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The Project Gutenberg EBook of The Last Link, by Ernst Haeckel
This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever. You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org
Title: The Last Link
Our Present Knowledge of the Descent of Man
Author: Ernst Haeckel
Commentator: Hans Gadow
Release Date: December 29, 2013 [EBook #44541]
Language: English
Character set encoding: ISO-8859-1
*** START OF THIS PROJECT GUTENBERG EBOOK THE LAST LINK ***
Produced by Chris Curnow, Les Galloway and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
THE LAST LINK
OUR PRESENT KNOWLEDGE OF THE
DESCENT OF MAN
BY
ERNST HAECKEL
(JENA)
WITH NOTES AND BIOGRAPHICAL SKETCHES
BY
HANS GADOW, F.R.S.
(CAMBRIDGE)
LONDON
ADAM AND CHARLES BLACK
1898
CONTENTS
PAGE
THE LAST LINK
INTRODUCTORY 1
COMPARATIVE ANATOMY 8
PALÆONTOLOGY 20
OTHER EVIDENCE 42
STAGES RECAPITULATED 47
BIOGRAPHICAL SKETCHES:
LAMARCK, SAINT-HILAIRE, CUVIER, BAER,
MUELLER, VIRCHOW, COPE, KOELLIKER, GEGENBAUR,
HAECKEL 80
THEORY OF CELLS 115
FACTORS OF EVOLUTION 117
GEOLOGICAL TIME AND EVOLUTION 135
NOTE
The address I delivered on August 26 at the Fourth International
Congress of Zoology at Cambridge, 'On our Present Knowledge of the
Descent of Man,' has, I find, from the high significance of the theme
and the general importance of the questions connected with it, excited
much interest, and has led to requests for its publication. Hence this
volume, edited by my friend Dr. H. Gadow, my pupil in earlier days,
who has not only revised the text, but has also enriched it by many
valuable additions and notes.
ERNST HAECKEL.
_Jena, December, 1898._
THE LAST LINK
At the end of the nineteenth century, the age of 'natural science,' the
department of knowledge that has made most progress is zoology. From
zoology has arisen the study of transformism, which now dominates the
whole of biology. Lamarck[1] laid its foundation in 1809, and forty
years ago Charles Darwin obtained for it a recognition which is now
universal. It is not my task to repeat the well-known principles of
Darwinism. I am not concerned to explain the scientific value of the
whole theory of descent. The whole of our biological study is pervaded
by it. No general problem in zoology and botany, in anatomy and
physiology, can be discussed without the question arising, How has this
problem originated? What are the real causes of its development?
[1] See note, p. 80.
This question was almost unknown seventy years ago, when Charles
Darwin, the great reformer of biology, began his academical career at
Cambridge as a student of theology. In the same year, 1828, Carl Ernst
von Baer[2] published in Germany his classical work on the embryology
of animals, the first successful attempt to elucidate by 'observation
and reflection' the mysterious origin of the animal body from the
egg, and to explain in every respect the 'history of the growing
individuality.' Darwin at that time had no knowledge of this great
advance, and he could not divine that forty years later embryology
would be one of the strongest supports of his own life's work--of that
very theory of transformism which, founded by Lamarck in the year of
Darwin's birth, was accepted with enthusiasm by Charles's grandfather
Erasmus. There is no doubt that of all the celebrated naturalists of
the nineteenth century Darwin achieved the greatest success, and we
should be justified in designating the last forty years as the Age of
Darwin.
[2] See note, p. 89.
In searching for the causes of this unexampled success, we must clearly
separate three sets of considerations: first, the comprehensive reform
of Lamarck's transformism, and its firm establishment by the many
arguments drawn from modern biology; secondly, the construction of the
new theory of selection, as established by Darwin, and independently
by Alfred Wallace (a theory called Darwinism in the proper sense);
thirdly, the deduction of anthropogeny, that most important conclusion
of the theory of descent, the value of which far surpasses all the
other truths in evolution.
It is the third point of Darwin's theory that I shall discuss here; and
I shall discuss it chiefly with the intention of examining critically
the evidence and the different conclusions which at present represent
our scientific knowledge of the descent of man and of the different
stages of his animal pedigree.
It is now generally admitted that this problem is the most important
of all biological questions. Huxley was right when in 1863 he called
it the question of questions for mankind. The problem which underlies
all others, and is more deeply interesting than any other, is as
to the place which man occupies in nature and his relations to the
universe of things. 'Whence our race has come; what are the limits of
our power over nature, and of nature's power over us; to what goal are
we tending--these are the problems which present themselves anew and
with undiminished interest to every man born into the world.' This
impressive view was explained by Huxley thirty-five years ago in his
three celebrated essays on 'Evidence as to Man's Place in Nature.' The
first is entitled 'On the Natural History of the Man-like Apes'; the
second, 'On the Relations of Man to the Lower Animals'; the third, 'On
some Fossil Remains of Man.' Darwin himself felt the burden of these
problems as much as Huxley; but in his chief work, 'On the Origin of
Species,' in 1859, he had purposely only just touched them, suggesting
that the theory of descent would shed light upon the origin of man and
his history. Twelve years later, in his celebrated work on 'The Descent
of Man, and Selection in Relation to Sex,' Darwin discussed fully and
ingeniously all the different sides of this 'question of questions'
from the morphological, historical, physiological, and psychological
points of view. As early as 1866 I myself had applied in the _Generelle
Morphologie der Organismen_ the theory of transformism to anthropology,
and had shown that the fundamental law of biogeny claims the same
value for man as for all the other animals. The intimate causal
connection between ontogeny and phylogeny, between the development of
the individual and the history of its ancestors, enables us to gain
a safe and certain knowledge of our ancestral series. I had at that
time distinguished in this series ten chief degrees of vertebrate
organization. I attributed the highest importance to the logical
connection of anthropogeny with transformism. If the latter be true,
the truth of the former is absolute. 'Our theory that man is descended
from lower vertebrates, and immediately from apes or primates, is a
case of special _deduction_ which follows with absolute certainty from
the general _induction_ of the theory of descent.' The full proof and
detailed explanation of this view was afterwards given in my 'History
of Natural Creation,' and especially in my 'Anthropogeny.'[3] Lastly,
it has received an ample scientific and critical foundation in the
third part of my 'Systematic Phylogeny.'[3]
[3] See notes, pp. 102, 106
During the forty years which have elapsed since Darwin's first
publication of his theories an enormous literature, discussing the
_general problems_ of transformism as well as its special application
to man, has been published. In spite of the wide divergence of the
different views, all agree in one main point: the natural development
of man cannot be separated from general transformism. There are only
two possibilities. Either all the various species of animals and
plants have been created independently by supernatural forces (and
in this case the creation of man also is a miracle); or the species
have been produced in a natural way by transmutation, by adaptation
and progressive heredity (and in this case man also is descended from
other vertebrates, and immediately from a series of primates). We are
absolutely convinced that only the latter theory is fully scientific.
