Discourses: Biological & Geological Part 19

1. The living population of all parts of the earth's surface which have yet been examined has undergone a succession of changes which, upon the whole, have been of a slow and gradual character.

2. When the fossil remains which are the evidences of these successive changes, as they have occurred in any two more or less distant parts of the surface of the earth, are compared, they exhibit a certain broad and general parallelism. In other words, certain forms of life in one locality occur in the same general order of succession as, or are _h.o.m.otaxial_ with, similar forms in the other locality.

3. h.o.m.otaxis is not to be held identical with synchronism without independent evidence. It is possible that similar, or even identical, faunae and florae in two different localities may be of extremely different ages, if the term "age" is used in its proper chronological sense. I stated that "geographical provinces, or zones, may have been as distinctly marked in the Palaeozoic epoch as at present; and those seemingly sudden appearances of new genera and species which we ascribe to new creation, may be simple results of migration."

4. The opinion that the oldest known fossils are the earliest forms of life has no solid foundation.

5. If we confine ourselves to positively ascertained facts, the total amount of change in the forms of animal and vegetable life, since the existence of such forms is recorded, is small. When compared with the lapse of time since the first appearance of these forms, the amount of change is wonderfully small. Moreover, in each great group of the animal and vegetable kingdoms, there are certain forms which I termed PERSISTENT TYPES, which have remained, with but very little apparent change, from their first appearance to the present time.

6. In answer to the question "What, then, does an impartial survey of the positively ascertained truths of palaeontology testify in relation to the common doctrines of progressive modification, which suppose that modification to have taken place by a necessary progress from more to less embryonic forms, from more to less generalised types, within the limits of the period represented by the fossiliferous rocks?" I reply, "It negatives these doctrines; for it either shows us no evidence of such modification, or demonstrates such modification as has occurred to have been very slight; and, as to the nature of that modification, it yields no evidence whatsoever that the earlier members of any long-continued group were more generalised in structure than the later ones."

I think that I cannot employ my last opportunity of addressing you, officially, more properly--I may say more dutifully--than in revising these old judgments with such help as further knowledge and reflection, and an extreme desire to get at the truth, may afford me.

1. With respect to the first proposition, I may remark that whatever may be the case among the physical geologists, catastrophic palaeontologists are practically extinct. It is now no part of recognised geological doctrine that the species of one formation all died out and were replaced by a brand-new set in the next formation. On the contrary, it is generally, if not universally, agreed that the succession of life has been the result of a slow and gradual replacement of species by species; and that all appearances of abruptness of change are due to breaks in the series of deposits, or other changes in physical conditions. The continuity of living forms has been unbroken from the earliest times to the present day.

2, 3. The use of the word "h.o.m.otaxis" instead of "synchronism" has not, so far as I know, found much favour in the eyes of geologists. I hope, therefore, that it is a love for scientific caution, and not mere personal affection for a bantling of my own, which leads me still to think that the change of phrase is of importance, and that the sooner it is made, the sooner shall we get rid of a number of pitfalls which beset the reasoner upon the facts and theories of geology.

One of the latest pieces of foreign intelligence which has reached us is the information that the Austrian geologists have, at last, succ.u.mbed to the weighty evidence which M. Barrande has acc.u.mulated, and have admitted the doctrine of colonies. But the admission of the doctrine of colonies implies the further admission that even ident.i.ty of organic remains is no proof of the synchronism of the deposits which contain them.

4. The discussions touching the _Eozoon,_ which commenced in 1864, have abundantly justified the fourth proposition. In 1862, the oldest record of life was in the Cambrian rocks; but if the _Eozoon_ be, as Princ.i.p.al Dawson and Dr. Carpenter have shown so much reason for believing, the remains of a living being, the discovery of its true nature carried life back to a period which, as Sir William Logan has observed, is as remote from that during which the Cambrian rocks were deposited, as the Cambrian epoch itself is from the tertiaries. In other words, the ascertained duration of life upon the globe was nearly doubled at a stroke.

5. The significance of persistent types, and of the small amount of change which has taken place even in those forms which can be shown to have been modified, becomes greater and greater in my eyes, the longer I occupy myself with the biology of the past.

