greater part of the body are quite
useless and devoid of physiological value; they are the last relic of
the thicker hairy coat of our simian ancestors. The sensory apparatus
presents a series of most remarkable rudimentary organs. We have seen
that the whole of the shell of the external ear, with its cartilages,
muscles, and skin, is in man a useless appendage, and has not the
physiological importance that was formerly ascribed to it. It is the
degenerate remainder of the pointed, freely moving, and more advanced
mammal ear, the muscles of which we still have, but cannot work them.
We found at the inner corner of our eye a small, curious, semi-lunar
fold that is of no use whatever to us, and is only interesting as the
last relic of the nictitating membrane, the third, inner eye-lid that
had a distinct physiological purpose in the ancient sharks, and still
has in many of the Amniotes.
The motor apparatus, in both the skeleton and muscular systems,
provides a number of interesting dysteleological arguments. I need
only recall the projecting tail of the human embryo, with its
rudimentary caudal vertebrae and muscles; this is totally useless in
man, but very interesting as the degenerate relic of the long tail of
our simian ancestors. From these we have also inherited various bony
processes and muscles, which were very useful to them in climbing
trees, but are useless to us. At various points of the skin we have
cutaneous muscles which we never use--remnants of a strongly-developed
cutaneous muscle in our lower mammal ancestors. This "panniculus
carnosus" had the function of contracting and creasing the skin to
chase away the flies, as we see every day in the horse. Another relic
in us of this large cutaneous muscle is the frontal muscle, by which
we knit our forehead and raise our eye-brows; but there is another
considerable relic of it, the large cutaneous muscle in the neck
(platysma myoides), over which we have no voluntary control.
Not only in the systems of animal organs, but also in the vegetal
apparatus, we find a number of rudimentary organs, many of which we
have already noticed. In the alimentary apparatus there are the
thymus-gland and the thyroid gland, the seat of goitre and the relic
of a ciliated groove that the Tunicates and Acrania still have in the
gill-pannier; there is also the vermiform appendix to the caecum. In
the vascular system we have a number of useless cords which represent
relics of atrophied vessels that were once active as blood-canals--the
ductus Botalli between the pulmonary artery and the aorta, the ductus
venosus Arantii between the portal vein and the vena cava, and many
others. The many rudimentary organs in the urinary and sexual
apparatus are particularly interesting. These are generally developed
in one sex and rudimentary in the other. Thus the spermaducts are
formed from the Wolffian ducts in the male, whereas in the female we
have merely rudimentary traces of them in Gaertner's canals. On the
other hand, in the female the oviducts and womb are developed from the
Mullerian ducts, while in the male only the lowest ends of them remain
as the "male womb" (vesicula prostatica). Again, the male has in his
nipples and mammary glands the rudiments of organs that are usually
active only in the female.
A careful anatomic study of the human frame would disclose to us
numbers of other rudimentary organs, and these can only be explained
on the theory of evolution. Robert Wiedersheim has collected a large
number of them in his work on The Human Frame as a Witness to its
Past. They are some of the weightiest proofs of the truth of the
mechanical conception and the strongest disproofs of the teleological
view. If, as the latter demands, man or any other organism had been
designed and fitted for his life-purposes from the start and brought
into being by a creative act, the existence of these rudimentary
organs would be an insoluble enigma; it would be impossible to
understand why the Creator had put this useless burden on his
creatures to walk a path that is in itself by no means easy. But the
theory of evolution gives the simplest possible explanation of them.
It says: The rudimentary organs are parts of the body that have fallen
into disuse in the course of centuries; they had definite functions in
our animal ancestors, but have lost their physiological significance.
On account of fresh adaptations they have become superfluous, but are
transmitted from generation to generation by heredity, and gradually
atrophy.
We have inherited not only these rudimentary parts, but all the organs
of our body, from the mammals--proximately from the apes. The human
body does not contain a single organ that has not been inherited from
the apes. In fact, with the aid of our biogenetic law we can trace the
origin of our various systems of organs much further, down to the
lowest stages of our ancestry. We can say, for instance, that we have
inherited the oldest organs of the body, the external skin and the
internal coat of the alimentary system, from the Gastraeads; the
nervous and muscular systems from the Platodes; the vascular system,
the body-cavity, and the blood from the Vermalia; the chorda and the
branchial gut from the Prochordonia; the articulation of the body from
the Acrania; the primitive skull and the higher sense-organs from the
Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot
from the Amphibia; the palate from the Reptiles; the hairy coat, the
mammary glands, and the external sexual organs from the Pro-mammals.
When we formulated "the law of the ontogenetic connection of
systematically related forms," and determined the relative age of
organs, we saw how it was possible to draw phylogenetic conclusions
from the ontogenetic succession of systems of organs.
With the aid of this important law and of comparative anatomy we were
also enabled to determine "man's place in nature," or, as we put it,
assign to man his position in the classification of the animal
kingdom. In recent zoological classification the animal world is
divided into twelve stems or phyla, and these are broadly sub-divided
into about sixty classes, and these classes into at least 300 orders.
In his whole organisation man is most certainly, in the first place, a
member of one of these stems, the vertebrate stem; secondly, a member
of one particular class in this stem, the Mammals; and thirdly, of one
particular order, the order of Primates. He has all the
characteristics that distinguish the Vertebrates from the other eleven
animal stems, the Mammals from the other sixty classes, and the
Primates from the 300 other orders of the animal kingdom. We may turn
and twist as we like, but we cannot get over this fact of anatomy and
classification. Of late years this fact has given rise to a good deal
of discussion, and especially of controversy as to the particular
anatomic relationship of man to the apes. The most curious opinions
have been advanced on this "ape-question," or "pithecoid-theory." It
is as well, therefore, to go into it once more and distinguish the
essential from the unessential. (Cf. Chapter 2.23.)
We start from the undisputed fact that man is in any case--whether we
accept or reject his special blood-relationship to the apes--a true
mammal; in fact, a placental mammal. This fundamental fact can be
proved so easily at any moment from comparative anatomy that it has
been universally admitted since the separation of the Placentals from
the lower mammals (Marsupials and Monotremes). But for every
consistent subscriber to the theory of evolution it must follow at
once that man descends from a common stem-form with all the other
Placentals, the stem-ancestor of the Placentals, just as we must admit
a common mesozoic ancestor of all the mammals. This is, however, to
settle decisively the great and burning question of man's place in
nature, whether or no we go on to admit a nearer or more distant
relationship to the apes. Whether man is or is not a member of the
ape-order (or, if you prefer, the primate-order.) in the phylogenetic
sense, in any case his direct blood-relationship to the rest of the
mammals, and especially the Placentals, is established. It is possible
that the affinities of the various orders of mammals to each other are
different from what we hypothetically assume to-day. But, in any case,
the common descent of man and all the other mammals from one stem-form
is beyond question. This long-extinct Promammal was probably evolved
from Proreptiles during the Triassic period, and must certainly be
regarded as the monotreme and oviparous ancestor of ALL the mammals.
