forms between some of the existing species, as _R. pachygnathus_ of
Pikermi, as Gaudry has shown, is about equally related to the two
species of modern African rhinoceros, and might have been (upon the
derivative hypothesis) the ancestor of both. In the same way the
Himalayan _R. sivalensis_ appears to be related to the modern _R.
indicus_ and _sondiacus_, and the _R. schleirmacheri_ to the Asiatic
two-horned species. One special line of variation indicated chiefly by
the ossification of the nasal septum culminated in the _R. tichorhinus_,
which became extinct only in the most recent geological epoch. The
history of this small group alone in its bearings upon evolution might
occupy many lectures; I must content myself now only with one
observation, borrowed from Mr. Boyd Dawkins, that in all modern
rhinoceroses the molar teeth have deeper crowns than in those which
existed prior to a certain epoch, so that the height of these teeth
alone will serve to distinguish a pleistocene from a pliocene form, in
other respects closely allied. The value of this observation will be
illustrated in the sequel.
The next line of modification from _Palæotherium_, is that which
culminates in the most specialised of mammals, the modern horse, an
animal we are so accustomed to look at that we scarcely ever notice the
most remarkably adaptive character of its structure for its special mode
of life. If we were not acquainted with the horse (and here of course I
include its immediate allies, the asses and zebras) we could scarcely
conceive of an animal whose only support was the tip of a single toe on
each extremity, to say nothing of the singular conformation of its teeth
and other organs. So striking have these characters appeared to many
zoologists that the animals possessing them have been reckoned as an
order apart called _Solidungula_; but palæontology has revealed that in
the structure of its skull, its teeth, its limbs, the horse is nothing
more than a modified _Palæotherium_; and though still with gaps in
certain places, many of the intermediate stages of these modifications
are already known to us, being the _Paloplotherium_, _Anchitherium_,
_Merychippus_, and _Hipparion_. On this very interesting point, which
looks more like a real genealogical history than any other known,
however, I need not dwell, as it was so fully treated of in a lecture
delivered in this theatre three years ago by Professor Huxley--a lecture
entitled the “Pedigree of the Horse.”
Lastly, there is _Macrauchenia_, a curiously modified Perissodactyle
found in pleistocene times in South America, apparently another
derivative of the palæotherium type, presenting resemblances (though
perhaps only analogical) to some of the artiodactyles, especially the
camels.
Directly intermediate forms between _Macrauchenia_ and the other animals
of its group are not yet known; but considering how little evidence we
have of the animal life of the middle or older tertiaries of South
America, this is not to be wondered at.
On the whole it will be seen that, taking actual anatomical characters
alone, palæontological research, even so far as it has yet been carried,
bridges over most of the gaps existing between the modern forms of
Perissodactyles, entirely abolishing, for instance, the order
_Solipedia_, as it is impossible to draw a satisfactory line where the
animal ceases to be equine and becomes a palæotheroid; some drawing it
between _Anchitherium_ and _Palæotherium_, some between _Anchitherium_
and _Hipparion_. Moreover, and this is most important, the lines from
the modern more specialised forms converge towards the ancient more
generalised forms; so that if we could get a side-view of what is shown
in the diagram, the earliest forms at the bottom and the latest at the
top, we should have lines (broken, it is true, here and there) diverging
from a common, or near a common centre, towards a circumference above--a
view, in fact, of the conventional genealogical tree.
We turn now to the _Artiodactyles_, represented at present by the
scattered groups before spoken of, clustering round two type-forms so
widely sundered in their structure and habits as the pig and the ox; but
the former history of this division yields a totally different state of
things. Of early eocene Artiodactyles we know very little at present;
but in the later divisions of the same epoch forms appeared, such as
_Anoplotherium_, _Dichobune_, _Chœropotamus_, and _Hyopotamus_, which
were certainly neither pigs nor ruminants, but which partook remarkably
of the characters of both. They had the complete number of teeth, _i.e._
incisors and canines, like modern pigs, but molars with indications of
the crescentic pattern so characteristic of ruminants. They had two or
four toes; but the metacarpals and metatarsals were not united to form a
cannon bone as in ruminants, and they wanted the horny appendages to the
head, so usually met with in the modern representatives of that group.
