in favour of the pedigree of the Horses.
The genus _Equus_ is represented as far back as the latter part of the
Miocene epoch; but in deposits belonging to the middle of that
epoch its place is taken by two other genera, _Hipparion_ and
_Anchitherium_[1]; and, in the lowest Miocene and upper Eocene, only
the last genus occurs. A species of _Anchitherium_ was referred by
Cuvier to the _Palaeotheria_ under the name of _P. aurelianense_. The
grinding-teeth are in fact very similar in shape and in pattern, and
in the absence of any thick layer of cement, to those of some species
of _Palaeotherium_, especially Cuvier's _Palaeotherium minus_, which
has been formed into a separate genus, _Plagiolophus_, by Pomel. But
in the fact that there are only six full-sized grinders in the lower
jaw, the first premolar being very small; that the anterior grinders
are as large as, or rather larger than, the posterior ones; that the
second premolar has an anterior prolongation; and that the posterior
molar of the lower jaw has, as Cuvier pointed out, a posterior lobe of
much smaller size and different form, the dentition of _Anchitherium_
departs from the type of the _Palaeotherium_, and approaches that of
the Horse.
[Footnote 1: Hermann von Meyer gave the name of _Anchitherium_ to _A.
Ezguerrae_; and in his paper on the subject he takes great pains
to distinguish the latter as the type of a new genus, from Cuvier's
_Palaeotherium d'Orléans._ But it is precisely the _Palaeotherium
d'Orléans_ which is the type of Christol's genus _Hipparitherium_; and
thus, though _Hipparitherium_ is of later date than _Anchitherium_,
it seemed to me to have a sort of equitable right to recognition
when this address was written. On the whole, however, it seems most
convenient to adopt _Anchitherium_.]
Again, the skeleton of _Anchitherium_ is extremely equine. M. Christol
goes so far as to say that the description of the bones of the
horse, or the ass, current in veterinary works, would fit those of
_Anchitherium._ And, in a general way, this may be true enough; but
there are some most important differences, which, indeed, are justly
indicated by the same careful observer. Thus the ulna is complete
throughout, and its shaft is not a mere rudiment, fused into one bone
with the radius. There are three toes, one large in the middle and one
small on each side. The femur is quite like that of a horse, and has
the characteristic fossa above the external condyle. In the British
Museum there is a most instructive specimen of the leg-bones, showing
that the fibula was represented by the external malleolus and by a
flat tongue of bone, which extends up from it on the outer side of the
tibia, and is closely ankylosed with the latter bone.[1] The hind toes
are three, like those of the fore leg; and the middle metatarsal bone
is much less compressed from side to side than that of the horse.
[Footnote 1: I am indebted to M. Gervais for a specimen which
indicates that the fibula was complete, at any rate, in some cases;
and for a very interesting ramus of a mandible, which shows that, as
in the _Palaeotheria_, the hindermost milk-molar of the lower jaw
was devoid of the posterior lobe which exists in the hindermost true
molar.]
In the _Hipparion_ the teeth nearly resemble those of the Horses,
though the crowns of the grinders are not so long; like those of the
Horses, they are abundantly coated with cement. The shaft of the
ulna is reduced to a mere style ankylosed throughout nearly its whole
length with the radius, and appearing to be little more than a ridge
on the surface of the latter bone until it is carefully examined. The
front toes are still three, but the outer ones are more slender than
in _Anchitherium_, and their hoofs smaller in proportion to that of
the middle toe: they are, in fact, reduced to mere dew-claws, and do
not touch the ground. In the leg, the distal end of the fibula is so
completely united with the tibia that it appears to be a mere process
of the latter bone, as in the Horses.
In _Equus_, finally, the crowns of the grinding-teeth become longer,
and their patterns are slightly modified; the middle of the shaft of
the ulna usually vanishes, and its proximal and distal ends ankylose
with the radius. The phalanges of the two outer toes in each foot
disappear, their metacarpal and metatarsal bones being left as the
"splints."
The _Hipparion_ has large depressions on the face in front of the
orbits, like those for the "larmiers" of many ruminants; but traces
of these are to be seen in some of the fossil horses from the Sewalik
Hills; and, as Leidy's recent researches show, they are preserved in
_Anchitherium_.