If we hold firmly to this fundamental and most important thesis, we
shall see the "ape-question" in a very different light from that in
which it is usually regarded. Little reflection is then needed to see
that it is not nearly so important as it is said to be. The origin of
the human race from a series of mammal ancestors, and the historic
evolution of these from an earlier series of lower vertebrate
ancestors, together with all the weighty conclusions that every
thoughtful man deduces therefrom, remain untouched; so far as these
are concerned, it is immaterial whether we regard true "apes" as our
nearest ancestors or not. But as it has become the fashion to lay the
chief stress in the whole question of man's origin on the "descent
from the apes," I am compelled to return to it once more, and recall
the facts of comparative anatomy and ontogeny that give a decisive
answer to this "ape-question."
The shortest way to attain our purpose is that followed by Huxley in
1863 in his able work, which I have already often quoted, Man's Place
in Nature--the way of comparative anatomy and ontogeny. We have to
compare impartially all man's organs with the same organs in the
higher apes, and then to examine if the differences between the two
are greater than the corresponding differences between the higher and
the lower apes. The indubitable and incontestable result of this
comparative-anatomical study, conducted with the greatest care and
impartiality, was the pithecometra-principle, which we have called the
Huxleian law in honour of its formulator--namely, that the differences
in organisation between man and the most advanced apes we know are
much slighter than the corresponding differences in organisation
between the higher and lower apes. We may even give a more precise
formula to this law, by excluding the Platyrrhines or American apes as
distant relatives, and restricting the comparison to the narrower
family-circle of the Catarrhines, the apes of the Old World. Within
the limits of this small group of mammals we found the structural
differences between the lower and higher catarrhine apes--for
instance, the baboon and the gorilla--to be much greater than the
differences between the anthropoid apes and man. If we now turn to
ontogeny, and find, according to our "law of the ontogenetic
connection of systematically related forms," that the embryos of the
anthropoid apes and man retain their resemblance for a longer time
than the embryos of the highest and the lowest apes, we are forced,
whether we like it or no, to recognise our descent from the order of
apes. We can assuredly construct an approximate picture in the
imagination of the form of our early Tertiary ancestors from the
foregoing facts of comparative anatomy; however we may frame this in
detail, it will be the picture of a true ape, and a distinct
catarrhine ape. This has been shown so well by Huxley (1863) that the
recent attacks of Klaatsch, Virchow, and other anthropologists, have
completely failed (cf. Chapter 2.23). All the structural characters
that distinguish the Catarrhines from the Platyrrhines are found in
man. Hence in the genealogy of the mammals we must derive man
immediately from the catarrhine group, and locate the origin of the
human race in the Old World. Only the early root-form from which both
descended was common to them.
It is, therefore, established beyond question for all impartial
scientific inquiry that the human race comes directly from the apes of
the Old World; but, at the same time, I repeat that this is not so
important in connection with the main question of the origin of man as
is commonly supposed. Even if we entirely ignore it, all that we have
learned from the zoological facts of comparative anatomy and ontogeny
as to the placental character of man remains untouched. These prove
beyond all doubt the common descent of man and all the rest of the
mammals. Further, the main question is not in the least affected if it
is said: "It is true that man is a mammal; but he has diverged at the
very root of the class from all the other mammals, and has no closer
relationship to any living group of mammals." The affinity is more or
less close in any case, if we examine the relation of the mammal class
to the sixty other classes of the animal world. Quite certainly the
whole of the mammals, including man, have had a common origin; and it
is equally certain that their common stem-forms were gradually evolved
from a long series of lower Vertebrates.
The resistance to the theory of a descent from the apes is clearly due
in most men to feeling rather than to reason. They shrink from the
notion of such an origin just because they see in the ape organism a
caricature of man, a distorted and unattractive image of themselves,
because it hurts man's aesthetic complacency and self-ennoblement. It
is more flattering to think we have descended from some lofty and
god-like being; and so, from the earliest times, human vanity has been
pleased to believe in our origin from gods or demi-gods. The Church,
with that sophistic reversal of ideas of which it is a master, has
succeeded in representing this ridiculous piece of vanity as
"Christian humility"; and the very men who reject with horror the
notion of an animal origin, and count themselves "children of God,"
love to prate of their "humble sense of servitude." In most of the
sermons that have poured out from pulpit and altar against the
doctrine of evolution human vanity and conceit have been a conspicuous
element; and, although we have inherited this very characteristic
weakness from the apes, we must admit that we have developed it to a
higher degree, which is entirely repudiated by sound and normal
intelligence. We are greatly amused at all the childish follies that
the ridiculous pride of ancestry has maintained from the Middle Ages
to our own time; yet there is a large amount of this empty feeling in
most men. Just as most people much prefer to trace their family back
to some degenerate baron or some famous prince rather than to an
unknown peasant, so most men would rather have as parent of the race a
sinful and fallen Adam than an advancing, and vigorous ape. It is a
matter of taste, and to that extent we cannot quarrel over these
genealogical tendencies. Personally, the notion of ascent is more
congenial to me than that of descent. It seems to me a finer thing to
be the advanced offspring of a simian ancestor, that has developed
progressively from the lower mammals in the struggle for life, than
the degenerate descendant of a god-like being, made from a clod, and
fallen for his sins, and an Eve created from one of his ribs. Speaking
of the rib, I may add to what I have said about the development of the
skeleton, that the number of ribs is just the same in man and woman.
In both of them the ribs are formed from the middle germinal layer,
and are, from the phylogenetic point of view, lower or ventral
vertebral arches.
But it is said: "That is all very well, as far as the human body is
concerned; on the facts quoted it is impossible to doubt that it has
really and gradually been evolved from the long ancestral series of
the Vertebrates. But it is quite another thing as regards man's mind,
or soul; this cannot possibly have been developed from the
vertebrate-soul."* (* The English reader will recognise here the
curious position of Dr. Wallace and of the late Dr.
Mivart.--Translator.) Let us see if we cannot meet this grave
stricture from the well-known facts of comparative anatomy,
physiology, and embryology. It will be best to begin with a
comparative study of the souls of various groups of Vertebrates. Here
we find such an enormous variety of vertebrate souls that, at first
sight, it seems quite impossible to trace them all to a common
"Primitive Vertebrate." Think of the tiny Amphioxus, with no real
brain but a simple medullary tube, and its whole psychic life at the
very lowest stage among the Vertebrates. The following group of the
Cyclostomes are still very limited, though they have a brain. When we
pass on to the fishes, we find their intelligence remaining at a very
low level. We do not see any material advance in mental development
until we go on to the Amphibia and Reptiles. There is still greater
advance when we come to the Mammals, though even here the minds of the
Monotremes and of the stupid Marsupials remain at a low stage. But
when we rise from these to the Placentals we find within this one vast
group such a number of important stages of differentiation and
progress that the psychic differences between the least intelligent
(such as the sloths and armadillos) and the most intelligent
Placentals (such as the dogs and apes) are much greater than the
psychic differences between the lowest Placentals and the Marsupials
or Monotremes. Most certainly the differences are far greater than the
differences in mental power between the dog, the ape, and man. Yet all
these animals are genetically-related members of a single natural
class.