From some of these central forms, or more probably from a still earlier
allied group indicated by the genus _Acotherulum_, or by some other
still undescribed remains from Mauremont, transitions can be traced
with few breaks, through the successively modified miocene genera
_Chærotherium_ and _Palæochærus_ to the genus _Sus_, or true pig, in
which the dentition undergoes some remarkable specialisations, as the
upturning of the upper canines, and great development and extremely
tuberculated character of the posterior molars, which are both
singularly exaggerated in some modern offsets of the pig family, the
first in the babirussa, and the second in the wart-hog (_Phacochærus_).
More distantly related to the true pigs are the hippopotamus on the one
hand, and the peccary on the other. In relation to the first, not found
anteriorly to the latest miocene, it is significant that the earliest
known forms had the more generalised number of incisor teeth (six)
instead of four as in the modern hippopotamus, and hence has been made
into a genus by itself, called _Hexaprotodon_.
The researches of Leidy into the ancient (miocene and early pliocene)
fauna of Nebraska have furnished evidence of a remarkable group of
animals now entirely extinct, the _Oreodontidæ_, the characters of which
are perfectly intermediate between those of the pigs and the ruminants;
animals with pig-like feet and complete number of incisors, canines, and
molars, but with the latter important set of teeth, formed precisely on
the same type as those of the deer. Within this particular group Leidy
has noted a curious series of slight modifications coinciding with the
successive age of the strata, in which the remains were found.
_Agriochærus_, the most ancient, approaches nearer to _Chæropotamus_,
has orbits open behind and very shallow-crowned teeth. Then follows
_Oreodon_ proper, and lastly _Merychyus_, more like the modern
ruminants.
To return to the European forms, in the genus _Gelocus_, where the union
of the two principal bones of the metapodium first occurs, Kowalevsky
has noticed the gradual way in which this change seems to have been
brought about in successive epochs of eocene and early miocene strata,
at first free in the young, and only coalescing in old animals,
afterwards coalescing at a much earlier age. The gradual perfecting of
the foot by the development of the ridge round the lower articular end
of the metapodium in later forms, the ridge being quite wanting in
early forms of the same group, has been noticed by the same author in
many different series of Ungulates.
During the miocene period the peculiar dental characteristics of the
modern ruminants, especially the loss of the upper incisors, were
developed, all selenodont artiodactyles henceforth showing it. Of this
early race of imperfect ruminants, still retaining many generalised
characters, especially in the skull, the cervical vertebræ, fibula,
stomach, etc., the chevrotains (_Tragulidæ_) are the survivors,
especially the West African _Hyomoschus_, which has existed almost
unchanged since the late miocene of Sansans and Steinheim. Then for the
first time the appendages called antlers were introduced, but only in a
comparatively rudimentary condition, with long pedicles and few
branches, as in the modern Muntjaks. It was not till pliocene and
especially pleistocene epochs that the wonderful and luxuriant variety
of cervine antlers reached their full development. As offsets of the
deer group, the giraffe, the gigantic Siwalik _Sivaitherium_, and the
_Helladotherium_ of Greece may be mentioned, the two latter having
become extinct, apparently without descendants.
Later still, the yet more specialised forms of hollow-horned ruminants
appear--forms which now dominate the earth, being of all Ungulates the
most widely diffused and most numerous in species, in individuals, and
in outward variety, though in essential structure all alike. One of
their principal characteristics is the modification of their molar teeth
in the same way as in the modern horses, to which in some respects they
seem to form a parallel group. The difference between the molar tooth of
a hollow-horned ruminant and that of a deer consists in the great
lengthening of the crown without any change in the pattern of the enamel
folds, and in the addition of cement to support these folds. This
alteration did not take place suddenly, and the crowns of teeth of the
artiodactyles before the time of deer were still shorter than in those
animals.