To prove its truth, we have to examine critically the strength of the
different arguments claimed for it.
I.
First, we have to consider the relative place which comparative
anatomy concedes to man in the 'natural system' of animals, for the
true value of our 'natural classification' is based upon its meaning
as a pedigree. All the minor and major groups of the system--the
classes, legions, orders, families, genera, and species--are only
different branches of the same pedigree. For man himself, his place
in the pedigree has been fixed since Lamarck,[4] in 1801, defined the
group of vertebrates. The most perfect[5] of these are the Mammalia;
and at the head of this class stands the order of Primates, in which
Linnæus, in 1735, united four 'genera'--Homo, Simia, Lemur, and
Vespertilio. If we exclude the last-named, the Chiroptera of modern
zoology, there remain three natural groups of Primates--the Lemures,
the Simiæ, and the Anthropi or Hominidæ. This is the classification of
the majority of zoologists; but if we compare man with the two chief
groups of monkeys--the Eastern monkeys (or Catarrhinæ) and the Western
or American monkeys (Platyrrhinæ)--there can be no doubt that the
former group is much more closely related to man than is the latter.
In the natural order of the Catarrhinæ we find united a long series
of lower and higher forms. The lowest, the Cynopitheci, appear still
closely related to the Platyrrhinæ and to the Lemures; while, on the
other hand, the tailless apes (Anthropomorphæ) approach man through
their higher organization. Hence one of our best authorities on the
Primates, Robert Hartmann,[6] proposed to subdivide the whole order of
the Simiæ into three groups: (1) Primarii, man together with the other
Anthropomorphæ, or tailless apes; (2) Simiæ, all the other monkeys; (3)
Prosimiæ, or Lemurs. This arrangement has received strong support from
the interesting discovery by Selenka that the peculiar placentation
of the human embryo is the same as in the great apes, and different
from that of all the other monkeys. Our choice between these different
classifications of Primates is best determined by the important thesis
of Huxley, in which, in 1863, he carried out a most careful and
critical comparison of all the anatomical gradations within this order.
In my opinion, this ingenious thesis--which I have called the Huxleyan
Law, or the 'Pithecometra-thesis of Huxley'--is of the utmost value.
It runs as follows: 'Thus, whatever system of organs be studied, the
comparison of their modifications in the ape-series leads to one and
the same result--that the structural differences which separate man
from the gorilla and the chimpanzee are not so great as those which
separate the gorilla from the lower apes.' If we accept the Huxleyan
law without prejudice, and apply it to the natural classification of
the Primates, we must concede that man's place is within the order
of the Simiæ. On examining this relation with care, and judging
with logical persistence, we may even go a step further. Instead of
the wider conception of 'Simiæ,' we must use the restricted term of
Catarrhinæ, and our Pithecometra-thesis has then to be formulated
as follows: _The comparative anatomy of all organs of the group of
Catarrhine Simiæ leads to the result that the morphological differences
between man and the great apes are not so great as are those between
the man-like apes and the lowest Catarrhinæ_. In fact, it is very
difficult to show why man should not be classed with the large apes in
the same zoological family. We all know a man from an ape; but it is
quite another thing to find differences which are absolute and not of
degree only. Speaking generally, we may say that man alone combines the
four following features: (1) Erect walk; (2) extremities differentiated
accordingly; (3) articulate speech; (4) higher reasoning power. Speech
and reason are obviously relative distinctions only--the direct result
of more brains and more brain-power, the so-called mental faculties.
The erect walk is not an absolutely distinguishing characteristic: the
large apes likewise walk on their feet only, supporting their bodies
by touching the ground with the backs of their hands--in fact, with
their knuckles--and this is a mode of progression very different from
that of the tailed monkeys, which walk upon the palms of their hands.
There are, however, two obvious differences in the development of the
muscles. In man alone the gastrocnemius and the soleus muscle are thick
enough to form the calf of the leg, and the glutæus maximus is enlarged
into the buttocks. A fourth glutæal muscle occurs occasionally in
man, while it is constantly present in apes as the so-called musculus
scansorius. Concerning the muscles of the whole body, we cannot do
better than quote Testut's summary: 'The mass of recorded observations
upon the muscular anomalies in man is so great, and the agreement of
many of these with the condition normal in apes is so marked, that the
gap which usually separates the muscular system of man from that of the
apes appears to be completely bridged over.'
[4] See note, p. 80.