When we consider these facts, and the further circumstance that
the Hipparions, the remains of which have been collected in immense
numbers, were subject, as M. Gaudry and others have pointed out, to
a great range of variation, it appears to me impossible to resist the
conclusion that the types of the _Anchitherium_, of the _Hipparion_,
and of the ancient Horses constitute the lineage of the modern Horses,
the _Hipparion_ being the intermediate stage between the other two,
and answering; to B in my former illustration.
The process by which the _Anchitherium_ has been converted into
_Equus_ is one of specialization, or of more and more complete
deviation from what might be called the average form of an ungulate
mammal. In the Horses, the reduction of some parts of the limbs,
together with the special modification of those which are left, is
carried to a greater extent than in any other hoofed mammals. The
reduction is less and the specialization is less in the _Hipparion_,
and still less in the _Anchitherium_; but yet, as compared with
other mammals, the reduction and specialization of parts in the
_Anchitherium_ remain great.
Is it not probable then, that, just as in the Miocene epoch, we find
an ancestral equine form less modified than _Equus_, so, if we go
back to the Eocene epoch, we shall find some quadruped related to the
_Anchitherium_, as _Hipparion_ is related to _Equus_, and consequently
departing less from the average form?
I think that this desideratum is very nearly, if not quite, supplied
by _Plagiolophus_, remains of which occur abundantly in some parts
of the Upper and Middle Eocene formations. The patterns of
the grinding-teeth of _Plagiolophus_ are similar to those of
_Anchitherium_, and their crowns are as thinly covered with cement;
but the grinders diminish in size forwards, and the last lower molar
has a large hind lobe, convex outwards and concave inwards, as in
_Palceotherium_. The ulna is complete and much larger than in any
of the _Equidae_, while it is more slender than in most of the true
_Palaeotheria_; it is fixedly united, but not ankylosed, with the
radius. There are three toes in the fore limb, the outer ones being
slender, but less attenuated than in the _Equidae_. The femur is more
like that of the _Palaeotheria_ than that of the horse, and has only
a small depression above its outer condyle in the place of the great
fossa which is so obvious in the _Equidae_. The fibula is distinct,
but very slender, and its distal end is ankylosed with the tibia.
There are three toes on the hind foot having similar proportions to
those on the fore foot. The principal metacarpal and metatarsal bones
are flatter than they are in any of the _Equidae_; and the metacarpal
bones are longer than the metatarsals, as in the _Palaeotheria_.
In its general form, _Plagiolophus_ resembles a very small and slender
horse[1], and is totally unlike the reluctant, pig-like creature
depicted in Cuvier's restoration of his _Palaeotherium minus_ in the
"Os semens Fossils."
[Footnote 1: Such, at least, is the conclusion suggested by the
proportions of the skeleton figured by Cuvier and De Blainville; but
perhaps something between a Horse and an Agouti would be nearest the
mark.]
It would be hazardous to say that _Plagiolophus_ is the exact radical
form of the Equine quadrupeds; but I do not think there can be any
reasonable doubt that the latter animals have resulted from the
modification of some quadruped similar to _Plagiolophus_.
We have thus arrived at the Middle Eocene formation, and yet
have traced back the Horses only to a three-toed stock; but these
three-toed forms, no less than the Equine quadrupeds themselves,
present rudiments of the two other toes which appertain to what I
have termed the "average" quadruped. If the expectation raised by
the splints of the Horses that, in some ancestor of the Horses, these
splints would be found to be complete digits, has been verified,
we are furnished with very strong reasons for looking for a no
less complete verification of the expectation that the three-toed.
_Plagiolophus_-like "avus" of the horse must have had a five-toed
"atavus" at some earlier period.
No such five-toed "atavus," however, has yet made its appearance among
the few middle and older Eocene _Mammalia_ which are known.
Another series of closely affiliated forms, though the evidence they
afford is perhaps less complete than that of the Equine series,
is presented to us by the _Dichobune_ of the Eocene epoch, the
_Cainotherium_ of the Miocene, and the _Tragulidae_, or so-called
"Musk-deer," of the present day.