We see this to a still more astonishing extent in the comparative
psychology of another class of animals, that is especially interesting
for many reasons--the insect class. It is well known that we find in
many insects a degree of intelligence that is found in man alone among
the Vertebrates. Everybody knows of the famous communities and states
of bees and ants, and of the very remarkable social arrangements in
them, such as we find among the more advanced races of men, but among
no other group of animals. I need only mention the social organisation
and government of the monarchic bees and the republican ants, and
their division into different conditions--queen, drone-nobles,
workers, educators, soldiers, etc. One of the most remarkable
phenomena in this very interesting province is the cattle-keeping of
the ants, which rear plant-lice as milch-cows and regularly extract
their honeyed juice. Still more remarkable is the slave-holding of the
large red ants, which steal the young of the small black ants and
bring them up as slaves. It has long been known that these political
and social arrangements of the ants are due to the deliberate
cooperation of the countless citizens, and that they understand each
other. A number of recent observers, especially Fritz Muller, Sir J.
Lubbock (Lord Avebury), and August Forel, have put the astonishing
degree of intelligence of these tiny Articulates beyond question.
Now, compare with these the mental life of many of the lower,
especially the parasitic insects, as Darwin did. There is, for
instance, the cochineal insect (Coccus), which, in its adult state,
has a motionless, shield-shaped body, attached to the leaves of
plants. Its feet are atrophied. Its snout is sunk in the tissue of the
plants of which it absorbs the sap. The whole psychic life of these
inert female parasites consists in the pleasure they experience from
sucking the sap of the plant and in sexual intercourse with the males.
It is the same with the maggot-like females of the fan-fly
(Strepsitera), which spend their lives parasitically and immovably,
without wings or feet, in the abdomen of wasps. There is no question
here of higher psychic action. If we compare these sluggish parasites
with the intelligent and active ants, we must admit that the psychic
differences between them are much greater than the psychic differences
between the lowest and highest mammals, between the Monotremes,
Marsupials, and armadillos on the one hand, and the dog, ape, or man
on the other. Yet all these insects belong to the same class of
Articulates, just as all the mammals belong to one and the same class.
And just as every consistent evolutionist must admit a common
stem-form for all these insects, so he must also for all the mammals.
If we now turn from the comparative study of psychic life in different
animals to the question of the organs of this function, we receive the
answer that in all the higher animals they are always bound up with
certain groups of cells, the ganglionic cells or neurona that compose
the nervous system. All scientists without exception are agreed that
the central nervous system is the organ of psychic life in the animal,
and it is possible to prove this experimentally at any moment. When we
partially or wholly destroy the central nervous system, we extinguish
in the same proportion, partially or wholly, the "soul" or psychic
activity of the animal. We have, therefore, to examine the features of
the psychic organ in man. The reader already knows the incontestable
answer to this question. Man's psychic organ is, in structure and
origin, just the same organ as in all the other Vertebrates. It
originates in the shape of a simple medullary tube from the outer
membrane of the embryo--the skin-sense layer. The simple cerebral
vesicle that is formed by the expansion of the head-part of this
medullary tube divides by transverse constrictions into five, and
these pass through more or less the same stages of construction in the
human embryo as in the rest of the mammals. As these are undoubtedly
of a common origin, their brain and spinal cord must also have a
common origin.
Physiology teaches us further, on the ground of observation and
experiment, that the relation of the "soul" to its organ, the brain
and spinal cord, is just the same in man as in the other mammals. The
one cannot act at all without the other; it is just as much bound up
with it as muscular movement is with the muscles. It can only develop
in connection with it. If we are evolutionists at all, and grant the
causal connection of ontogenesis and phylogenesis, we are forced to
admit this thesis: The human soul or psyche, as a function of the
medullary tube, has developed along with it; and just as brain and
spinal cord now develop from the simple medullary tube in every human
individual, so the human mind or the psychic life of the whole human
race has been gradually evolved from the lower vertebrate soul. Just
as to-day the intricate structure of the brain proceeds step by step
from the same rudiment in every human individual--the same five
cerebral vesicles--as in all the other Craniotes; so the human soul
has been gradually developed in the course of millions of years from a
long series of craniote-souls. Finally, just as to-day in every human
embryo the various parts of the brain differentiate after the special
type of the ape-brain, so the human psyche has proceeded historically
from the ape-soul.
It is true that this Monistic conception is rejected with horror by
most men, and the Dualistic idea, which denies the inseparable
connection of brain and mind, and regards body and soul as two totally
different things, is still popular. But how can we reconcile this view
with the known facts of evolution? It meets with difficulties equally
great and insuperable in embryology and in phylogeny. If we suppose
with the majority of men that the soul is an independent entity, which
has nothing to do with the body originally, but merely inhabits it for
a time, and gives expression to its experiences through the brain just
as the pianist does through his instrument, we must assign a point in
human embryology at which the soul enters into the brain; and at death
again we must assign a moment at which it abandons the body. As,
further, each human individual has inherited certain personal features
from each parent, we must suppose that in the act of conception pieces
were detached from their souls and transferred to the embryo. A piece
of the paternal soul goes with-the spermatozoon, and a piece of the
mother's soul remains in the ovum. At the moment of conception, when
portions of the two nuclei of the copulating cells join together to
form the nucleus of the stem-cell, the accompanying fragments of the
immaterial souls must also be supposed to coalesce.
On this Dualistic view the phenomena of psychic development are
totally incomprehensible. Everybody knows that the new-born child has
no consciousness, no knowledge of itself and the surrounding world.
Every parent who has impartially followed the mental development of
his children will find it impossible to deny that it is a case of
biological evolutionary processes. Just as all other functions of the
body develop in connection with their organs, so the soul does in
connection with the brain. This gradual unfolding of the soul of the
child is, in fact, so wonderful and glorious a phenomenon that every
mother or father who has eyes to observe is never tired of
contemplating it. It is only our manuals of psychology that know
nothing of this development; we are almost tempted to think sometimes
that their authors can never have had children themselves. The human
soul, as described in most of our psychological works, is merely the
soul of a learned philosopher, who has read a good many books, but
knows nothing of evolution, and never even reflects that his own soul
has had a development.