[5] _Perfect_, in the sense of highest stage of evolution, may seem a
_petitio principii_. Leaving aside the consideration that no living
creature is absolutely perfect, in the sense that its organization
cannot become more efficient or proficient, we have here to deal with
relative perfection of the whole organization. A fish or a snake is in
its way more specialized than a mammal; but specialization does not
necessarily mean height of development: it generally means life in a
comparatively narrow groove. The acts of giving birth and nourishing
the young with the mother's milk is a much higher stage than the act
of laying eggs and letting them run their chance. The development of
a hairy coat goes along with heightened temperature of the blood,
subsequent greater independence of the surrounding temperature, and
increased steady activity of the brain and other nerve-centres. The
brain of the Mammalia, in its minute structure, is much more complex.
This rule applies to some of the principal sense organs, chiefly the
nose and the ear. The skeleton, not so much as a whole as in the
various bones and joints, is more neatly finished, and built up more
in conformity with 'scientific principles,' than is the case even with
birds, in spite of their marvellous specialization. The same is the
case with the vascular system, notably the heart and the veins, and
with the excretory organs. In all of these many imperfections, still
to be found in the other classes, have been corrected in Mammalia. The
Primates take an easy first by their hands, and among them the apes and
man himself by their brains.
[6] 'Die menschenähnlichen Affen und ihre Organisation im Vergleich zur
menschlichen.' 1883.
There are, for example, the muscles of the ear. In most people the
majority, or even all of them, are no longer movable at will, while in
the apes they are still in use. The important point, however, is that
these muscles are still present in man, although often in a reduced
condition. They are the following: (1) Musculus auricularis anterior
or attrahens auris, which is frequently much reduced and no longer
reaches the ear at all, being then absolutely useless; (2) Musculus
auricularis superior or attollens auris, more constant than the former;
(3) Musculus auricularis posterior or retrahens auris, likewise often
functional. Occasionally smaller slips differentiated from these
three muscles are present, and as so-called intrinsic muscles are
restricted to the ear itself; their function is, or was, that of
curling up or opening the external ear.
[Illustration: OUTLINES OF THE LEFT EAR OF--
1. _Lemur macaco_; 2. _Macacus rhesus_, the Rhesus monkey; 3.
Cercopithecus, a macaque; 4. human embryo of six months; 5. man, with
Darwin's point well retained: the dotted outline is that of the ear of
a baboon; 6. orang-utan (after G. Schwalbe):[7] ^x the original tip of
the ear; 7. human ear with the principal muscles.
[7] G. Schwalbe, 'In wiefern ist die menschliche Ohrmuschel ein
rudimentäres Organ?'--In what Respects is the Human Outer Ear a
Rudimentary Organ? (_Archiv f. Anatomie und Physiologie_, 1889).]
In connection with the ear, I may touch upon another interesting
and most suggestive little feature which is present in many
individuals--namely, 'Darwin's point.' This is the last remnant of the
original tip of the ear, before the outer, upper, and hinder rim became
doubled up or folded in. It is a feature quite useless, and absolutely
impossible of interpretation, excepting as the vestige of such previous
ancestral conditions as are normal in the monkeys.
In some cases the reduction of muscles has proceeded further in apes
than in man--for example, the muscles of the little toe. Another
instance is afforded by the coccyx or vestige of the tail; this is
still furnished with muscles which are now in man, as well as in
the apes, quite useless, and vary considerably with every sign of
degeneration, most so in the orang-utan.
Darwin has mentioned the frequent action of the 'snarling muscle,' by
which, in sneering, our upper canine teeth are exposed, like those of a
dog prepared to fight.
Monkeys and apes possess vocal sacs, especially large in the
orang-utan; survivals of them, although no longer used, persist in man
in the shape of a pair of small diverticula, the pouches of Morgagni,
between the true and the false vocal cords.
'In the native Australians, the dental formula appears least removed
from the hypothetical original type, for in it are still found complete
rows of splendid teeth, with powerfully-developed canines and molars,
the latter being either uniform, or even increasing in size, as we
proceed backwards, in such a way that the wisdom tooth is the largest
of the series. This is decidedly a pithecoid characteristic which is
always found in apes. The upper incisors of the Malay, apart from their
prognathous disposition, have occasionally a distinctly pithecoid
form, their anterior surface being convex, and their lingual surface
slightly concave. The ancestors of Europeans seem to have had the same
form of teeth, for the oldest existing fragments of skulls from the
Mammoth age (_e.g._, the jaws from La Naulette, in Belgium) reveal
tooth-forms which must be classed with those of the lowest races of
to-day.'[8]
[8] Wiedersheim, 'Der Bau des Menschen als Zeugniss für seine
Vergangenheit.' Freiburg, 1888. Translated: 'The Structure of Man an
Index to his Past History.' London, 1895.
Now we are able to apply this fundamental Pithecometra-thesis directly
to the classification of the Primates and to the phylogeny of man,
which is intimately connected with it, because in this order, as in
all the other groups of animals, the natural system is the clear
expression of true phylogenetic affinity. Four results follow from our
thesis: (1) The Primates, as the highest legion or order of mammals,
form one natural, monophyletic group. All the Lemures, Simiæ, and
Homines descend from one common ancestral form, from a hypothetical
'Archiprimas.' (2) The Lemures are the older and lower of the natural
groups of the Primates; they stand between the oldest Placentalia
(Prochoriata) and the true Simiæ. (3) All the Catarrhinæ, or Eastern
Simiæ, form one natural monophyletic group. Their hypothetical
common ancestor, the Archipithecus, may have descended directly or
indirectly from a branch of the Lemures. (4) Man is descended directly
from one series of extinct Catarrhine ancestors. The more recent
ancestors of this series were tailless anthropoids (similar to the
Anthropopithecus), with five sacral vertebræ. The more remote ancestors
were tailed Cercopitheci, with three or four sacral vertebræ.
These four theses possess, in my opinion, absolute certainty.
They are independent of all future anatomical, embryological, and
palæontological discoveries which may possibly throw more light upon
the details of our phyletic anthropogenesis.
II.