The _Tragulidae_ have no incisors in the upper jaw, and only six
grinding-teeth on each side of each jaw; while the canine is moved up
to the outer incisor, and there is a diastema, in the lower jaw. There
are four complete toes on the hind foot, but the middle metatarsals
usually become, sooner or later, ankylosed into a cannon bone. The
navicular and the cuboid unite, and the distal end of the fibula is
ankylosed with the tibia.
In _Cainotherium_ and _Dichobune_ the upper incisors are fully
developed. There are seven grinders; the teeth form a continuous
series without a diastema. The metatarsals, the navicular and cuboid,
and the distal end of the fibula, remain free. In the _Cainotherium_,
also, the second metacarpal is developed, but is much shorter than the
third, while the fifth is absent or rudimentary. In this respect it
resembles _Anoplotherium secundarium_. This circumstance, and the
peculiar pattern of the upper molars in _Cainotherium_, lead me
to hesitate in considering it as the actual ancestor of the modern
_Tragulidae_. If _Dichobune_ has a four-toed fore foot (though I am
inclined to suspect that it resembles _Cainotherium_), it will be a
better representative of the oldest forms of the Traguline series; but
_Dichobune_ occurs in the Middle-Eocene, and is, in fact, the oldest
known artiodactyle mammal. Where, then, must we look for its five-toed
ancestor?
If we follow down other lines of recent and tertiary _Ungulata_, the
same question presents itself. The Pigs are traceable back through
the Miocene epoch to the Upper Eocene, where they appear in the
two well-marked forms of _Hyopotamus_ and _Chaeropotamus_; but
_Hyopotamus_ appears to have had only two toes.
Again, all the great groups of the Ruminants, the _Bovidae,
Antilopidae, Camelopardalidae_, and _Cervidae_, are represented in
the Miocene epoch, and so are the Camels. The Upper Eocene
_Anoplotherium_, which is intercalary between the Pigs and the
_Tragulidae_, has only two or, at most, three toes. Among the scanty
mammals of the Lower Eocene formation we have the perissodactyle
_Ungulata_ represented by _Coryphodon, Hyra-cotherium_, and
_Pliolophus_. Suppose for a moment, for the sake of following out
the argument, that _Pliolophus_ represents the primary stock of the
Perissodactyles, and _Dichobune_ that of the Artiodactyles (though
I am far from saying that such is the case), then we find, in the
earliest fauna of the Eocene epoch to which our investigations carry
us, the two divisions of the _Ungulata_ completely differentiated, and
no trace of any common stock of both, or of five-toed predecessors to
either. With the case of the Horses before us, justifying a belief in
the production of new animal forms by modification of old ones, I see
no escape from the necessity of seeking for these ancestors of the
_Ungulata_ beyond the limits of the Tertiary formations.
I could as soon admit special creation, at once, as suppose that the
Perissodactyles and Artiodactyles had no five-toed ancestors. And when
we consider how large a portion of the Tertiary period elapsed before
_Anchitherium_ was converted into _Equus_, it is difficult to escape
the conclusion that a large proportion of time anterior to the
Tertiary period must have been expended in converting the common stock
of the _Ungulata_ into Perissodactyles and Artiodactyles.
The same moral is inculcated by the study of every other order of
Tertiary monodelphous _Mammalia_. Each of these orders is represented
in the Miocene epoch: the Eocene formation, as I have already said,
contains _Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora,_
and _Cetacea_. But the _Cheiroptera_ are extreme modifications of the
_Insectivora_, just as the _Cetacea_ are extreme modifications of
the Carnivorous type; and therefore it is to my mind incredible
that monodelphous _Insectivora_ and _Carnivora_ should not have been
abundantly developed, along with _Ungulata_, in the Mesozoic epoch.
But if this be the case, how much further back must we go to find the
common stock of the monodelphous _Mammalia_? As to the _Didelphia_,
if we may trust the evidence which seems to be afforded by their
very scanty remains, a Hypsiprymnoid form existed at the epoch of the
Trias, contemporaneously with a Carnivorous form. At the epoch of the
Trias, therefore, the _Marsupialia_ must have, already existed long
enough to have become differentiated into carnivorous and herbivorous
forms. But the _Monotremata_ are lower forms than the _Didelphia,_
which last are intercalary between the _Ornithodelphia_ and the
_Monodelphia_. To what point of the Palaeozoic epoch, then, must we,
upon any rational estimate, relegate the origin of the _Monotremata_?