When these Dualistic philosophers are consistent they must assign a
moment in the phylogeny of the human soul at which it was first
"introduced" into man's vertebrate body. Hence, at the time when the
human body was evolved from the anthropoid body of the ape (probably
in the Tertiary period), a specific human psychic element--or, as
people love to say, "a spark of divinity"--must have been suddenly
infused or breathed into the anthropoid brain, and been associated
with the ape-soul already present in it. I need not insist on the
enormous theoretical difficulties of this idea. I will only point out
that this "spark of divinity," which is supposed to distinguish the
soul of man from that of the other animals, must be itself capable of
development, and has, as a matter of fact, progressively developed in
the course of human history. As a rule, reason is taken to be this
"spark of divinity," and is supposed to be an exclusive possession of
humanity. But comparative psychology shows us that it is quite
impossible to set up this barrier between man and the brute. Either we
take the word "reason" in the wider sense, and then it is found in the
higher mammals (ape, dog, elephant, horse) just as well as in most
men; or else in the narrower sense, and then it is lacking in most men
just as much as in the majority of animals. On the whole, we may still
say of man's reason what Goethe's Mephistopheles said:--
Life somewhat better might content him
But for the gleam of heavenly light that Thou hast given him.
He calls it reason; thence his power's increased
To be still beastlier than any beast.
If, then, we must reject these popular and, in some respects,
agreeable Dualistic theories as untenable, because inconsistent with
the genetic facts, there remains only the opposite or Monistic
conception, according to which the human soul is, like any other
animal soul, a function of the central nervous system, and develops in
inseparable connection therewith. We see this ontogenetically in every
child. The biogenetic law compels us to affirm it phylogenetically.
Just as in every human embryo the skin-sense layer gives rise to the
medullary tube, from the anterior end of which the five cerebral
vesicles of the Craniotes are developed, and from these the mammal
brain (first with the characters of the lower, then with those of the
higher mammals); and as the whole of this ontogenetic process is only
a brief, hereditary reproduction of the same process in the
phylogenesis of the Vertebrates; so the wonderful spiritual life of
the human race through many thousands of years has been evolved step
by step from the lowly psychic life of the lower Vertebrates, and the
development of every child-soul is only a brief repetition of that
long and complex phylogenetic process. From all these facts sound
reason must conclude that the still prevalent belief in the
immortality of the soul is an untenable superstition. I have shown its
inconsistency with modern science in the eleventh chapter of The
Riddle of the Universe.
Here it may also be well to point out the great importance of
anthropogeny, in the light of the biogenetic law, for the purposes of
philosophy. The speculative philosophers who take cognizance of these
ontogenetic facts, and explain them (in accordance with the law)
phylogenetically, will advance the great questions of philosophy far
more than the most distinguished thinkers of all ages have yet
succeeded in doing. Most certainly every clear and consistent thinker
must derive from the facts of comparative anatomy and ontogeny we have
adduced a number of suggestive ideas that cannot fail to have an
influence on the progress of philosophy. Nor can it be doubted that
the candid statement and impartial appreciation of these facts will
lead to the decisive triumph of the philosophic tendency that we call
"Monistic" or "Mechanical," as opposed to the "Dualistic" or
"Teleological," on which most of the ancient, medieval, and modern
systems of philosophy are based. The Monistic or Mechanical philosophy
affirms that all the phenomena of human life and of the rest of nature
are ruled by fixed and unalterable laws; that there is everywhere a
necessary causal connection of phenomena; and that, therefore, the
whole knowable universe is a harmonious unity, a monon. It says,
further, that all phenomena are due solely to mechanical or efficient
causes, not to final causes. It does not admit free-will in the
ordinary sense of the word. In the light of the Monistic philosophy
the phenomena that we are wont to regard as the freest and most
independent, the expressions of the human will, are subject just as
much to rigid laws as any other natural phenomenon. As a matter of
fact, impartial and thorough examination of our "free" volitions shows
that they are never really free, but always determined by antecedent
factors that can be traced to either heredity or adaptation. We
cannot, therefore, admit the conventional distinction between nature
and spirit. There is spirit everywhere in nature, and we know of no
spirit outside of nature. Hence, also, the common antithesis of
natural science and mental or moral science is untenable. Every
science, as such, is both natural and mental. That is a firm principle
of Monism, which, on its religious side, we may also denominate
Pantheism. Man is not above, but in, nature.
It is true that the opponents of evolution love to misrepresent the
Monistic philosophy based on it as "Materialism," and confuse the
philosophic tendency of this name with a wholly unconnected and
despicable moral materialism. Strictly speaking, it would be just as
proper to call our system Spiritualism as Materialism. The real
Materialistic philosophy affirms that the phenomena of life are, like
all other phenomena, effects or products of matter. The opposite
extreme, the Spiritualistic philosophy, says, on the contrary, that
matter is a product of energy, and that all material forms are
produced by free and independent forces. Thus, according to one-sided
Materialism, the matter is antecedent to the living force; according
to the equally one-sided view of the Spiritist, it is the reverse.
Both views are Dualistic, and, in my opinion, both are false. For us
the antithesis disappears in the Monistic philosophy, which knows
neither matter without force nor force without matter. It is only
necessary to reflect for some time over the question from the strictly
scientific point of view to see that it is impossible to form a clear
idea of either hypothesis. As Goethe said, "Matter can never exist or
act without spirit, nor spirit without matter."
The human "spirit" or "soul" is merely a force or form of energy,
inseparably bound up with the material sub-stratum of the body. The
thinking force of the mind is just as much connected with the
structural elements of the brain as the motor force of the muscles
with their structural elements. Our mental powers are functions of the
brain as much as any other force is a function of a material body. We
know of no matter that is devoid of force, and no forces that are not
bound up with matter. When the forces enter into the phenomenon as
movements we call them living or active forces; when they are in a
state of rest or equilibrium we call them latent or potential. This
applies equally to inorganic and organic bodies. The magnet that
attracts iron filings, the powder that explodes, the steam that drives
the locomotive, are living inorganics; they act by living force as
much as the sensitive Mimosa does when it contracts its leaves at
touch, or the venerable Amphioxus that buries itself in the sand of
the sea, or man when he thinks. Only in the latter cases the
combinations of the different forces that appear as "movement" in the
phenomenon are much more intricate and difficult to analyse than in
the former.
Our study has led us to the conclusion that in the whole evolution of
man, in his embryology and in his phylogeny, there are no living
forces at work other than those of the rest of organic and inorganic
nature. All the forces that are operative in it could be reduced in
the ultimate analysis to growth, the fundamental evolutionary function
that brings about the forms of both the organic and the inorganic. But
growth itself depends on the attraction and repulsion of homogeneous
and heterogeneous particles. Seventy-five years ago Carl Ernst von
Baer summed up the general result of his classic studies of animal
development in the sentence: "The evolution of the individual is the
history of the growth of individuality in every respect." And if we go
deeper to the root of this law of growth, we find that in the long run
it can always be reduced to that attraction and repulsion of animated
atoms which Empedocles called the "love and hatred" of the elements.