The next question is, how the facts of palæontology agree with these
most important results of comparative anatomy and ontogeny. The fossils
are the true historical 'medals of creation,' the palpable evidence of
the historical succession of all those innumerable organic forms which
have peopled the globe for many millions of years. Here the question
arises, If the known fossil specimens of Mammalia, and particularly
of Primates, give proof of these Pithecometra-theses, do they confirm
directly the descent of man from ape-like creatures? The answer to this
question is, in my opinion, affirmative.
It is true that the gaps in the palæontological evidence, here as
elsewhere, are many and keenly felt. In the order of the Primates
they are greater than in many other orders, chiefly because of the
arboreal life of our ancestors. The explanation is very simple. It is
really due to a long chain of favourable coincidences if the skeleton
of a vertebrate, covered as it was with flesh and skin, and containing
still more perishable viscera, is petrified at all. The body may be
devoured by other creatures, and its bones scattered about; or it rots
away and crumbles to pieces. Many animals hide in thick undergrowth
when death approaches them; and, leading an almost entirely arboreal
life, the Primates are especially likely to disappear without being
fossilized. It is only when the body is quickly covered with sand, or
is embedded in suitable lime or silica containing mud, that the process
of petrifaction can come to pass. Even then it is only by great good
luck that we come across such a fossil. Very few countries have been
searched systematically, and the areas that have been searched amount
to little in comparison with the whole surface of the land, even if we
leave out of account the fact that more than two-thirds of the globe
are covered by water.
These deplorable deficiencies of empirical palæontology are balanced
on the other side by a growing number of positive facts, which possess
an inestimable value in human phylogeny. The most interesting and most
important of these is the celebrated fossil _Pithecanthropus erectus_,
discovered in Java in 1894 by Dr. Eugène Dubois.[9] Three years ago
this now famous ape-like man provoked an animated discussion at the
third International Zoological Congress at Leyden. I may therefore
be allowed to say a few words as to its scientific significance.
Unfortunately, the fossil remains of this creature are very scanty: the
skull-cap, a femur, and two teeth. It is obviously impossible to form
from these scanty remains a complete and satisfactory reconstruction of
this remarkable Pliocene Primate.
[9] _Pithecanthropus erectus._ 'Eine menschenähnliche Uebergangsform
aus Java' ('A Human-like Transitional Form'). Batavia, 1894.
The more important points are the following: The remains in question
rested upon a conglomerate which lies upon a bed of marine marl and
sand of Pliocene age. Together with the bones of Pithecanthropus were
found those of Stegodon, Leptobos, Rhinoceros, Sus, Felis, Hyæna,
Hippopotamus, Tapir, Elephas, and a gigantic Pangolin. It is remarkable
that the first two of these genera are now extinct, and that neither
hippopotamus nor hyæna exists any longer in the Oriental region. If we
may judge from these fossil remains, the bones of Pithecanthropus are
not younger than the oldest Pleistocene, and probably belong to the
upper Pliocene. The teeth are like those of man. The femur, also, is
very human, but shows some resemblances to that of the gibbons. Its
size, however, indicates an animal which stood when erect not less
than 5 feet 6 inches high. The skull-cap also is very human, but with
very prominent eyebrow ridges, like those of the famous Neanderthal
cranium. It is certainly not that of an idiot. It had an estimated
cranial capacity of about 1,000 cubic centimetres--that is to say, much
more than that of the largest ape, which possesses not more than 600
c.c. The crania of female Australians and Veddahs measure not more than
1,100, some even less than 1,000 c.c.; but, as these Veddah women stand
only about 4 feet 9 inches high, the computed cranial capacity of the
much taller Pithecanthropus is comparatively very low indeed.[10]
[10] On the day after the delivery of this address Dr. Dubois exhibited
the cranium of Pithecanthropus, from which he had removed the stony
matrix which filled the inside, in order to examine the impression left
by the cerebral convolutions. He was able to show that they also are
very human, and more highly developed than those of the recent apes. [
Illustration: The upper figure represents the outlines
of the skull of Pithecanthropus, as restored by Manouvier.[11] The
lower figure shows the comparative size and shape of Pithecanthropus,
the Neanderthal skull, a specimen of the Cro-Magnon race of neolithic
France, and a Young Chimpanzee before the full development of the
supraorbital crests.]
[11] L. Manouvier: 'Deuxième étude sur le Pithecanthropus erectus comme
précurseur présumé de l'homme.' (_Bulletins de la Soc. d'Anthropologie
de Paris_, 1895.)
The final result of the long discussion at Leyden was that, of twelve
experts present, three held that the fossil remains belonged to a low
race of man; three declared them to be those of a man-like ape of great
size; the rest maintained that they belonged to an intermediate
form, which directly connected primitive man with the anthropoid
apes. This last view is the right one, and accords with the laws of
logical inference. _Pithecanthropus erectus_ of Dubois is truly a
Pliocene remainder of that famous group of highest Catarrhines which
were the immediate pithecoid ancestors of man. He is, indeed, the
long-searched-for 'missing link,' for which, in 1866, I myself had
proposed the hypothetical genus Pithecanthropus, species Alalus.
It must, however, be admitted that this opinion is still strongly
combated by some distinguished authorities. At the Leyden Congress it
was attacked by the illustrious pathologist Rudolf Virchow.[12] He,
however, is one of the minority of leading men of science who set
themselves to refute the theory of Evolution in every possible way. For
thirty years he has defended the thesis: 'It is quite certain that man
is not a descendant of apes.' He declares any intermediate form to be
unimaginable save in a dream.
[12] See Notes, p. 93.
Virchow went to the Leyden Congress with the set purpose of disproving
that the bones found by Dubois belonged to a creature which linked
together apes and man. First, he maintained that the skull was that
of an ape, while the thigh belonged to man. This insinuation was at
once refuted by the expert palæontologists, who declared that without
the slightest doubt the bones belonged to one and the same individual.