Among the deer themselves, as Lartet observed, the most ancient have
very short-crowned molars, and the depressions on the surface are so
shallow that the bottom is always visible, while in the Cervidæ of the
more recent tertiary periods, and especially the pleistocene and living
species, these same cavities are so deep that, whatever be the state of
dentition, the bottom cannot be seen. This (he says) is a perfectly
reliable rule for distinguishing the ancient from the more modern forms
of deer, and can be applied to other animals as well as the Cervidæ.
From it he surmises that the duration of the life of modern is greater
than that of ancient deer. The same careful observer also remarks that
in many natural groups a gradual progress is observed in the volume of
the brain and complexity of its surface, as deduced from casts of the
interior of the skull, from which fact he concludes that a gradual
growth of vital energy and intelligence has occurred as the effect of
the tendency towards improvement, of which the cause is always acting,
and the limits indefinite.
Thus the history of the Even-toed Ungulates tells the same story as that
of the Perissodactyles. The modern forms are placed along lines which
converge towards a common centre. Moreover, the lines of both groups, to
a certain extent, approximate; but within the limits of our knowledge
they do not meet. Both artiodactyles and perissodactyles existed low
down in the eocene, just as Carnivores, Insectivores, bats, rodents, and
other great groups then existed with boundary lines as distinctly marked
as now.
Was the order, according to which the introduction of new forms seems to
have taken place since that epoch, then entirely changed? or did it
continue as far back as the period when these lines would have been
gradually fused into a common centre?
Here we are landed in the region of pure speculation; but bolder
travellers than I have endeavoured to penetrate its mysteries, as may be
seen by a perusal of Professor Huxley’s presidential address to the
Geological Society for 1870.
I have so far confined myself within the region of the known, and shown
that at least in one group of animals the facts which we have as yet
acquired point to the former existence of intermediate forms, so
numerous that they go far to discredit the view of the sudden
introduction of new species.
They also show many cases of gradual modification of particular organs,
probably always to the benefit of the race, and also a general progress
from lower to higher or more specialised types: though, as in all other
cases of progress (human civilisation, for instance), attended with many
exceptions, some local and temporary, some only apparent.
Whether the inferences which seem to me to follow from these facts are
true or not may still be an open question; for the sake of the stimulus
that an open question of this sort lends to scientific research I am
very glad that it is so; but if true, if we are led by them to the
conclusion that the world we live in is a world of gradual growth and
progress, and orderly evolution, what grander view of the Creation and
the history of that world can we have opened to us?
XII
A CENTURY’S PROGRESS IN ZOOLOGICAL KNOWLEDGE[31]
On the 10th of January 1778 died the great Swedish naturalist, Charles
Linné, more commonly known as Linnæus, a name which will ever be
mentioned with respect and regard in an assembly devoted to the
cultivation of the sciences of Zoology and Botany, as, whatever may be
the future progress of those sciences, the numerous writings of Linnæus,
and especially the publication of the _Systema Naturæ_, can never cease
to be looked upon as marking an era in their development. That work
contained a systematic exposition of all that was known on these
subjects expressed in language the most terse and precise. The
accumulated knowledge of all the workers at Zoology, Botany, and
Mineralogy since the world began, was here collected together by patient
industry, and welded into a complete and harmonious whole by penetrating
genius.
Exactly a century has passed since Linnæus died. What of the progress of
the subjects to which he devoted his long and laborious life? This one
century is a brief space compared with the ages which have passed since
man began to dwell upon the earth, surrounded by living objects, which
have, more and more as time rolled on, awakened his curiosity,
stimulated his faculties to observe, and impelled him to record the
knowledge so gained for the benefit of those to come. How does it stand
in comparison with those which preceded it, in the contributions it has
thus acquired and recorded?