Consider how long a time has elapsed since the Miocene epoch. Yet, at that time there is reason to believe that every important group in every order of the _Mammalia_ was represented. Even the comparatively scanty Eocene fauna yields examples of the orders _Cheiroptera, Insectivora, Rodentia_, and _Perissodactyla_; of _Artiodactyla_ under both the Ruminant and the Porcine modifications; of _Caranivora, Cetacea_, and _Marsupialia_.

Or, if we go back to the older half of the Mesozoic epoch, how truly surprising it is to find every order of the _Reptilia_, except the _Ophidia_, represented; while some groups, such as the _Ornithoseclida_ and the _Pterosauria_, more specialised than any which now exist, abounded.

There is one division of the _Amphibia_ which offers especially important evidence upon this point, inasmuch as it bridges over the gap between the Mesozoic and the Palaeozoic formations (often supposed to be of such prodigious magnitude), extending, as it does, from the bottom of the Carboniferous series to the top of the Trias, if not into the Lias. I refer to the Labyrinthodonts. As the Address of 1862 was pa.s.sing through the press, I was able to mention, in a note, the discovery of a large Labyrinthodont, with well-ossified vertebrae, in the Edinburgh coal-field.

Since that time eight or ten distinct genera of Labyrinthodonts have been discovered in the Carboniferous rocks of England, Scotland, and Ireland, not to mention the American forms described by Princ.i.p.al Dawson and Professor Cope. So that, at the present time, the Labyrinthodont Fauna of the Carboniferous rocks is more extensive and diversified than that of the Trias, while its chief types, so far as osteology enables us to judge, are quite as highly organised. Thus it is certain that a comparatively highly organised vertebrate type, such as that of the Labyrinthodonts, is capable of persisting, with no considerable change, through the period represented by the vast deposits which const.i.tute the Carboniferous, the Permian, and the Tria.s.sic formations.

The very remarkable results which have been brought to light by the sounding and dredging operations, which have been carried on with such remarkable success by the expeditions sent out by our own, the American, and the Swedish Governments, under the supervision of able naturalists, have a bearing in the same direction. These investigations have demonstrated the existence, at great depths in the ocean, of living animals in some cases identical with, in others very similar to, those which are found fossilised in the white chalk. The _Globigerinoe_, Cyatholiths, Coccospheres, Discoliths in the one are absolutely identical with those in the other; there are identical, or closely a.n.a.logous, species of Sponges, Echinoderms, and Brachiopods. Off the coast of Portugal, there now lives a species of _Beryx_, which, doubtless, leaves its bones and scales here and there in the Atlantic ooze, as its predecessor left its spoils in the mud of the sea of the Cretaceous epoch.

Many years ago[1] I ventured to speak of the Atlantic mud as "modern chalk," and I know of no fact inconsistent with the view which Professor Wyville Thomson has advocated, that the modern chalk is not only the lineal descendant of the ancient chalk, but that it remains, so to speak, in the possession of the ancestral estate; and that from the Cretaceous period (if not much earlier) to the present day, the deep sea has covered a large part of what is now the area of the Atlantic. But if _Globigerina_, and _Terebratula caput-serpentis_ and _Beryx_, not to mention other forms of animals and of plants, thus bridge over the interval between the present and the Mesozoic periods, is it possible that the majority of other living things underwent a "sea-change into something new and strange" all at once?

[Footnote 1: See an article in the _Sat.u.r.day Review_, for 1858, on "Chalk, Ancient and Modern."]

6. Thus far I have endeavoured to expand and to enforce by fresh arguments, but not to modify in any important respect, the ideas submitted to you on a former occasion. But when I come to the propositions touching progressive modification, it appears to me, with the help of the new light which has broken from various quarters, that there is much ground for softening the somewhat Brutus-like severity with which, in 1862, I dealt with a doctrine, for the truth of which I should have been glad enough to be able to find a good foundation. So far, indeed, as the _Invertebrata_ and the lower _Vertebrata_ are concerned, the facts and the conclusions which are to be drawn from them appear to me to remain what they were. For anything that, as yet, appears to the contrary, the earliest known Marsupials may have been as highly organised as their living congeners; the Permian lizards show no signs of inferiority to those of the present day; the Labyrinthodonts cannot be placed below the living Salamander and Triton; the Devonian Ganoids are closely related to _Polypterus_ and to _Lepidosiren_.

But when we turn to the higher _Vertebrata_, the results of recent investigations, however we may sift and criticise them, seem to me to leave a clear balance in favour of the doctrine of the evolution of living forms one from another. Nevertheless, in discussing this question, it is very necessary to discriminate carefully between the different kinds of evidence from fossil remains which are brought forward in favour of evolution.