Thus the evolution of man is directed by the same "eternal, iron laws"
as the development of any other body. These laws always lead us back
to the same simple principles, the elementary principles of physics
and chemistry. The various phenomena of nature only differ in the
degree of complexity in which the different forces work together. Each
single process of adaptation and heredity in the stem-history of our
ancestors is in itself a very complex physiological phenomenon. Far
more intricate are the processes of human embryology; in these are
condensed and comprised thousands of the phylogenetic processes.
In my General Morphology, which appeared in 1866, I made the first
attempt to apply the theory of evolution, as reformed by Darwin, to
the whole province of biology, and especially to provide with its
assistance a mechanical foundation for the science of organic forms.
The intimate relations that exist between all parts of organic
science, especially the direct causal nexus between the two sections
of evolution--ontogeny and phylogeny--were explained in that work for
the first time by transformism, and were interpreted philosophically
in the light of the theory of descent. The anthropological part of the
General Morphology (Book 7) contains the first attempt to determine
the series of man's ancestors (volume 2 page 428). However imperfect
this attempt was, it provided a starting-point for further
investigation. In the thirty-seven years that have since elapsed the
biological horizon has been enormously widened; our empirical
acquisitions in paleontology, comparative anatomy, and ontogeny have
grown to an astonishing extent, thanks to the united efforts of a
number of able workers and the employment of better methods. Many
important biological questions that then appeared to be obscure
enigmas seem to be entirely settled. Darwinism arose like the dawn of
a new day of clear Monistic science after the dark night of mystic
dogmatism, and we can say now, proudly and gladly, that there is
daylight in our field of inquiry.
Philosophers and others, who are equally ignorant of the empirical
sources of our evidence and the phylogenetic methods of utilising it,
have even lately claimed that in the matter of constructing our
genealogical tree nothing more has been done than the discovery of a
"gallery of ancestors," such as we find in the mansions of the
nobility. This would be quite true if the genealogy given in the
second part of this work were merely the juxtaposition of a series of
animal forms, of which we gathered the genetic connection from their
external physiognomic resemblances. As we have sufficiently proved
already, it is for us a question of a totally different thing--of the
morphological and historical proof of the phylogenetic connection of
these ancestors on the basis of their identity in internal structure
and embryonic development; and I think I have sufficiently shown in
the first part of this work how far this is calculated to reveal to us
their inner nature and its historical development. I see the essence
of its significance precisely in the proof of historical connection. I
am one of those scientists who believe in a real "natural history,"
and who think as much of an historical knowledge of the past as of an
exact investigation of the present. The incalculable value of the
historical consciousness cannot be sufficiently emphasised at a time
when historical research is ignored and neglected, and when an "exact"
school, as dogmatic as it is narrow, would substitute for it physical
experiments and mathematical formulae. Historical knowledge cannot be
replaced by any other branch of science.
It is clear that the prejudices that stand in the way of a general
recognition of this "natural anthropogeny" are still very great;
otherwise the long struggle of philosophic systems would have ended in
favour of Monism. But we may confidently expect that a more general
acquaintance with the genetic facts will gradually destroy these
prejudices, and lead to the triumph of the natural conception of
"man's place in nature." When we hear it said, in face of this
expectation, that this would lead to retrogression in the intellectual
and moral development of mankind, I cannot refrain from saying that,
in my opinion, it will be just the reverse; that it will promote to an
enormous extent the advance of the human mind. All progress in our
knowledge of truth means an advance in the higher cultivation of the
human intelligence; and all progress in its application to practical
life implies a corresponding improvement of morality. The worst
enemies of the human race--ignorance and superstition--can only be
vanquished by truth and reason. In any case, I hope and desire to have
convinced the reader of these chapters that the true scientific
comprehension of the human frame can only be attained in the way that
we recognise to be the sole sound and effective one in organic science
generally--namely, the way of Evolution.
INDEX.
Abiogenesis.
Accipenser.
Abortive ova.
Achromatin.
Achromin.
Acoela.
Acoustic nerve, the.
Acquired characters, inheritance of.
Acrania, the.
Acroganglion, the.
Adam's apple, the.
Adapida.
Adaptation.
After-birth, the.
Agassiz, L.
Age of life.
Alimentary canal, evolution of the.
structure of the.
Allantoic circulation, the.
Allantois, development of the.
Allmann.
Amblystoma.
Amitotic cleavage.
Ammoconida.
Ammolynthus.
Amnion, the.
formation of the.
Amniotic fluid, the.
Amoeba, the.
Amphibia, the.
Amphichoerus.
Amphigastrula.
Amphioxus, the.
circulation of the.
coelomation of the.
embryology of the.
structure of the.
Amphirhina.
Anamnia, the.
Anatomy, comparative.
Animalculists.
Animal layer, the.
Annelids, the.
Annelid theory, the.
Anomodontia.
Ant, intelligence of the.
Anthropithecus.
Anthropogeny.
Anthropoid apes, the.
Anthropology.
Anthropozoic period.
Antimera.
Anura.
Anus, the.
Anus, formation of the.
Aorta, the.
development of the.
Ape and man.
Ape-man, the.
Apes, the.
Aphanocapsa.
Aphanostomum.
Appendicaria.
Appendix vermiformis, the.
Aquatic life, early prevalence of.
Ararat, Mount.
Archenteron.
Archeolithic age.
Archicaryon.
Archicrania.
Archigastrula.
Archiprimas.
Arctopitheca.
Area, the germinative.
Aristotle.
Arm, structure of the.
Arrow-worm, the.
Arterial arches, the.
cone, the.
Arteries, evolution of the.
Articulates, the.
skeleton of the.
Articulation.
Aryo-Romanic languages, the.
Ascidia, the.
embryology of the.
Ascula.
Asexual reproduction.
Atlas, the.
Atrium, the.
(heart), the.
Auditory nerve, the.
Auricles of the heart.
Autolemures.
Axolotl, the.
Bacteria.
Baer, K.E. von.
Balanoglossus.
Balfour, F.
Batrachia.
Bdellostoma Stouti.
Bee, generation of the.
Beyschlag, W., on evolution.
Bilateral symmetry.
origin of.
Bimana.
Biogenetic law, the.
Biogeny.
Bionomy.
Bird, evolution of the.
ovum of the.
Bischoff, W.
Bladder, evolution of the.
Blastaea, the.
Blastocoel, the.
Blastocrene, the.
Blastocystis, the.
Blastoderm, the.
Blastodermic vesicle, the.
