RBH left a comment to a previous posting that inspired me to put some material together to address his (or anyone’s) reservations on the subject of intermediate fossil forms and the pre-Darwin (creationist) geologists.

Another really helpful post, Troy. Thanks!

Thanks RB!

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Addendum

I realized during my discussions with RBH in the comments to this post that my main point behind writing about this subject might not be transparent to the average reader who doesn’t eat, sleep, and breath the creation/evolution debate. So I add this preface to give the reader a context for why I am going on at length about early 19th century geologists.

My point in all this is less about understanding the often vague and sometimes even contradictory views of the pre-Darwin scientists (as worthy as that subject of study is) and more about countering the arguments from modern antievolutionists that intermediate fossils do not exist and that those paleontologists who claim that they do, do so only because they are reading their “evolutionary beliefs” into the evidence.

If the pre-Darwin creationist geologists saw intermediates this tends, strongly I think, to falsify that argument. The same applies to the overall pattern of the fossil record and the geologic column that illustrates it (which is also frequently claimed by antievolutionists to be a evolutionary invention).

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RBH: I do have one reservation. You wrote

The changing pattern of the fossil record and the existence of intermediate fossil forms was recognized by scientists (who were creationists) long before Darwin brought evolutionary theory into the scientific mainstream.

The changing pattern in the fossil record was surely observed; Cuvier in France and Owen in England — both eminent comparative anatomists in the first half of the 19th century — were very clear on that.

Indeed, Cuvier, Owen and just about every other geologist/paleontologist in the world at the time.

RBH: But Owen opposed Darwin’s hypothesis of species transmutation and common ancestry specifically because he did not see transitional/intermediate forms in the fossil record to which he had access.

I can’t speak much about Cuvier, but Owen is a little difficult to pigeon-hole into modern categories (perhaps a theistic evolutionist of sorts). He did oppose Darwin, particularly Darwin’s mechanism of natural selection but seemed to have been open to the idea of some sort of secondary causation for living things (as opposed to their direct creation by God).

For example in his book Palæontology (1861) after summarizing the Darwin/Wallace case for evolution and concluding that there has been no observed instance of a species arising through their mechanisms, Owen nonetheless goes on to make the case for why he believed that there must be some sort of secondary mechanism by which species originate:

Facts that oppose some of the sumises on the origin of species have been elsewhere pointed out by the writer [Owen]. The generalisations, based on rigorous and extensive observation of facts, which have impressed him [Owen] with a conviction of a continuously operative secondary creational power, originating the succession of species, are the following: that of irrelative or vegetative repetition; that of unity of plan, as demonstrated in the articulate and vertebrate types of organisation; the facts of congenital varieties; the phenomena of parthenogenesis; the analogies of transitory embryonal stages in a higher animal, to the mature forms of lower animals; the great palæontological fact of the successive coming in of new species, from the period of the oldest deposits in which organic remains have been found; such species being limited in time, and never reappearing after once dying out; the many instances of retention of structures in palæozoic species, which are embryonal and transitory in later species of the same order or class; the progressive departure from a general to a special type, as exemplified in the series of species from their first introduction to the present time.

The inductive demonstration of the nature and mode of operation of such secondary continuously operative species-producing force will henceforth be the great aim of the philosophical naturalist. (Owen 1861, pp. 443-444, emphasis mine)

Similarly in the third volume of his On the Anatomy of Vertebrates (1868) while discussing fossil series of perissodactyls Owen wrote:

If the alternative ––species by miracle or by law? ––be applied to Palæotherium, Palopotherium, Anchitherium, Hipparion, Equus, I accept the latter, without misgiving, and recognise such law as continuously operative throughout tertiary time. (Owen 1868, p. 793)

It is true that at times Owen wrote about a lack of intermediate forms, for example there is this passage from his review of the Origin of Species:

The geological record, it is averred, is so imperfect! But what human record is not? Especially must the record of past organisms be much less perfect than of present ones. We freely admit it. But when Mr. Darwin, in reference to the absence of the intermediate fossil forms required by his hypothesis — and only the zootomical zoologist can approximatively appreciate their immense numbers — the countless hosts of transitional links which, on ‘natural selection,’ must certainly have existed at one period or another of the world’s history — when Mr. Darwin exclaims what may be, or what may not be, the forms yet forthcoming out of the graveyards of strata, we would reply, that our only ground for prophesying of what may come, is by the analogy of what has come to light. We may expect, e.g., a chambered-shell from a secondary rock; but not the evidence of a creature linking on the cuttle-fish to the lump-fish.