Every fossil which takes an intermediate place between forms of life already known, may be said, so far as it is intermediate, to be evidence in favour of evolution, inasmuch as it shows a possible road by which evolution may have taken place. But the mere discovery of such a form does not, in itself, prove that evolution took place by and through it, nor does it const.i.tute more than presumptive evidence in favour of evolution in general. Suppose A, B, C to be three forms, while B is intermediate in structure between A and C. Then the doctrine of evolution offers four possible alternatives. A may have become C by way of B; or C may have become A by way of B; or A and C may be independent modifications of B; or A, B, and C may be independent modifications of some unknown D. Take the case of the Pigs, the _Anoplothcridoe_, and the Ruminants. The _Anoplothcridoe_ are intermediate between the first and the last; but this does not tell us whether the Ruminants have come from the Pigs, or the Pigs from Ruminants, or both from _Anoplothcridoe_, or whether Pigs, Ruminants, and _Anoplotlicridoe_ alike may not have diverged from some common stock.

But if it can be shown that A, B, and C exhibit successive stages in the degree of modification, or specialisation, of the same type; and if, further, it can be proved that they occur in successively newer deposits, A being in the oldest and C in the newest, then the intermediate character of B has quite another importance, and I should accept it, without hesitation, as a link in the genealogy of C. I should consider the burden of proof to be thrown upon any one who denied C to have been derived from A by way of B, or in some closely a.n.a.logous fas.h.i.+on; for it is always probable that one may not hit upon the exact line of filiation, and, in dealing with fossils, may mistake uncles and nephews for fathers and sons.

I think it necessary to distinguish between the former and the latter cla.s.ses of intermediate forms, as _intercalary types_ and _linear types_.

When I apply the former term, I merely mean to say that as a matter of fact, the form B, so named, is intermediate between the others, in the sense in which the _Anoplotherium_ is intermediate between the Pigs and the Ruminants--without either affirming, or denying, any direct genetic relation between the three forms involved. When I apply the latter term, on the other hand, I mean to express the opinion that the forms A, B, and C const.i.tute a line of descent, and that B is thus part of the lineage of C.

From the time when Cuvier's wonderful researches upon the extinct Mammals of the Paris gypsum first made intercalary types known, and caused them to be recognised as such, the number of such forms has steadily increased among the higher _Mammalia_. Not only do we now know numerous intercalary forins of _Ungulata_, but M. Gaudry's great monograph upon the fossils of Pikermi (which strikes me as one of the most perfect pieces of palaeontological work I have seen for a long time) shows us, among the Primates, _Mesopithecus_ as an intercalary form between the _Semnopitheci_ and the _Macaci_; and among the _Carnivora_, _Hyoenictis_ and _Ict.i.therium_ as intercalary, or, perhaps, linear types between the _Viverridoe_ and the _Hyoenidoe_.

Hardly any order of the higher _Mammalia_ stands so apparently separate and isolated from the rest as that of the _Cetacea_; though a careful consideration of the structure of the pinnipede _Carnivora_, or Seals, shows, in them, many an approximation towards the still more completely marine mammals. The extinct _Zeuglodon_, however, presents us with an intercalary form between the type of the Seals and that of the Whales.

The skull of this great Eocene sea-monster, in fact, shows by the narrow and prolonged interorbital region; the extensive union of the parietal bones in a sagittal suture; the well-developed nasal bones; the distinct and large incisors implanted in premaxillary bones, which take a full share in bounding the fore part of the gape; the two-fanged molar teeth with triangular and serrated crowns, not exceeding five on each side in each jaw; and the existence of a deciduous dent.i.tion--its close relation with the Seals. While, on the other hand, the produced rostral form of the snout, the long symphysis, and the low coronary process of the mandible are approximations to the cetacean form of those parts.

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 cla.s.s; 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 cla.s.ses 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 Rutimeyer 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 acc.u.mulate 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 dent.i.tion 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'Orleans_. But it is precisely the _Paloeotherium d'Orleans_ 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 a.s.s, 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 circ.u.mstance 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 const.i.tute the lineage of the modern Horses, the _Hipparion_ being the intermediate stage between the other two, and answering to B in my former ill.u.s.tration.

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 princ.i.p.al 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 "Oss.e.m.e.ns 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.