The investigation of the occurrence of the classes and of the orders
of the _Sauropsida_ in time points in exactly the same direction.
If, as there is great reason to believe, true Birds existed in the
Triassic epoch, the ornithoscelidous forms by which Reptiles passed
into Birds must have preceded them. In fact there is, even at present,
considerable ground for suspecting the existence of _Dinosauria_ in
the Permian formations; but, in that case, lizards must be of still
earlier date. And if the very small differences which are observable
between the _Crocodilia_ of the older Mesozoic formations and those of
the present day furnish any sort of approximation towards an estimate
of the average rate of change among the _Sauropsida_, it is almost
appalling to reflect how far back in Palaeozoic times we must go,
before we can hope to arrive at that common stock from which the
_Crocodilia, Lacertilia, Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have
been derived.
The _Amphibia_ and _Pisces_ tell the same story. There is not a
single class of vertebrated animals which, when it first appears,
is represented by analogues of the lowest known members of the same
class. Therefore, if there is any truth in the doctrine of evolution,
every class must be vastly older than the first record of its
appearance upon the surface of the globe. But if considerations of
this kind compel us to place the origin of vertebrated animals at
a period sufficiently distant from the Upper Silurian, in which the
first Elasmobranchs and Ganoids occur, to allow of the evolution of
such fishes as these from a Vertebrate as simple as the _Amphioxus_,
I can only repeat that it is appalling to speculate upon the extent to
which that origin must have preceded the epoch of the first recorded
appearance of vertebrate life.
Such is the further commentary which I have to offer upon the
statement of the chief results of palaeontology which I formerly
ventured to lay before you.
But the growth of knowledge in the interval makes me conscious of
an omission of considerable moment in that statement, inasmuch as it
contains no reference to the bearings of palaeontology upon the theory
of the distribution of life; nor takes note of the remarkable manner
in which the facts of distribution, in present and past times, accord
with the doctrine of evolution, especially in regard to land animals.
That connection between palaeontology and geology and the present
distribution of terrestrial animals, which so strikingly impressed
Mr. Darwin, thirty years ago, as to lead him to speak of a "law of
succession of types," and of the wonderful relationship on the same
continent between the dead and the living, has recently received much
elucidation from the researches of Gaudry, of Rütimeyer, of Leidy,
and of Alphonse Milne-Edwards, taken in connection with the
earlier labours of our lamented colleague Falconer; and it has been
instructively discussed in the thoughtful and ingenious work of Mr.
Andrew Murray "On the Geographical Distribution of Mammals."[1]
[Footnote 1: The paper "On the Form and Distribution of the
Land-tracts during the Secondary and Tertiary Periods respectively;
and on the Effect upon Animal Life which great Changes in Geographical
Configuration have probably produced," by Mr. Searles V. Wood, jun.,
which was published in the _Philosophical Magazine_, in 1862, was
unknown to me when this Address was written. It is well worthy of the
most careful study.]
I propose to lay before you, as briefly as I can, the ideas to which a
long consideration of the subject has given rise in my own mind.
If the doctrine of evolution is sound, one of its immediate
consequences clearly is, that the present distribution of life
upon the globe is the product of two factors, the one being the
distribution which obtained in the immediately preceding epoch, and
the other the character and the extent of the changes which have taken
place in physical geography between the one epoch and the other; or,
to put the matter in another way, the Fauna and Flora of any given
area, in any given epoch, can consist only of such forms of life as
are directly descended from those which constituted the Fauna and
Flora of the same area in the immediately preceding epoch, unless the
physical geography (under which I include climatal conditions) of
the area has been so altered as to give rise to immigration of living
forms from some other area.
The evolutionist, therefore, is bound to grapple with the following
problem whenever it is clearly put before him:--Here are the Faunae of
the same area during successive epochs. Show good cause for believing
either that these Faunae have been derived from one another by gradual
modification, or that the Faunae have reached the area in question
by migration from some area in which they have undergone their
development.