Blastoporus, the.
Blastosphere, the.
Blastula, the.
the mammal.
Blood, importance of the.
recent experiments in mixture of.
structure of the.
Blood-cells, the.
Blood-vessels, the.
development of the.
of the vertebrate.
origin of the.
Boniface VIII, Bull of.
Bonnet.
Borneo nosed-ape, the.
Boveri, Theodor.
Brachytarsi.
Brain and mind.
evolution of the.
in the fish.
in the lower animals.
structure of the.
Branchial arches, evolution of the.
cavity, the.
system, the.
Branchiotomes.
Breasts, the.
Bulbilla.
Calamichthys.
Calcolynthus.
Capillaries, the.
Caracoideum, the.
Carboniferous strata.
Carcharodon.
Cardiac cavity, the.
Cardiocoel, the.
Catallacta.
Caryobasis.
Caryokinesis.
Caryolymph.
Caryolyses.
Caryon.
Caryoplasm.
Catarrhinae, the.
Catastrophic theory, the.
Caudate cells.
Cell, life of the.
nature of the.
size of the.
Cell theory, the.
Cenogenesis.
Cenogenetic structures.
Cenozoic period, the.
Central body, the.
Central nervous system, the.
Centrolecithal ova.
Centrosoma, the.
Ceratodus, the.
Cerebellum, the.
Cerebral vesicles, evolution of the.
Cerebrum, the.
Cestracion Japonicus.
Chaetognatha.
Chick, importance of the, in embryology.
Child, mind of the.
Chimpanzee, the.
Chiromys.
Chiroptera.
Chirotherium.
Chondylarthra.
Chorda, the.
evolution of the.
Chordaea, the.
Chordalemma, the.
Chordaria.
Chordula, the.
Choriata, the.
Chorion, the.
development of the.
frondosum.
laeve.
Choroid coat, the.
Chorology.
Chromacea.
Chromatin.
Chroococcacea.
Chroococcus, the.
Church, opposition of, to science in Middle Ages.
Chyle.
Chyle-vessels.
Cicatricula, the.
Ciliated cells.
Cinghalese gynecomast.
Circulation in the lancelet.
Circulatory system, evolution of the.
structure of the.
Classification.
evolutionary value of.
Clitoris, the.
Cloaca, the.
Cnidaria.
Coccyx, the.
Cochineal insect, the.
Cochlea, the.
Coecilia.
Coecum [Caecum], the.
Coelenterata.
Coelenteria.
Coeloma, the.
Coelomaea, the.
Coelomaria.
Coelomation.
Coelom-theory, the.
Coelomula, the.
Colon, the.
Comparative anatomy.
Conception, nature of.
Conjunctiva, the.
Conocyema.
Convoluta.
Copelata, the.
Copulative organs, evolution of the.
Corium, the.
Cornea, the.
Corpora cavernosa, the.
Corpora quadrigemina.
Corpora striata.
Corpus callosum, the.
Corpus vitreum, the.
Corpuscles of the blood.
Craniology.
Craniota, the.
Cranium, the.
Creation.
Cretaceous strata.
Crossopterygii.
Crustacea, the.
Cryptocoela.
Cryptorchism.
Crystalline lens, the.
development of the.
Cutaneous glands.
Cuttlefish, embryology of the.
Cuvier, G.
Cyanophycea.
Cyclostoma, the.
ova of the.
Cyemaria.
Cynopitheca.
Cynthia.
Cytoblastus, the.
Cytodes.
Cytoplasm.
Cytosoma.
Cytula, the.
Dalton.
Darwin, C.
Darwin, E.
Darwinism.
Decidua, the.
Deciduata.
Deduction, nature of.
Degeneration theory, the.
Dentition of the ape and man.
Depula.
Descent of Man.
Design in organisms.
Deutoplasm.
Devonian strata.
Diaphragm, the.
evolution of the.
Dicyema.
Dicyemida.
Didelphia.
Digonopora.
Dinosauria.
Dipneumones.
Dipneusta.
ova of the.
Dipnoa.
Directive bodies.
Discoblastic ova.
Discoplacenta.
Dissatyrus.
Dissection, medieval decrees against.
Dohrn, Anton.
Dollinger.
Dorsal furrow, the.
shield, the.
zone, the.
Dromatherium.
Dualism.
Dubois, Eugen.
Ductus Botalli, the.
Ductus venosus Arantii.
Duodenum, the.
Duration of embryonic development.
of man's history.
Dysteleology.
proofs of.
Ear, evolution of the.
structure of the.
uselessness of the external.
Ear-bones, the.
Earth, age of the.
Echidna hystrix.
Ectoblast.
Ectoderm, the.
Edentata.
Efficient causes.
Egg of the bird.
or the chick, priority of the.
Elasmobranchs, the.
Embryo, human, development of the.
Embryology.
evolutionary value of.
Embryonic development, duration of.
disk, the.
spot, the.
Encephalon, the.
Endoblast.
Endothelia.
Enterocoela.
Enteropneusta.
Entoderm, the.
Eocene strata.
Eopitheca.
Epiblast.
Epidermis, the.
Epididymis, the.
Epigastrula.
Epigenesis.
Epiglottis, the.
Epiphysis, the.
Episoma.
Episomites.
Epispadia.
Epithelia.
Epitheria.
Epovarium, the.
Equilibrium, sense of.
Esthonychida.
Eustachian tube, the.
Eutheria.
Eve.
Evolution theory, the.
inductive nature of.
Eye, evolution of the.
structure of the.
Eyelid, the third.
Eyelids, evolution of the.
Fabricius ab Aquapendente
Face, embryonic development of the.
Fat glands in the skin.
Feathers, evolution of.
Fertilisation.
place of.
Fin, evolution of the.
Final causes.
Flagellate cells.
Floating bladder, the.
evolution of the.
Foetal circulation.
Food-yelk, the.
Foot, evolution of the.
of the ape and man.
Fore brain, the.
Fore kidneys, the.
Fossiliferous strata, list of.
Fossils.
scarcity of.
Free will.
Friedenthal, experiments of.
Frog, the.
ova of the.
Frontonia.
Function and structure.
Furcation of ova.
Gaertner's duct.
Ganglia, commencement of.
Ganglionic cell, the.
Ganoids.
Gastraea, the.
formation of the.
Gastraea theory, the.
Gastraeads.
Gastremaria.
Gastrocystis, the.
Gastrophysema.
Gastrotricha.
Gastrula, the.
Gastrulation.
Gegenbaur, Carl.
on evolution.
on the skull.
Gemmation.
General Morphology.
Genesis.
Genital pore, the.
Geological evolution, length of.
periods.
Geology, methods of.
rise of.
Germ-plasm, theory of.
Germinal disk.
layers, the.
scheme of the.
spot, the.
vesicle, the.