Next, Virchow explained that certain exostoses or growths observable on
the thigh proved its human nature, since only under careful treatment
the patient could have healed the original injury. Thereupon Professor
Marsh, the celebrated palæontologist, exhibited a number of thigh-bones
of wild monkeys which showed similar exostoses and had healed without
hospital treatment. As a last argument the Berlin pathologist declared
that the deep constriction behind the upper margin of the orbits
proved that the skull was that of an ape, as such never occurred in
man. It so happened that a few weeks later Professor Nehring of Berlin
demonstrated exactly the same formation on a human prehistoric skull
received by him from Santos, in Brazil.
Virchow was, in fact, just as unlucky in Leyden in his fight with our
pliocene ancestor as he had been unfortunate in his opinion on the
famous skulls of Neanderthal, Spy, La Naulette, etc., every one of which
he explained as a pathological abnormality. It would be a very curious
coincidence indeed if all these and other fossil human remains were
those of idiots or otherwise abnormal individuals, provided they are
old and low enough in their organization to be of phylogenetic value to
the unbiassed zoologist.
As the sworn adversary of Evolution, transformism, and Darwinism in
particular, but a believer in the constancy of species, the great and
renowned pathologist has been driven to the incredible contention that
all variations of organic forms are pathological.
Four years ago, as honorary president of the Anthropological Congress
at Vienna, he attacked Darwinism in the severest manner, and declared
that 'man may be as well descended from the elephant or from the sheep
as from the ape.' Such attacks on the theory of transformism indicate a
failure to understand the principles of the theory of Evolution and to
appreciate the significance of palæontology, comparative anatomy, and
ontogeny.
The thousands of other objections which have been made during the last
forty years (chiefly by outsiders) may be passed over in silence. They
do not require serious refutation. In spite of, or perhaps because of,
these attacks, the theory of Evolution stands established more firmly
than ever.
It is easy for the outsider to exult over the difficulties which our
problem implies--difficulties which we who have given our lives to the
study understand likewise, and try our best not only to bridge over,
but also to point out. Anyhow, we do not conceal them; while those who
reject the explanation offered by Evolution make the most of the gaps,
and pass silently over the far more numerous points favourable to our
theory.
How fruitful during the last thirty years the astonishing progress in
our palæontological knowledge has been for our Pithecometra-thesis is
best shown by a short glance at the growth of our knowledge of fossil
Primates. Cuvier,[13] the founder of palæontology, continued up to the
time of his death, in 1832, to assert that fossil remains of monkeys
and lemurs did not exist. The only skull of a fossil lemuroid which
he described (namely, Adapis) he declared to be that of an ungulate.
Not until 1836 were the first fragments of extinct monkeys found in
India; it was two years later, near Athens, that the skeleton of
_Mesopithecus penthelicus_ was discovered. Other remains of lemurs were
found in 1862. But during the last twenty years the number of fossil
Primates has been augmented by the remarkable discoveries of Gaudry,
Filhol, Milne Edwards, Seeley, Schlosser, and others in Europe; of
Marsh, Cope, Osborn, Leidy, Ameghino, in South America; and Forsyth
Major in Madagascar.[14] These tertiary remains, chiefly of Eocene and
Miocene date, fill many gaps between existing genera of Primates, and
afford us quite a clear insight into the phyletic development of this
order during the millions of years of the Cænozoic age.
[13] See notes, p. 87.
[14]
F. AMEGHINO: 'Contribucion al conocimiento de los mamíferos
de la república Argentina.' In _Actas de la Academia nacional de
Sciencias en Cordoba_, 1889.--Another article in _Revista Argentina de
Historia natural_. Buenos Aires, 1891.
A. GAUDRY: 'Animaux fossiles et géologie de l'Attique.'
1862.--'Le Dryopithèque.' _Mém. Soc. géol. de France_:
'Paléontologie.' 1890.
O. MARSH: 'Introduction and Succession of Vertebrate Life in
America.' Address, Amer. Assoc. Adv. Sci., Nashville, 1887.
H. F. OSBORN: 'The Rise of the Mammalia in North America.'
Address, Amer. Assoc. Adv. Sci., Madison, 1893.
L. RUETIMEYER: 'Ueber die Herkunft unserer Thierwelt,' Basel,
1867.
C. S. FORSYTH MAJOR: 'Fossil Monkeys from Madagascar.'
_Geological Magazine_, 1896.
M. SCHLOSSER: 'Ueber die Beziehungen der ausgestorbenen
Saeugethierfaunen und ihr Verhaeltniss zur Saeugethierfauna der
Gegenwart.' Biolog. Centralblatt, 1888.
The most important difference between the two groups of existing
monkeys is indicated by their dentition. Adult man possesses, like
all the other Catarrhine Simiæ, thirty-two teeth, whilst the American
monkeys (the Platyrrhinæ) have thirty-six teeth--namely, one pair of
premolars more in the upper and lower jaws. Comparative odontology
leads us to the phylogenetic conclusion that this number has been
produced by reduction from a still older form with forty-four teeth.
This typical dental formula (three incisors, one canine, four
premolars, and three molars, in each half-jaw) is common to all those
most important older mammals which in the beginning of the Eocene
period constituted the four large groups of Lemuravida, Condylarthra,
Esthonychida, and Ictopsida. These are the four ancestral groups
of the four main orders of Placentalia--namely, of the Primates,
Ungulata, Rodentia, and Carnassia. They seem to be so closely related
by their primitive organization that they may be united in one common
super-order, Prochoriata.
With a considerable degree of probability, we are led to formulate
the further hypothesis that all the orders of Placentalia--from the
lowest Prochoriata upwards to man--have descended from some unknown
common ancestor living in the Cretaceous period, and that this oldest
placental form originated from some Jurassic group of marsupials.