It may be not without interest in commencing our work at this meeting to
cast our eyes back and take stock, as it were, of the knowledge of a
hundred years ago, and of that of the present time, and see what
advances have been made; to look at the living world as it was known to
Linnæus and as it is known to ourselves. The _Systema Naturæ_, the last
edition of which, revised by the author, was published in 1766, will be
a convenient basis for the comparison; but as the subject is one which,
even in a most superficial outline, might reach such lengths as would
well tire out the most patient of audiences, and absorb time which will
be more profitably occupied by the valuable contributions which are
forthcoming from other members of the Association, I will merely take a
small section of the work, about 100 pages out of the first of the four
volumes, those devoted to the first class MAMMALIA. The comparison of
this part is perhaps the easiest, as the contrast is the least striking,
and the progress has been comparatively the slowest. The knowledge of
large, accessible, and attractive-looking animals had naturally preceded
that of minute and obscure organisms, and hence, while in many other
departments the advance has altogether revolutionised the knowledge of
Linnæus, in the Vertebrated Classes, especially the one of which I shall
now speak, it has only extended and reformed it.
In taking the _Systema Naturæ_ of Linnæus, the comparison is certainly
carried back somewhat beyond the hundred years which have elapsed since
his death, and the brilliant contributions to the knowledge of the
Mammalia of Buffon and Daubenton, just then beginning to be known, and
the systematic compilation of Erxleben (published in 1777), are ignored;
but for the present purpose, especially considering the limited time at
my disposal, it will be best not to go beyond the actual text of the
work in question.
Before considering systematically the different groups into which
Linnæus divides the class, I must remark in passing upon what is the
greatest, and indeed most marvellous difference between the knowledge of
Zoology of our time and that of Linnæus. Now we know that the animals at
present existing upon the earth are merely the survivors of an
immensity of others, different in form, characters, and mode of life,
which have peopled the earth through vast ages of time, and to which
numerically our existing forms are but infinitesimally small, and that
the knowledge we already possess of great numbers fully justifies the
expectation of an enormous further advance in this direction. In the
time of Linnæus the existence in any past time of a species having no
longer living representatives on the earth, though perhaps the
speculation of a few philosophical minds, had not been received among
the certainties of science, and at all events found no place in the
great work we are now considering.
In the twelfth edition of the _Systema Naturæ_ we find the class
MAMMALIA divided into seven orders: I. _Primates_, II. _Bruta_, III.
_Feræ_, IV. _Glires_, V. _Pecora_, VI. _Belluæ_, VII. _Cete_. These
orders contain forty genera without any intermediate subdivisions. The
genera are again divided into species, of which the total number is 220.
The first order, PRIMATES, contains four genera: _Homo_, _Simia_,
_Lemur_, and _Vespertilio_.
The vexed question of man’s place in the zoological system was thus
settled by Linnæus. He belongs to the class _Mammalia_, and the order
_Primates_, the same order which includes all known monkeys, lemurs, and
bats: he differs only generically from these animals. But then we must
remember that the Linnæan genera were not our genera; they correspond
usually to what we call families, sometimes to entire orders. So that
practically man’s position is much the same as that to which, after
several vicissitudes, as his separation as an order by Blumenbach and
Cuvier, or as a sub-class by Owen, he has returned in the systems of
nearly all the zoologists of the present day who treat of him as a
subject for classification upon zoological and not metaphysical grounds.