[…] The last ichthyosaurus, by which the genus disappears in the chalk, is hardly distinguishable specifically from the first ichthyosaurus, which abruptly introduces that strange form of sea-lizard in the lias. The oldest Pterodactyle is as thorough and complete a one as the latest. No contrast can be more remarkable, nor, we believe, more instructive, than the abundance of evidence of the various species of ichthyosaurus throughout the marine strata of the oolite and cretaceous periods, and the utter blank in reference to any form calculated to enlighten us as to whence the ichthyosaurus came, or what it graduated into, before or after these periods. (Owen 1860)

But I take this with a grain of salt simply because it was written in response to idea of evolution (in this case Darwin’s book). I’ll have more to say on this in a few moments.

I feel safe in making such a statement because of the many times Owen referred to existence of intermediate fossil forms outside the context of evolution (at least Lamarckian or Darwinian evolution); he makes repeated references to their existence in a number of his writings. Here are some examples:

Our knowledge of the progression of Mammalian life in Europe during this period, is derived exclusively from continental fossils. These teach us that one or two of the generic forms most frequent in the older tertiary strata still linger on the earth, but that the rest of the eocene Mammalia had been superseded by a new race, some of which present characters intermediate between those of eocene and those of Pliocene genera.  The Dinotherium and narrow-toothed Mastodon, for example, diminish the interval between the Lophiodon and the Elephant; the Anthracotherium and the Hippohyus, that between Chæropotamus and Hippopotamus; and the Acerotherium was a link connecting Palæotherium with Rhinoceros. With these and other forms, as Halitherium, a kind of Dugong wit molar teeth like those of the Hippopotamus, there likewise appear a few genera that predominate in the pliocene strata, and which are still represented on the earth;  though by species quite distinct from those that existed during either of the tertiary periods. (Owen 1846, pp. XXI-XXII, emphasis mine)

What concerns, however, the palæontologist and zoologist chiefly, is the satisfactory recognition of the genus Lophiodon, together with the Tapirotherium, the Palæotherium, Hippotherium, Acerotherium, Maerauchenia, Elasmotherium, and Coryphodon, as links filling up the now broken series of perissodactyle or odd-toed Ungulates, represented by the existing genera Rhinoceros, Hyrax, Tapirus, and Equus.

The progress of palæontological discovery, and especially the additions from our British eocene strata, have tended considerably to complete that other and parallel chain of Ungulata, a portion of which Cuvier designated as ‘les Pachydermes à doigt pairs,’ and which are represented in the actual creation by the widely dissevered links, Sus, Dicotyles, and Hippopotamus.

The genera Adapis, Chæropotamus, Hyracotherium, Meryeopotamus, Hyopotamus, Anthraeotherium, Diehodon, with Xiphodon, Diehobune, Chalicotherium and Anoplotherium, tend not only to bridge over the chasms now separating the few existing forms of artiodactyle or even-toed Pachyderms, but to connect them so closely with the Ruminants as to render it inconsistent with such knowledge to regard the Pecora of Linnaeus as the equivalent and very circumscribed order which they are represented to be in our existing Systems of Zoology. (Owen 1848, pp. 128-129, emphasis mine)

Regularly radiated crinoids are found in the Palæozoic formations, such as Cariocrinus ornatus, in the Upper Silurian; Olivanites Verneuilli, in the Devonian system; Platyerinus and Poteriocrinus, in the Carboniferous; as well as the armless and stemless crinoids, ––perhaps an intermediate link between them and the echinites,––Pentremites; and true echinites, Cidarites Nerie, C. Munsterianus, etc. (Owen 1857, p. 112, emphasis mine)

The Lepidosiren and Archegosaurus are intermediate gradations, one having more the piscine, the other more of the reptilian, characters. The Archegosaurus conducts the march of development from the ganoid fishes to the labyrinthodont type, the Lepidosiren to the perennibranchiate type. Both illustrate the artificiality of the supposed class-distinction between fishes and reptiles, and the unity of the “Hæmatocrya,” or cold-blooded Vertebrata, as a natural group. (Owen 1861, p. 218, emphasis mine)

Succession of Species, broken or linked? –To the hypothesis that existing are modifications of extinct species Cuvier replied, that, in every mooted form of transmutation, the species were made to alter by small degrees, and that, therefore, traces of such gradual modifications were due from the fossil world: ––‘You ought,’ he said, ‘to be able to show, e.g., the intermediate forms between the Palæotherium and existing hoofed quadrupeds.’