Among these numerous fossil Lemures which have been discovered within
the last twenty years, there exist, indeed, all the connecting forms
of the older series of Primates, all the 'missing links' sought for by
comparative odontology.
The oldest Lemures of the tertiary age are the Eocene Pachylemures,
or Hyopsodina. They possess the complete dentition of the
Prochoriata--namely, forty-four teeth (3.1.4.3/3.1.4.3). Then follow
the Eocene Palæolemures, or Adapida, with forty teeth, they having lost
one pair of incisors in each jaw. To these are attached the younger
Autolemures, or Stenopida, with thirty-six teeth, they thus possessing
already the same dentition as the Platyrrhinæ. The characteristic
dentition of the Catarrhinæ is derived from this formula by the loss of
another premolar.
These relations are so clear and so closely connected with a
gradual transformation of the whole skull, and with the progressive
differentiation of the Primate-form, that we are justified in saying
that the pedigree of the Primates, from the oldest Eocene Lemures
upwards to man, is now so well known, its principal features so firmly
fixed within the Tertiary age, that there is no missing link whatever.
Quite different, and much more incomplete, is the palæontological
evidence, if we go further back into the Secondary or Mesozoic age,
and look there for the older ancestors of the mammalian series. There
we meet everywhere with wide gaps, and the scarce fragments of fossil
Mesozoic mammals (excessively rare in the Cretaceous formation) are too
poor to permit definite conclusions as to their systematic position.
Indeed, comparative anatomy and ontogeny lead us to the hypothesis
that the oldest Cretaceous Mammalia--the Prochoriata--are descended
from Jurassic marsupials, and these again from Monotremes. We may
also suppose with high probability that among the unknown Cretaceous
Prochoriata there have been Lemuravida and forms intermediate between
these and the Jurassic Amphitheriidæ, and that these marsupials in
their turn are descendants of Pantotheria or similar monotreme-like
creatures of the Triassic age. Any certain evidence for these
hypotheses is at present still wanting. One important fact, however,
is established--namely, that these interesting and oldest Mammalia--the
Pantotheria of Marsh, the Triassic Dromatheriidæ, and the Jurassic
Triconodontidæ of Osborn--were small insectivorous mammals with a very
primitive organization. Probably they were Monotremes, and may be
derived directly from Permian Sauromammalia, an ill-defined mixture of
Mammalia and Reptilia.
This generalized characteristic supports our view that _the whole
class of Mammalia is monophyletic_, and that all its members, from
the oldest Monotremes upwards to man, have descended from one common
ancestor living in the older Triassic, or perhaps in the Permian,
age. To acquire full conviction of this important conception, we have
only to think of the hair and the glands of our human skin, of our
diaphragm, the heart and the blood corpuscles without a nucleus, our
skull with its squamoso-mandibular articulation. All these singular
and striking modifications of the vertebrate organization are common
to mammals, and distinguish them clearly from the other Craniota. This
characteristic combination and correlation proves that they have been
developed only _once_ in the history of the vertebrate stem, and that
they have been transferred by heredity from one common ancestor to all
the members of the class of Mammalia.
The next step, as we trace our human phylogeny to its origin, leads us
further back into the lower Vertebrata, into that obscure Palæozoic
age the immeasurable length of which (much greater than that of the
Mesozoic) may, according to one of the newest geological calculations,
have comprised about one thousand millions of years.[15]
[15] See note, 'Geological Time and Evolution' p. 134.
The first important fact we have to face here is the complete absence
of mammalian remains. Instead of these we find in the later Palæozoic
period, the Permian, air-breathing _reptiles_ as the earliest
representatives of Amniota. They belong to the most primitive order
of that class, the Tocosauria; and besides them there were the
Theromorpha, which approach the Mammalia in a remarkable manner. These
reptiles in turn were preceded, in the Carboniferous period, by true
Amphibia, most of them belonging to the armour-clad Stegocephali.
These interesting Progonamphibia were the oldest Tetrapoda, the first
vertebrates which had adapted themselves to the terrestrial mode of
life; in them the swimming fin of fishes and Dipneusta was transformed
into the pentadactyle extremities characteristic of quadrupeds.
To appreciate the high importance of this metamorphosis, we need only
compare the skeleton of our own human limbs with that of the living
Amphibia. We find in the latter the same characteristic composition as
in man: the same shoulder and pelvic girdle; the same single bone, the
humerus or the femur, followed by the same pair of bones in the forearm
and leg; then the same skeletal elements composing the wrist and the
ankle regions; and, lastly, the same five fingers and toes.
The arrangement of these bones, peculiar and often complicated, but
everywhere essentially the same in all the Tetrapoda, is a striking
evidence that man is a descendant from the oldest pentadactyle Amphibia
of the Carboniferous period. In man the pentadactyle type has been
better preserved by constant heredity than in many other Mammalia,
notably the Ungulata.
The oldest Carboniferous Amphibia, the armour-clad Stegocephali, and
especially the remarkable Branchiosauri discovered by Credner, are
now regarded by all competent zoologists as the indubitable common
ancestral group of all Tetrapoda, comprising both Amphibia and Amniota.
