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.