The progress of Palæontology since 1830 has brought to light many missing links unknown to the founder of the science. My own share in the labour led me, after a few years’ research, to discern what I believe, and still hold, to be a tendency to a more generalised, or less specialised, organisation as species recede in date of existence from the present time.

[…] The discovery of the remains of the Hipparion supplied one of the links required by Cuvier, between the Palæotherium and the Horse of the present day, and it is still more significant of the fact of filiation of species that the remains of such three-toed Horses are found only in deposits of that tertiary period which intervene between the older palæotherian on and the newer strata in which the modern Horse first appears to have lost its lateral hooflets. (Owen 1868, pp. 789-791, emphasis mine)

Other pre-Darwin (overt) creationists also recognized the existence of extinct animals intermediate in forms between existing groups. This is from Louis Agassiz of Harvard University who remained a staunch opponent of Darwin’s theory of descent with modification even when most in the scientific community had turned his way:

I have now to introduce the class of Reptiles. – They are animals very dissimilar in their structure and appearance. At first one can scarcely understand the likeness existing between a snake and a turtle. Their skeletons in their external form are so totally different that a common characteristic is by no means easy to perceive. It seem almost impossible that such heterogenous animals as frogs, snakes, lizards and tortoises should belong to one natural division; nevertheless the class of reptiles is the most natural group of the Animal Kingdom. The extreme differences we notice between the groups just named, disappear more and more when we examine the distinct types of those animals which lived in former epochs and are now extinct. We have now some animals which, by their form, stand intermediate between lizards, or crocodiles, and tortoises. We have other forms even intermediate between the snakes and lizards. (Agassiz 1847, p. 45, emphasis mine)

David Thomas Ansted was Professor of geology at King’s College, London.

Two great natural families of fishes, one of them entirely, and the other almost extinct, seem to have occupied at this time [Carboniferous] the place of the great marine reptiles which succeeded and displaced them. These two families are nearly allied to each other and present many remarkable and close analogies to the true saurians or reptiles, and for this reason the one first determined was named Sauroid.

The sauroid or reptilian fishes, although met with throughout in the rocks of the secondary epoch, and often very abundant, nowhere attain so great a magnitude, or offer such perfect types of their development, as in the earlier seas, whose inhabitants we are now considering.

So intimate is the resemblance, and so nearly perfect the passage between fishes and reptiles through these sauroid fishes, that very little is wanting to complete our knowledge of the numerous extinct forms, in spite of the rarity of existing species with which to compare them. (Ansted 1847, pp. 96-97, emphasis mine)

This quote from the Rev. William Buckland, who was the head of geology at Oxford University in the early to mid-19th century:

The place of the genus Palæotherium (see Plates 3 and 4) is intermediate between the rhinoceros, the horse, and tapir. Eleven or twelve species have already been discovered; some as large as a rhinoceros, others varying from the size of a horse to that of a hog. The bones of the nose show that, like the tapir, they had a short fleshy trunk. These animals probably lived and died upon the margins of the then existing lakes and rivers, and their dead carcases may have been drifted to the bottom in seasons of flood. (Buckland 1837, I:70, emphasis mine)

This numerical preponderance of Pachydermata, among the earliest fossil Mammalia, beyond the proportion they bear among existing quadrupeds, is a remarkable fact, much insisted on by Cuvier; because it supplies, from the relics of a former world, many intermediate forms which do not occur in the present distribution of that important Order. As the living genera of Pachydermata are more widely separated from one another, than those of any other Order of Mammalia, it is important to fill these vacant intervals with the fossil genera of a former state of the earth; thus supplying links that appeared deficient in the grand continuous chain which connects all past and present forms of organic life, as parts of one great system of Creation. (Buckland ibid. p. 75, emphasis mine)

[Note: Buckland’s reference here to “Pachydermata” is almost certainly to a taxonomic grouping coined by Cuvier that is no longer considered valid. It included not only elephants but pretty much all hoofed mammals as well. ]

Again from Buckland, only in more general terms:

When we discover a constant and regular assemblage of organic Remains, commencing with one series of strata, and ending with another, which contains a different assemblage, we have herein the surest grounds whereon to establish those Divisions which are called geologic formations, and we find many such Divisions succeeding one another, when we investigate the mineral deposits on the surface of the Earth. The study of these Remains presents to the Zoologist a large amount of extinct species and genera, bearing important relations to existing forms of animals and vegetables, and often supplying links that had hitherto appeared deficient, in the great chain whereby all animated beings are held together in a series of near and gradual connexions.