But whence this most remote group of Tetrapoda? That difficult question
is answered by the marvellous progress of modern palæontology, and
the answer is in complete harmony with the older results arrived
at by comparative anatomy and ontogeny. Thirty-four years ago Carl
Gegenbaur,[16] the great living master of comparative anatomy, had
demonstrated in a series of works how the skeletal parts of the various
classes of Vertebrata, especially the skull and the limbs, still
represent a continuous scale of phyletic gradations. Apart from the
Cyclostomes, there are the fishes, and among them the Elasmobranchi
(sharks and rays), which have best preserved the original structure in
all its essential parts of organization. Closely connected with the
Elasmobranchi are the Crossopterygii, and with these the Dipneusta or
Dipnoi. Among the latter the highest importance attaches to the ancient
Australian Ceratodus. Its organization and development is now, at last,
becoming well known. This transitional group of Dipnoi, 'fishes with
lungs' but without pentadactyle limbs, is the morphological bridge
which joins the Ganoids and the oldest Amphibia. With this chain
of successive groups of Vertebrata, constructed anatomically, the
palæontological facts agree most satisfactorily. Selachians and Ganoids
existed in the Silurian times, Dipnoi in the Devonian, Amphibia in the
Carboniferous, Reptilia in the Permian, Mammalia in the Trias. These
are historical facts of first rank. They connote in the most convincing
manner that remarkable ascending scale in the series of vertebrates
for our knowledge of which we are indebted to the works of Cuvier and
Blainville, Meckel, Johannes Mueller and Gegenbaur, Owen and Huxley.
The historical succession of the classes and orders of the Vertebrata
in the course of untold millions of years is definitely fixed by the
concordance of those leading works, and this invaluable acquisition is
much more important for the foundation of our human pedigree than would
be a complete series of all possible skeletons of Primates.
[16] See note, p. 97.
Greater and more frequent difficulties arise if we penetrate further
into the most remote part of the human phylogeny, and attempt to derive
the vertebrate stem from an older stem of invertebrate ancestors. None
of those had a skeleton which could be petrified; and the same remark
applies to the lowest classes of Vertebrata--to the Cyclostomes and
the Acrania. Palæontology, therefore, can tell us nothing about them;
and we are limited to the other two great documents of phylogeny--the
results of comparative anatomy and ontogeny. The value of their
evidence is, however, so great that every competent zoologist can
perceive the most important features of the most remote portion of our
phylogeny.
Here the first place belongs to the invaluable results which modern
comparative ontogeny has gained by the aid of the biogenetic law or
the theory of recapitulation. The foundation-stones of vertebrate
embryology had been laid by the works of Von Baer, Bischoff,[17] Remak,
and Koelliker;[18] but the clearest light was thrown upon it by the
famous discoveries of Kowalevsky[19] in 1866. He proved the identity
of the first developmental stages of Amphioxus and the Ascidians, and
thereby confirmed the divination of Goodsir, who had already announced
the close affinity of Vertebrates and Tunicates. The acknowledgment of
this affinity has proved of increasing importance, and has abolished
the erroneous hypothesis that the Vertebrata may have arisen from
Annelids or from other Articulata. Meanwhile, from 1860 to 1872, I
myself had been studying the development of the Spongiæ, Medusæ,
Siphonophora, and other Coelenterata. Their comparison led me to the
statements embodied in the 'Gastræatheorie,' the first abstract of
which was published in 1872 in my monograph of the Calcispongiæ.
[17] Wilhelm Bischoff of Munich: works on the history of the
development of the rabbit, dog, guinea-pig, roe-deer. 1840-1854.
[18] See note, p. 96.
[19] 'Ueber die Entwicklung der einfachen Ascidien,' Mém. Acad. St.
Petersbourg, vii. ser., tome x. (1866). Other papers in 'Archiv f.
Mikroskop. Anatomie,' vii. (1871); xiii. (1877).
These ideas were carried on and expanded during the subsequent ten
years by the help of many excellent embryologists--first of all by E.
Ray Lankester and Francis Balfour. The most fruitful result of these
widely extended researches was the conclusion that the first stages of
embryonic development are essentially the same in all the different
Metazoa, and that we may derive from these facts certain views on
the common descent of all from one ancestral form. The unicellular
egg[20] repeats the stage of our Protozoan ancestors; the Blastula
is equivalent to an ancestral coenobium of Magosphæra or Volvox;
the Gastrula is the hereditary repetition of the Gastræa, the common
ancestor of all the Metazoa.
[20] See note, p. 115--Theory of cells.
Man agrees in all these respects with the other vertebrates, and must
have descended with them from the same common root.
Particularly obscure is that part of our phylogeny which extends from
the Gastræa to Amphioxus. The morphological importance of this last
small creature had been perceived by Johannes Mueller, who in 1842
gave the first accurate description of it. It would not, of course, be
correct to proclaim the modern Amphioxus the common ancestor of all the
vertebrates; but he must be regarded as closely related to them, and
as the only survivor of the whole class of Acrania. If the Amphioxidæ
had through some unfortunate accident become extinct, we should not
have been able to gain anything like a positive glimpse at our most
remote vertebrate ancestor. On the one hand, Amphioxus is closely
connected with the early larva of the Cyclostomes, which are the
oldest Craniota, and the pre-Silurian ancestors of the fishes. On the
other hand, the ontogeny of Amphioxus is in harmony with that of the
Ascidians, and if this agreement is not merely coincidental, but due to
relationship, we are justified in reconstructing for both Ascidians
and Amphioxus one common ancestral group of chordate animals, the
hypothetical _Prochordonia_. The modern Copelata give us a remote idea
of their structure. The curious Balanoglossus, the only living form of
Enteropneusta, seems to connect these Prochordonia with the Nemertina
and other Vermalia, which we unite in one large class--Frontonia.
No doubt these pre-Cambrian Vermalia, and the common root of all
Metazoa, the Gastræades, were connected during the Laurentian period
by a long chain of intermediate forms, and probably among these
were some older forms of Rotatoria and Turbellaria; but at present
it is not possible to fill this wide gap with hypotheses that are
satisfactory, and we have to admit that here indeed are many missing
links in the older history of the Invertebrata. Still, every zoologist
who is convinced of the truth of transformism, and is accustomed to
phylogenetic speculations, knows very well that their results are most
unequal, often incomplete.
III.