Germinative area, the.
Giant gorilla, the.
Gibbon, the.
Gill-clefts and arches.
formation of the.
Gill-crate, the.
Gills, disappearance of the.
Gloeocapsa.
Gnathostoma.
Goethe as an evolutionist.
Goitre.
Gonads, the.
formation of the.
Gonidia.
Gonochorism, beginning of.
Gonoducts.
Gonotomes.
Goodsir.
Gorilla, the.
Graafian follicles, the.
Gregarinae.
Gullet-ganglion, the.
Gut, evolution of the.
Gyrini.
Gynecomastism.
Hag-fish, the.
Hair, evolution of the.
on the human embryo and infant.
Hair, restriction of, by sexual selection.
Haliphysema.
Halisauria.
Haller, Albrecht.
Halosphaera viridis.
Hand, evolution of the.
of the ape and man.
Hapalidae.
Harderian gland, the.
Hare-lip.
Harrison, Granville.
Hartmann.
Harvey.
Hatschek.
Hatteria.
Head-cavity, the.
Head-plates, the.
Heart, development of the.
of the ascidia.
position of the.
Helmholtz.
Helminthes.
Hepatic gut, the.
Heredity, nature of.
Hermaphrodism.
Hertwig.
Hesperopitheca.
His, W.
Histogeny.
History of Creation.
Holoblastic ova.
Homoeosaurus.
Homology of the germinal layers.
Hoof, evolution of the.
Hunterian ligament, the.
Huxleian law, the.
Huxley, T.H.
Hydra, the.
Hydrostatic apparatus in the fish.
Hylobates.
Hylodes Martinicensis.
Hyoid bone, the.
Hypermastism.
Hyperthelism.
Hypoblast.
Hypobranchial groove, the.
Hypodermis, the.
Hypopsodina.
Hyposoma, the.
Hyposomites.
Hypospadia.
Ichthydina.
Ichthyophis glutinosa.
Ictopsida.
Ileum, the.
Immortality, Aristotle on.
Immortality of the soul.
Impregnation-rise, the.
Indecidua.
Indo-Germanic languages.
Induction and deduction.
Inheritance of acquired characters.
Insects, intelligence of.
Interamniotic cavity, the.
Intestines, the.
Invagination.
Iris, the.
Jacchus.
Java, ape-man of.
Jaws, evolution of the.
Jurassic strata.
Kant, dualism of.
Kelvin, Lord, on the origin of life.
Kidneys, the.
formation of the.
Klaatsch.
Kolliker.
Kowalevsky.
Labia, the.
Labyrinth, the.
Lachrymal glands.
Lamarck, J.
theories of.
Lamprey, the.
ova of the.
Lancelet, the.
description of the.
Languages, evolution of.
Lanugo of the embryo.
Larynx, the.
evolution of the.
Latebra, the.
Lateral plates, the.
Laurentian strata.
Lecithoma, the.
Leg, evolution of the.
structure of the.
Lemuravida.
Lemurogona.
Lemurs, the.
Lepidosiren.
Leucocytes.
Life, age of.
Limbs, evolution of the.
Limiting furrow, the.
Linin.
Liver, the.
Long-nosed ape, the.
Love, importance of in nature.
Lungs, the.
evolution of the.
Lyell, Sir C.
Lymphatic vessels, the.
Lymph-cells, the.
Macrogonidion.
Macrospores.
Magosphaera planula.
Male womb, the.
Mallochorion, the.
Mallotheria.
Malpighian capsules.
Mammal, characters of the.
gastrulation of the.
Mammals, unity of the.
Mammary glands, the.
Man and the ape, relation of.
origin of.
Man's Place in Nature.
Mantle, the.
Mantle-folds, the.
Marsupials, the.
ova of the.
Materialism.
Mathematical method, the.
Mechanical causes.
embryology.
Meckel's cartilage.
Medulla capitis, the.
oblongata, the.
spinalis, the.
Medullary groove, the.
tube, the.
formation of the.
Mehnert, E., on the biogenetic law.
Meroblastic ova.
Merocytes.
Mesentery, the.
Mesocardium, the.
Mesoderm, the.
Mesogastria.
Mesonephridia, the.
Mesonephros, the.
Mesorchium, the.
Mesovarium, the.
Mesozoic period, the.
Metogaster, the.
Metagastrula, the.
Metamerism.
Metanephridia, the.
Metanephros, the.
Metaplasm.
Metastoma.
Metatheria.
Metazoa.
Metovum, the.
Microgonidian.
Microspores.
Middle ear, the.
Migration, effect of.
Milk, secretion of the.
Mind, evolution of.
in the lower animals.
Miocene strata.
Mitosis.
Monera.
Monism.
Monodelphia.
Monogonopora.
Monopneumones.
Monotremes.
ova of the.
Monoxenia Darwinii.
Morea, the.
Morphology.
Morula, the.
Motor-germinative layer, the.
Mouth, development of the.
structure of the.
Mucous layer, the.
Mullerian duct, the.
Muscle-layer, the.
Muscles, evolution of the.
of the ear, rudimentary.
Myotomes.
Myxinoides, the.
Nails, evolution of the.
Nasal pits.
Natural philosophy.
selection.
Navel, the.
Necrolemurs.
Nectocystis, the.
Nemertina.
Nephroduct, evolution of the.
Nephrotomes.
Nerve-cell, the.
Nerves, animals without.
Nervous system, evolution of the.
Neurenteric canal, the.
Nictitating membrane, the.
Nose, the, in man and the ape.
development of the.
structure of the.
Notochorda, the.
Nuclein.
Nucleolinus.
Nucleolus, the.
Nucleus of the cell.
Oesophagus, the.
Oken.
Oken's bodies.
Oligocene strata.
Olynthus.
On the generation of animals.
Ontogeny.
defective evidence of.
Opaque area, the.
Opossum, the.
ova of the.
Optic nerve, the.
Optic thalami.
vesicles.
Orang, the.
Ornithodelphia.
Ornithorhyncus.
Ornithostoma.
Ossicles of the ear.
Otoliths.
Ova, number of.
of the lancelet.
Ovaries, evolution of the.
Oviduct, origin of the.
Ovolemma, the.
Ovulists.
Ovum, discovery of the.
nature of the.
size of the.
Pachylemurs, the.
Pacinian corpuscles.
Paleontology.
evolutionary evidence of.
incompleteness of.
rise of.
Paleozoic age, the.
Palingenesis.
Palingenetic structures.
Palaehatteria.
Panniculus carnosus, the.
Paradidymis, the.
Parietal zone, the.
Parthenogenesis.
Pastrana, Miss Julia.
Pedimana.
Pellucid area, the.
Pelvic cavity, the.
Pemmatodiscus gastrulaceus.
Penis-bone, the.