This discovery, amid the relics of past creations, of links that seemed wanting in the present system of organic nature, affords to natural Theology an important argument, in proving the unity and universal agency of a common great first cause; since every individual in such an uniform and closely connected series, is thus shown to be an integral part of one grand design.

The non-discovery of such links indeed, would form but a negative and feeble argument against the common origin of organic beings, widely separated from one another; because, for aught we know, the existence of intervals may have formed part of the original design of a common creator; and because such apparent voids may perhaps exist only in our own imperfect knowledge; but the presence of such links throughout all past and present modifications of being, shows an unity of design which proves the unity of the intelligence in which it originated. (Buckland ibid, pp. 94-95, emphasis mine)

And that was just from stuff dug up from stuff available on the internet.

RBH: Cuvier was dead before Darwin published but rejected Lamarck’s evolutionary proposal on similar grounds. They rejected the underlying species transmutation hypothesis (and therefore common ancestry) for what they regarded as a good scientific reason: they didn’t see intermediate forms in the fossil record.

Actually I think the problem was a combination of Lamarck’s problematic ideas and ‘creationist blinders’ if you will, both of which were heavily under the influence of the idea of the Great Chain of Being (also known as the scala naturae). The Great Chain paradigm led these otherwise competent scientists to reject intermediate forms as evidence for evolution not because intermediates were completely lacking but because their ideas of what “evolution” was and what evidence for it would supposedly look like, were quite different from what Darwin and later evolutionists were and are.

For example here is a passage written by the (respected amateur) geologist Hugh Miller where on the one hand he practically gushes about how many intermediate fossils there are but the turns around and argues that they actually work against the idea of the ‘transmutation theory’ using the “sauroid fish” mentioned above as a case in point:

Geology abounds with creatures of the intermediate class: there are none of its links more numerous than its connecting links; and hence its interest, as a field of speculation, to the assertors of the transmutation of races. But there is a fatal incompleteness in the evidence, that destroys its character as such. It supplies in abundance those links of generic connection which, as it were marry together dissimilar races; but it furnishes no genealogical link to show that the existence of one race derive their lineage from the existence of another. The scene shifts as we pass from formation to formation; we are introduced in each to a new dramatis personæ; and there exists no such proofs of there being at once different and yet the same, as those produced in the Winter’s Tale, to show that the grown shepherdess of the one scene is identical with the exposed infant of the scene that went before. Nay, the reverse is wellnigh as strikingly the case, as if the grown shepherdess had been introduced into the earlier scenes of the drama, and the child into it concluding scenes.

The argument is a very simple one. Of all the vertebrata, fishes rank lowest, and in geological history appear first. We find their remains in the Upper and Lower Silurians, in the Lower, Middle, and Upper Old Red Sandstone, in the Mountain Limestone, and in the Coal Measures; and in the latter formation the first reptiles appear. Fish seem to have been the master existences of two great systems, mayhap of three, ere the age of reptiles began. Now, fishes differ very much among themselves: some rank nearly as low as worms, some nearly as high as reptiles; and if fish could have risen into reptiles, and reptiles into mammalia, we would necessarily expect to find lower orders of fish passing into higher, and taking precedence of the higher in their appearance in point of time, just as in the Winter’s Tale we see the infant preceding the adult. If such be not the case ―if fish made their first appearance, not in their least perfect, but in their most perfect state― not in their nearest approximation to the worm, but in their nearest approximation to the reptile ―there is no room for progression, and the argument falls. Now, it is a geological fact, that it is fish of the higher orders that appear first on the stage, and that they are found to occupy exactly the same level during the vast period represented by five succeeding formations. There is no progression. If fish rose into reptiles, if must have been by sudden transformation ―it must have been as if a man who had stood still for half a lifetime should bestir himself all at once, and take seven leagues at a stride. There is no getting rid of miracle in the case ―there is no alternative between creation and metamorphosis. The infidel substitutes progression for Deity ―geology robs him of his God. (Miller 1852, pp. 75-77, emphasis mine)