Yet since the time of Linnæus the whole science of Anthropology has been
created. There is certainly an attempt at the division of the species
_Homo sapiens_ into six varieties in the _Systema Naturæ_, but it has
scarcely any scientific basis. Zoological Anthropology may be said to
have commenced with Blumenbach, who, it is interesting to recall as an
evidence of the rapid growth of the science, was a contemporary with
most of us in this room, for he died as lately as 1840, although his
first work on the subject, _De generis humani varietate nativa_, was
published three years before the death of Linnæus, too late, however, to
influence the work we are now speaking of. The scientific study of the
natural history of man is therefore, we may say, but one century old. To
what it has grown during that time you are probably aware. Scarcely an
important centre of civilisation in the world but has a special Society
devoted to its cultivation. It forms by itself a special department of
the Biological Section of our Association--a department of such
importance that on this occasion no less distinguished a person than a
former most eminent President of the whole Association was thought fit
to take charge of it. From him you will doubtless hear what is its
present scope, aim, and compass. I need only remind you that except the
one cardinal point of the zoological relation of man to other forms of
life, which Linnæus appears to have appreciated with intuitive
perception, all else that you will now hear in that department was not
dreamt of in his philosophy.
As might naturally be supposed, apes and monkeys have, for various
reasons, attracted the attention of observers of nature from very early
times, and consequently Linnæus was able to give rather a goodly list of
species of these animals, amounting to thirty-three; but of their mutual
affinities, and of the important structural differences which exist
between many of them, he seems to have had no idea, his three divisions
being simply regulated by the condition of the tail, whether absent,
short, or long.
We now know that the so-called Anthropoid or man-like apes, the gorilla,
chimpanzee, orang, and gibbons, form a group apart from all the others
of such importance that everything related to their history, structure,
and habits has been most assiduously studied, and there is now an
immense literature devoted to this group alone. Nothing could better
illustrate the advances we have made in a hundred years, than the
contrast of our present knowledge of these forms with that of Linnæus.
It is true that, as shown in the most interesting story of the gradual
development of our knowledge relating to them in the first chapter of
Huxley’s _Man’s Place in Nature_, the animal now called gorilla was,
without doubt, the pongo, well known to and clearly described by our
countryman, Andrew Battle, a contemporary of Shakespeare; and that a
really accurate and scientific account of the anatomy of the chimpanzee
had been published as far back as 1699 by Dr. Edward Tyson, who, as the
first English comparative anatomist, I am proud to claim as in some sort
a predecessor in the chair I have the honour to hold in London, as he is
described on the title-page of his work as “Reader of Anatomy at
Chirurgeons’ Hall.”
Linnæus was, however, not acquainted with these, and his second species
of the genus _Homo_, _H. troglodytes_, and his first of the genus
_Simia_, _S. satyrus_, were both made up of vague and semi-fabulous
accounts of the animals now known as chimpanzees and orangs, but
hopelessly confounded together. Of the gorilla, and what is stranger
still, of any of the large genus of gibbons, or long-armed apes of
South-eastern Asia, he had at the time he revised the _Systema_ no idea.
The remaining monkeys, we now know, fall into three very distinct
sections: the _Cercopithecidæ_ of the Old World, and the _Cebidæ_ and
_Hapalidæ_ of the New, or by whatever other names we may like to
designate them. Although members of all three groups appear in the list
in the _Systema_, they are all confusedly mixed together. Even that the
American monkeys belong to a totally different stock from those of the
Old World does not seem to have been suspected.
The genus _Lemur_ of Linnæus comprehends five species, of which the
first four were all the then known forms of a most interesting section
of the Mammalia. These animals, mostly inhabitants of the great island
of Madagascar, though some are found in the African continent, and
others in some of the Southern and Eastern parts of Asia, constitute a
well-defined group, but one of which the relations are very uncertain.
At one time, as in the system of Linnæus, they were closely associated
with the monkeys. As more complete knowledge of their organisation has
been gradually attained, the interval which separates them structurally
from those animals has become continually more evident, and since they
cannot be placed within the limits of any of the previously constituted
orders, it has been considered advisable by some naturalists to increase
the ordinal divisions in their behalf and to allow them to take rank as
a distinct group, related to the _Primates_ on the one hand, and to the
_Carnivora_ and _Insectivora_ on the other. The knowledge of their
relations, however, bids fair to be greatly increased by the discoveries
of fossil forms lately made both in France and America, some of which
seem to carry their affinities even to the _Ungulata_.