Penis, varieties of the.
Peramelida.
Periblastic ova.
Peribranchial cavity, the.
Pericardial cavity, the.
Perichorda, the.
formation of the.
Perigastrula.
Permian strata.
Petromyzontes, the.
Phagocytes.
Pharyngeal ganglion, the.
Pharynx, the.
Philology, comparison with.
Philosophie Zoologique.
Philosophy and evolution.
Phycochromacea.
Phylogeny.
Physemaria.
Physiology, backwardness of.
Phytomonera.
Pineal eye, the.
Pinna, the.
Pithecanthropus.
Pithecometra-principle, the.
Placenta, the.
Placentals, the.
characters of the.
gastrulation of the.
Planocytes.
Plant-louse, parthenogenesis of the.
Planula, the.
Plasma-products.
Plasson.
Plastids.
Plastidules.
Platodaria.
Platodes, the.
Platyrrhinae.
Pleuracanthida.
Pleural ducts.
Pliocene strata.
Polar cells.
Polyspermism.
Preformation theory, the.
Primary period, the.
Primates, the.
Primatoid.
Primitive groove, the.
gut, the.
kidneys, the.
mouth, the.
segments.
streak, the.
vertebrae.
Primordial period, the.
Prochordata.
Prochordonia, the.
Prochoriata.
Prochorion, the.
Proctodaeum, the.
Procytella primordialis.
Prodidelphia.
Progaster, the.
Progonidia.
Promammalia.
Pronephridia, the.
Pronucleus femininus.
masculinus.
Properistoma.
Prorenal canals of the lancelet.
duct, the.
evolution of the.
Proselachii.
Prosimiae, the.
Prospermaria.
Prospondylus.
Prostoma.
Protamniotes.
Protamoeba.
Proterosaurus, the.
Protists.
Protonephros.
Protophyta.
Protoplasm.
Protopterus.
Prototheria.
Protovertebrae.
Protozoa.
Provertebral cavity, the.
plates, the.
Pseudocoela.
Pseudopodia.
Pseudova.
Psychic life, evolution of the.
Psychology.
Pterosauria.
Pylorus, the.
Quadratum, the.
Quadrumana.
Quaternary period.
Rabbit, ova of the.
Radiates, the.
Rathke's canals.
Rectum, the.
Regner de Graaf.
Renal system, evolution of the.
Reproduction, nature of.
Reptiles.
Respiratory organs, evolution of the.
pore, the.
Retina, the.
Rhabdocoela.
Rhodocytes.
Rhopalura.
Rhyncocephala.
Ribs, the.
number of the.
Rudimentary ear-muscles.
organs.
list of.
toes.
Sacculus, the.
Sagitta.
coelomation of.
Salamander, the.
ova of the.
Sandal-shape of embryo.
Satyrus.
Sauromammalia.
Sauropsida.
Scatulation theory, the.
Schizomycetes.
Schleiden, M.
Schwann, T.
Sclerotic coat, the.
Sclerotomes.
Scrotum, the.
Scyllium, nose of the.
Sea-squirt, the.
Secondary period, the.
Segmentation.
Segmentation-cells.
Segmentation-sphere, the.
Selachii.
skull of the.
Selection, theory of.
Selenka.
Semnopitheci.
Sense-organs, evolution of the.
number of the.
origin of the.
Sensory nerves.
Serocoelom, the.
Serous layer, the.
Sex-organs, early vertebrate form of the.
evolution of the.
Sexual reproduction, simplest forms of.
selection.
Shark, the.
nose of the.
ova of the.
placenta of the.
skull of the.
Shoulder-blade, the.
Sickle-groove, the.
Sieve-membrane, the.
Silurian strata.
Simiae, the.
Siphonophorae, embryology of the.
Skeleton, structure of the.
Skeleton-plate, the.
Skin, the.
evolution of.
function of the.
Skin-layer, the.
Skull, evolution of the.
structure of the.
vertebral theory of the.
Smell, the sense of.
Soul, evolution of the.
nature of the.
phylogeny of the.
seat of the.
Sound, sensations of.
Sozobranchia.
Space, sense of.
Species, nature of the.
Speech, evolution of.
Spermaducts.
Spermaries, evolution of the.
Spermatozoon, the.
discovery of the.
Spinal cord, development of the.
structure of the.
Spirema, the.
Spiritualism.
Spleen, the.
Spondyli.
Sponges, classification of the.
ova of the.
Spontaneous generation.
Stegocephala.
Stem-cell, the.
Stem-zone, the.
Stomach, evolution of the.
structure of the human.
Strata, thickness of.
Struggle for life, the.
Subcutis, the.
Sweat glands.
Tactile corpuscles.
Tadpole, the.
Tail, evolution of the.
rudimentary, in man.
Tailed men.
Taste, the sense of.
Teeth, evolution of the.
--of the ape and man.
Teleostei.
Telolecithal ova.
Temperature, sense of.
Terrestrial life, beginning of.
Tertiary period, the.
Theoria generationis, the.
Theories, value of.
Theromorpha.
Third eyelid, the.
Thyroid gland, the.
Time-variations in ontogeny.
Tissues, primary and secondary.
Toad, the.
Tocosauria.
Toes, number of the.
Tori genitales, the.
Touch, the sense of.
Tracheata.
Tread, the.
Tree-frog, the.
Triassic strata.
Triton taeniatus.
Troglodytes.
Tunicates, the.
Turbellaria.
Turbinated bones, the.
Tympanic cavity, the.
Umbilical, cord, the.
vesicle, the.
Unicellular ancestor of all animals.
--animals.
Urachus, the.
Urinary system, evolution of the.
Urogenital ducts.
Uterus masculinus, the.
Utriculus, the.
Vasa deferentia.
Vascular layer, the.
system, evolution of the.
structure of the.
Vegetative layer, the.
Veins, evolution of the.
Ventral pedicle, the.
Ventricles of the heart.
Vermalia.
Vermiform appendage, the.
Vertebrae.
Vertebraea.
Vertebral arch, the.
column, the.
evolution of the.
structure of the.
Vertebrates, character of the.
descent of the.
Vertebration.
Vesico-umbilical ligament, the.
Vesicula prostatica, the.
Villi of the chorion.
Virchow, R.
on the ape-man.
on the evolution of man.
Virgin-birth.
Vitalism.
Vitelline duct, the.
Volvocina.
Wallace, A.R.
Water, organic importance of.
Water vessels.
Weismann's theories.
Wolff, C.F.
Wolffian bodies.
Wolffian duct, the.
Womb, evolution of the.
Yelk, the.
Yelk-sac, the.
Zona pellucida, the.
Zonoplacenta.
Zoomonera.
Zoophytes.
End of Project Gutenberg's The Evolution of Man, V.2, by Ernst Haeckel