The geologist Edward Hitchcock quoted William Buckland to much the same effect (if more succinctly):

Hence we learn that the hypothesis of Lamarck is without foundation, which supposes there has been a transmutation of species from less to more perfect, since the beginning of organic life on the globe… “The Sauroid fishes,” says Dr. Buckland, (Bridgewater Treatise, Vol. 1. p. 294,) “occupy a higher place in the scale of organization than the ordinary forms of bony fishes; yet we find examples of Sauroids of the greatest magnitude and in abundant numbers, in the carboniferous and secondary formations, whilst they almost disappear and are replaced by less perfect forms in the tertiary strata, and present only two genera among existing fishes. ―In this, as in many other cases, a kind of retrograde development, from complex to simple forms may be said to have taken place.” (Hitchcock 1845, p. 93, emphasis mine)

So you see, they were finding Paleozoic fossils of lobe-finned fish (Sarcopterygii, which we now know to be the group ancestral to tetrapods) and recognizing that they shared many similarities with “reptiles” (which in those days included amphibians), while on the other hand they saw more recent fossiliferous rocks as well as today’s oceans, lakes and rivers, chock full of ray finned fish (Actinopterygii) which despite their mind boggling diversity, are less like “reptiles” and since they viewed everything through the Great Chain of Being paradigm with all its attendant nonsense about “higher” “more perfect” types (which in their minds meant more like us) and it’s ladder-like ascension from one type to the next higher type (as opposed to a branching bush) they couldn’t see them as evidence for evolution.

To us lobe-finned fish are simply a group of fish that were once much more diverse, one branch of which gave rise to the tetrapods, and then declined in diversity leaving only a few species of lungfish (Dipnoi) and coelacanths as living representatives. Lobe-fins are not “higher” or more “perfect” than ray-finned fish (“ordinary bony fishes”) so their earlier appearance in time is not a problem for the idea of evolution.  In fact by modern evolutionary standards, if we were to pick a “more perfect” fish (but we shouldn’t), it would be the ray-fins based on their diversity. They seem to be better at being fish that the lobe-fins are or ever were. The graph below illustrates the differences in “fish” diversity through time. Notice that even at their greatest diversity lobe-finned fish don’t even compare to ray-finned fish (which are mostly teleosts).

Here is another example of evolution denial while simultaneously speaking of intermediate forms. The authors (Henry De La Beche & William Conybeare) went on at some length in a footnote to deny that the evidence they were reporting should be thought to support the transmutation theory (main text in bold, footnote normal):

The points of analogy between the newly discovered animal [Plesiosaurus] and the Ichthyosaurus are sufficiently numerous and important to evince the propriety of their being referred to the same great natural family, a family on every account highly deserving an attentive examination, its members being not only unknown in the recent state, but presenting many peculiarities of general structure, of which no other examples had been previously observed ; and of that most interesting description which affords intermediate forms, and as it were a transition between different races, and adds new links to the connected chain of organized beings, †

†When alluding to the regular gradation, and, as it were, the linked and concatenated series of animal forms, we would wish carefully to guard against the absurd and extravagant application which has sometimes been made of this notion. In the original formation of animated beings, the plan evidently to be traced throughout is this. That every place capable of supporting animal life should be so filled, and that every possible mode of sustenance should be taken advantage of; hence every possible variety of structure became necessary, many of them such as to involve a total change of parts, but others again, such as required nothing beyond a modification of similar parts, slight indeed in external appearance, yet important in subserving the peculiar habits and economy of the different animals; in these cases the unity of general design was preserved, while the requisite peculiarity of organisation was superinduced; nor can there be any where found a more striking proof of the infinite riches of creative design, or of the infinite wisdom which guided their application. Some physiologists however (and Lamarck is more especially censurable on this account) have most ridiculously imagined that the links hence arising represent real transitions from one branch to another of the animal kingdom ; that through a series of such links, and by means of the constant tendency of the vital fluids, urged by animal appetencies to perfect old organs and develop new ones, that which was once a polypus became successively a mollusca, a fish, a quadruped; an idea so monstrous, and so completely at variance with the structure of the peculiar organs considered in the detail (which is in the great majority of instances such that no conceivable appetency could have any conceivable tendency to produce it) and no less so with the evident permanency of all animal forms, that nothing less than the credulity of a material philosophy could have been brought for a single moment to entertain it ―nothing less than its bigotry to defend it. (De La Beche & Conybeare 1821, pp.560-561)