Let us now recapitulate the ancestral chain of man, as it is set forth
in the accompanying diagram (p. 55), which represents our present
knowledge of our descent. For simplicity's sake the many side-issues
or branches which lead to groups not in the main line of our descent
have been left out, or have been indicated merely. Many of the stages
are of course hypothetical, arrived at by the study of comparative
anatomy and ontogeny; but an example for each of them has been taken
from those living or fossil creatures which seem to be their nearest
representatives.
1. The most remote ancestors of all living organisms were living beings
of the simplest imaginable kind, organisms without organs, like
the still existing _Monera_. Each consisted of a simple granule of
protoplasm, a structureless mass of albuminous matter or plasson, like
the recent Chromaceæ and Bacteriæ. The morphological value of these
beings is not yet that of a cell, but that of a cytode, or cell without
a nucleus. Cytoplasm and nucleus were still undifferentiated.
I assume that the first Monera owe their existence to spontaneous
creation out of so-called anorganic combinations, consisting of carbon,
hydrogen, oxygen, and nitrogen. An explanation of this hypothesis I
have given in my 'Generelle Morphologie.'
The Monera probably arose early in the Laurentian period. The oldest
are the Phytomonera, with vegetable metabolism. They possessed the
power (characteristic of plants) of forming albumin by synthesis from
carbon, water, and ammonia. From some of these plasma-forming Monera
arose the plasmophagous Zoomonera with animal metabolism, living
directly upon the produce of their plasmodomous or plasma-forming
sisters. This is the first instance of the great principle of division
of labour.
2. The second stage is that of the _simple and single cell_, a bit
of protoplasm with a nucleus. Such unicellular organisms are still
very common. The _Amoebæ_ are their simplest representatives. The
morphological value of such beings is the same as that of the egg
of any animal. The naked egg cells of the sponges creep about in an
amoeboid fashion, scarcely distinguishable from Amoeba. The same
remark applies to the egg-cell of man himself in its early stages
before it is enclosed in a membrane. The first unicellular organisms
arose from Monera through differentiation of the inner nucleus from the
outer protoplasm.
3. Repeated division of the unicellular organism produces the
_Synamoebium_, or community of Amoebæ, provided the divisional
products, or new generations of the original cell, do not scatter,
but remain together. The existence of such a _Coenobium_, a number
of equal and only loosely-connected cells, as a separate stage in the
ancestral history of animals, is made highly probable by the fact that
the eggs of all animals undergo after fertilization such a process of
repeated self-division, or 'cleavage,' until the single egg cell is
transformed into a heap of cells closely packed together, not unlike a
mulberry (_morula_)--hence _morula_ stage in ontogeny.
4. The morula of most animals further changes into a _Blastula_, a
hollow ball filled with fluid, the wall being formed by a single layer
of cells, the blastoderm or germinal layer. This modification is
brought about by the action of the cells--they conveying nourishing
fluid into the interior of the whole cell colony and thereby
being themselves forced towards the surface. The Blastula of most
Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ,
or hair-like processes, the vibrating motion of which causes the whole
organism to rotate and advance in the water. Living representatives of
such Blastæads, namely, globular gelatinous colonies of cells enclosing
a cavity, are Volvox and Magosphæra.
5. The Blastula of most animals assumes a new larval form called
_Gastrula_, in which the essential characteristics are that a portion
of the blastoderm by invagination converts the Blastula into a cup
with double walls, enclosing a new cavity, the primitive gut. This
invagination or bulging-in obliterates the original inner cavity of
the Blastula. The outer layer of the Gastrula is the ectoderm, the
inner the endoderm; both pass into each other at the blastoporus, or
opening of the gut cavity. The Gastrula is a stage in the embryonic
development of the various great groups of animals, and some such
primitive form as ancestral to all Metazoa is thus indicated. This
hypothetical _Gastræa_ is still very essentially represented by the
lower Coelenterates--_e.g._, Olynthus, Hydra.
6. The sixth stage--that of the _Platodes_, or flat-worms--is very
hypothetical. They are bilateral gastræads, with a flattened oblong
body, furnished with ciliæ, with a primitive nervous system, simple
sensory and reproductive organs, but still without appendages, body
cavity, vent, and blood-vessels. The nearest living representatives of
such creatures are the acoelous Turbellarians--_e.g._, Convoluta, a
free-swimming, ciliated creature.
7. The next higher stage is represented by such low animals as the
_Gastrotricha_--_e.g._, Chætonotus among the Rotatoria, which differ
from the rhabdocoelous Turbellarians chiefly by the formation of
a vent and the beginnings of a coelom, or cavity, between gut and
body wall. The addition of a primitive vascular system and a pair of
nephridia, or excretory organs, is first met with in the _Nemertines_.
8. These, together with the _Enteropneusta_ (Balanoglossus), are
comprised under the name of Frontonia, or Rhynchelminthes, and form the
highest group of the Vermalia.
The Enteropneusta especially fix our attention, because they alone,
although essentially 'worms,' exhibit certain characteristics which
make it possible to bridge over the gulf which still separates the
Invertebrata from the vertebrate phylum. The anterior portion of the
gut is transformed into a breathing apparatus--hence Gegenbaur's
term of Enteropneusta, or Gut-breathers. Moreover, Balanoglossus and
Cephalodiscus possess another modification of the gut--namely, a
peculiar diverticulum, which, in the present state of our knowledge,
may be looked upon as the forerunner of the chorda dorsalis.
9. Stage of _Prochordonia_, as indicated by the larval form, called
Chordula, which is common to the Tunicata and all the Vertebrata.
These two groups possess three most important features: (_a_) A chorda
dorsalis, a stiff rod lying in the long axis of the body, dorsally from
the gut and below the central nervous system. This latter, for the
first time in the animal kingdom, appears in the shape of a spinal
cord. (_b_) The use of the anterior portion of the gut for respiratory
purposes. (_c_) The larval development of the Tunicata is essentially