Chapter 23

The scapula resembles that of the cetacean _Hyperoodon_, but the supra- spinous fossa is larger and more seal-like; as is the humerus, which differs from that of the _Cetacea_ in presenting true articular surfaces for the free jointing of the bones of the fore-arm. In the apparently complete absence of hinder limbs, and in the characters of the vertebral column, the _Zeuglodon_ lies on the cetacean side of the boundary line; so that upon the whole, the Zeuglodonts, transitional as they are, are conveniently retained in the cetacean order. And the publication, in 1864, of M. Van Beneden's memoir on the Miocene and Pliocene _Squalodon_, furnished much better means than anatomists previously possessed of fitting in another link of the chain which connects the existing _Cetacea_ with _Zeuglodon_. The teeth are much more numerous, although the molars exhibit the zeuglodont double fang; the nasal bones are very short, and the upper surface of the rostrum presents the groove, filled up during life by the prolongation of the ethmoidal cartilage, which is so characteristic of the majority of the _Cetacea_.

It appears to me that, just as among the existing _Carnivora_, the walruses and the eared seals are intercalary forms between the fissipede Carnivora and the ordinary seals, so the Zeuglodonts are intercalary between the _Carnivora_, as a whole, and the _Cetacea_. Whether the Zeuglodonts are also linear types in their relation to these two groups cannot be ascertained, until we have more definite knowledge than we possess at present, respecting the relations in time of the _Carnivora_ and _Cetacea_.

Thus far we have been concerned with the intercalary types which occupy the intervals between Families or Orders of the same class; but the investigations which have been carried on by Professor Gegenbaur, Professor Cope, and myself into the structure and relations of the extinct reptilian forms of the _Ornithoscelida_ (or _Dinosauria_ and _Compsognatha_) have brought to light the existence of intercalary forms between what have hitherto been always regarded as very distinct classes of the vertebrate sub-kingdom, namely _Reptilia_ and _Aves_. Whatever inferences may, or may not, be drawn from the fact, it is now an established truth that, in many of these _Ornithoscelida_, the hind limbs and the pelvis are much more similar to those of Birds than they are to those of Reptiles, and that these Bird-reptiles, or Reptile-birds, were more or less completely bipedal.

When I addressed you in 1862, I should have been bold indeed had I suggested that palaeontology would before long show us the possibility of a direct transition from the type of the lizard to that of the ostrich. At the present moment, we have, in the _Ornithoscelida_, the intercalary type, which proves that transition to be something more than a possibility; but it is very doubtful whether any of the genera of _Ornithoscelida_ with which we are at present acquainted are the actual linear types by which the transition from the lizard to the bird was effected. These, very probably, are still hidden from us in the older formations.

Let us now endeavour to find some cases of true linear types, or forms which are intermediate between others because they stand in a direct genetic relation to them. It is no easy matter to find clear and unmistakable evidence of filiation among fossil animals; for, in order that such evidence should be quite satisfactory, it is necessary that we should be acquainted with all the most important features of the organisation of the animals which are supposed to be thus related, and not merely with the fragments upon which the genera and species of the palaeontologist are so often based. M. Gaudry has arranged the species of _Hyoenidoe, Proboscidea, Rhinocerotidoe_, and _Equidoe_ in their order of filiation from their earliest appearance in the Miocene epoch to the present time, and Professor Rütimeyer has drawn up similar schemes for the Oxen and other _Ungulata_--with what, I am disposed to think, is a fair and probable approximation to the order of nature. But, as no one is better aware than these two learned, acute, and philosophical biologists, all such arrangements must be regarded as provisional, except in those cases in which, by a fortunate accident, large series of remains are obtainable from a thick and widespread series of deposits. It is easy to accumulate probabilities--hard to make out some particular case in such a way that it will stand rigorous criticism.

After much search, however, I think that such a case is to be made out 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_;[2] and, in the lowest Miocene and upper Eocene, only the last genus occurs. A species of _Anchitherium_ was referred by Cuvier to the _Paloeotheria_ 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 _Paloeotherium_, especially Cuvier's _Paloeotherium 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 _Paloeotherium_, and approaches that of the Horse.

[Footnote 2: Hermann von Meyer gave the name of _Anchitherium_ to _A. Ezquerroe_; 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 _Paloeotherium d'Orléans_. But it is precisely the _Paloeotherium 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.[3] 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 3: 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 ramps of a mandible, which shows that, as in the _Paloeotheria_, 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 specialisation, 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 specialisation is less in the _Hipparion_, and still less in the _Anchitherium_; but yet, as compared with other mammals, the reduction and specialisation 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 _Palueotherium_. The ulna is complete and much larger than in any of the _Equidoe_, while it is more slender than in most of the true _Paloeotheria_; 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 _Equidoe_. The femur is more like that of the _Paloeotheria_ 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 _Equidoe_. 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 _Equidoe_; and the metacarpal bones are longer than the metatarsals, as in the _Paloeotheria_.

In its general form, _Plagiolophus_ resembles a very small and slender horse,[4] and is totally unlike the reluctant, pig-like creature depicted in Cuvier's restoration of his _Paloeotherium minus_ in the "Ossemens Fossiles."

[Footnote 4: 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 _Tragulidoe_, or so-called "Musk- deer," of the present day.

The _Tragulidoe_; 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 _Tragulidoe_. If _Dichobune_ has a fore-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 _Hyopopotamus_ and _Choeropotamus_; but _Hyopotamus_ appears to have had only two toes.

Again, all the great groups of the Ruminants, the _Bovidoe, Antilopidoe, Camelopardalidoe_, and _Cervidoe_, are represented in the Miocene epoch, and so are the Camels. The Upper Eocene _Anoplotherium_, which is intercalary between the Pigs and the _Tragulidoe_, 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, Hyracotherium_, 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 Rutimeyer, 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."[5]

[Footnote 5: The paper "On the Form and Distribution of the Landtracts 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 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.