I propose to attempt to deal with this problem, so far as it is
exemplified by the distribution of the terrestrial _Vertebrata_, and I
shall endeavour to show you that it is capable of solution in a sense
entirely favourable to the doctrine of evolution.
I have elsewhere[1] stated at length the reasons which lead me to
recognize four primary distributional provinces for the terrestrial
_Vertebrata_ in the present world, namely,--first, the _Novozelanian_,
or New-Zealand province; secondly, the _Australian_ province,
including Australia, Tasmania, and the Negrito Islands; thirdly,
_Austro-Columbia_, or South America _plus_ North America as far as
Mexico; and fourthly, the rest of the world, or _Arctogaea_, in which
province America north of Mexico constitutes one sub-province, Africa
south of the Sahara a second, Hindostan a third, and the remainder of
the Old World, a fourth.
[Footnote 1: "On the Classification and Distribution of the
Alectoromorphae;" Proceedings of the Zoological Society, 1868.]
Now the truth which Mr. Darwin perceived and promulgated as "the law
of the succession of types" is, that, in all these provinces, the
animals found in Pliocene or later deposits are closely affined to
those which now inhabit the same provinces; and that, conversely, the
forms characteristic of other provinces are absent. North and South
America, perhaps, present one or two exceptions to the last rule, but
they are readily susceptible of explanation. Thus, in Australia, the
later Tertiary mammals are marsupials (possibly with exception of the
Dog and a Rodent or two, as at present). In Austro-Columbia the later
Tertiary fauna exhibits numerous and varied forms of Platyrrhine
Apes, Rodents, Cats, Dogs, Stags, _Edentata_, and Opossums; but, as
at present, no Catarrhine Apes, no Lemurs, no _Insectivora_, Oxen,
Antelopes, Rhinoceroses, nor _Didelphia_ other than Opossums. And in
the wide-spread Arctogaeal province, the Pliocene and later mammals
belong to the same groups as those which now exist in the province.
The law of succession of types, therefore, holds good for the present
epoch as compared with its predecessor. Does it equally well apply to
the Pliocene fauna when we compare it with that of the Miocene epoch?
By great good fortune, an extensive mammalian fauna of the latter
epoch has now become known, in four very distant portions of the
Arctogaeal province which do not differ greatly in latitude. Thus
Falconer and Cautley have made known the fauna of the sub-Himalayas
and the Perim Islands; Gaudry that of Attica; many observers that of
Central Europe and France; and Leidy that of Nebraska, on the eastern
flank of the Rocky Mountains. The results are very striking. The total
Miocene fauna comprises many genera, and species of Catarrhine Apes,
of Bats, of _Insectivora_; of Arctogaeal types of _Rodentia_; of
_Proboscidea_; of equine, rhinocerotic, and tapirine quadrupeds; of
cameline, bovine, antilopine, cervine, and traguline Ruminants; of
Pigs and Hippopotamuses; of _Viverridae_ and _Hyaenidae_ among other
_Carnivora_; with _Edentata_ allied to the Arctogaeal _Orycteropus_
and _Manis_, and not to the Austro-Columbian Edentates. The only type
present in the Miocene, but absent in the existing, fauna of Eastern
Arctogaea, is that of the _Didelphidae_, which, however, remains in
North America.
But it is very remarkable that while the Miocene fauna of the
Arctogaeal province, as a whole, is of the same character as the
existing fauna of the same province, as a whole, the component
elements of the fauna were differently associated. In the Miocene
epoch, North America possessed Elephants, Horses, Rhinoceroses, and
a great number and variety of Ruminants and Pigs, which are absent
in the present indigenous fauna; Europe had its Apes, Elephants,
Rhinoceroses, Tapirs, Musk-deer, Giraffes, Hyaenas, great Cats,
Edentates, and Opossum-like Marsupials, which have equally vanished
from its present fauna; and in Northern India, the African types of
Hippopotamuses, Giraffes, and Elephants were mixed up with what
are now the Asiatic types of the latter, and with Camels, and
Semnopithecine and Pithecine Apes of no less distinctly Asiatic forms.