I want to give a note of caution here. While many of fossil ‘intermediate links’ pre-Darwin geologists spoke of are indeed what we would consider examples of evolutionary intermediates others were only intermediate in the context of the Great Chain paradigm. The De La Beche & Conybeare paper quoted above is a good example. They speak of Plesiosaurus as being intermediate between crocodiles and ichthyosaurs, which might be true in a superficial sense; however they do not form what we would consider a legitimate fossil series. Crocodiles belong to the  Archosauria (along with dinosaurs and pterosaurs) branch of diapsid reptiles, Plesiosaurs (depending on who you ask) belong to either the Lepidosauria (along with lizards, snakes and tuataras) or their own separate clade and Ichthyosaurs seem usually to be classified in their own seperate clade as well. Given that the ancestors of crocodiles diverged from the lines leading to Plesiosaurs and Ichthyosaurs (probably sometime in the late Paleozoic) long before there was such a thing as crocodiles proper (late Triassic), crocodiles can hardly be ancestral to either. Never mind the actual details of their comparative anatomies.

RBH: The same might be said at least in part for the late 19th century/early 20th century mutationists. They rejected Darwinian incrementalism in favor of a saltationist account.

I think you’ll find that the later evolutionist objections were more about finer grade intermediates at the species level (I believe Miller is alluding to this problem in the first paragraph quoted above as well). In other words their problem was not that they saw no intermediate fossil forms between Classes, Orders and Families, but saw that there were relatively few between species. They felt that Darwin’s mechanism of natural selection should, if it was the primary mechanism for evolution, have produced more species level fossil intermediates given how slowly they believed it to work at the time.

This was the problem that Gould and Eldredge ultimately addressed with the theory of punctuated equilibria (though earlier evolutionists, even Darwin, foreshadowed their thinking).

Thanks RB, for giving me an excuse to look some of this up.

References

Agassiz, Louis (1847) An Introduction to the Study of Natural History, In a Series of Lectures Delivered in the Hall of the College of Physicians and Surgeons, New York, Lecture X, p.45, Greeley & McElrath, NY

Ansted, David Thomas (1847) The Ancient World: Or, Picturesque Sketches of Creation, Published by J. Van Voorst

Buckland, William, (1837) Geology and Mineralogy Considered With Reference to Natural Theology Vol. I  (Bridgewater Treatise Number VI) Carey, Lea and Blanchard

De La Beche, Henry Thomas & Conybeare, William Daniel (1821) “Notice of the discovery of a new Fossil Animal, forming a link between the Ichthyosaurus and Crocodile, together with general remarks on the Osteology of the Ichthyosaurus“, Transactions of the Geological Society of London 1st Series Vol. 5 Part 2, pp.558-594 pp.559-594

Hitchcock, Edward (1845) Elementary Geology (3rd Edition) Mark H. Newman, NY

Miller, Hugh (1852) The Old Red Sandstone (5th Edition) Johnstone and Hunter, Edinburgh (Amazon.com)

Owen, Richard (1846) A History of British Fossil Mammals, and Birds, J. Van Voorst

Owen, Richard (1848) “Description of Teeth and portions of Jaws of two extinct Anthracotheroid Quadrupeds (Hyopotamus vectianus and Hyopbovinus) discovered by the Marchioness of Hastings in the Eocene Deposits on the N. W. coast of the Isle of Wight: with an attempt to develope Cuvier’s idea of the Classification of Pachyderms by the Number of their Toes”, Quarterly Journal of the Geological Society 4:103-141

Owen, Richard (1860) ‘Darwin on the Origin of Species’, Edinburgh Review, 3:487-532

Owen, Richard (1861) Palæontology, Or, A Systematic Summary of Extinct Animals and Their Geological Relations: Or A Systematic Summary of Extinct Animals and Their Geological Relations (2nd Edition), Adam and Charles Black

Owen, Richard (1868) On the Anatomy of Vertebrates Vol. III (Mammals), Longmans, Green, and Company