THE RISE AND PROGRESS OF PALAEONTOLOGY

THIS IS ESSAY #2 FROM "SCIENCE AND HEBREW TRADITION"

By Thomas Henry Huxley

That application of the sciences of biology and geology, which iscommonly known as palaeontology, took its origin in the mind of thefirst person who, finding something like a shell, or a bone, naturallyimbedded in gravel or rock, indulged in speculations upon the natureof this thing which he had dug out--this "fossil"--and upon the causeswhich had brought it into such a position. In this rudimentary form, ahigh antiquity may safely be ascribed to palaeontology, inasmuch as weknow that, 500 years before the Christian era, the philosophic doctrinesof Xenophanes were influenced by his observations upon the fossilremains exposed in the quarries of Syracuse. From this time forth notonly the philosophers, but the poets, the historians, the geographersof antiquity occasionally refer to fossils; and, after the revival oflearning, lively controversies arose respecting their real nature. But hardly more than two centuries have elapsed since this fundamentalproblem was first exhaustively treated; it was only in the last centurythat the archaeological value of fossils--their importance, I mean, asrecords of the history of the earth--was fully recognised; the firstadequate investigation of the fossil remains of any large group ofvertebrated animals is to be found in Cuvier's "Recherches sur lesOssemens Fossiles, " completed in 1822; and, so modern is stratigraphicalpalaeontology, that its founder, William Smith, lived to receive thejust recognition of his services by the award of the first WollastonMedal in 1831. 

But, although palaeontology is a comparatively youthful scientificspeciality, the mass of materials with which it has to deal is alreadyprodigious. In the last fifty years the number of known fossil remainsof invertebrated animals has been trebled or quadrupled. The work ofinterpretation of vertebrate fossils, the foundations of which wereso solidly laid by Cuvier, was carried on, with wonderful vigour andsuccess, by Agassiz in Switzerland, by Von Meyer in Germany, andlast, but not least, by Owen in this country, while, in later years, amultitude of workers have laboured in the same field. In many groups ofthe animal kingdom the number of fossil forms already known is as greatas that of the existing species. In some cases it is much greater; andthere are entire orders of animals of the existence of which we shouldknow nothing except for the evidence afforded by fossil remains. Withall this it may be safely assumed that, at the present moment, we arenot acquainted with a tittle of the fossils which will sooner or laterbe discovered. If we may judge by the profusion yielded within the lastfew years by the Tertiary formations of North America, there seems to beno limit to the multitude of mammalian remains to be expected from thatcontinent; and analogy leads us to expect similar riches in EasternAsia, whenever the Tertiary formations of that region are as carefullyexplored. Again, we have, as yet, almost everything to learn respectingthe terrestrial population of the Mesozoic epoch; and it seems as ifthe Western territories of the United States were about to prove asinstructive in regard to this point as they have in respect of tertiarylife. My friend Professor Marsh informs me that, within two years, remains of more than 160 distinct individuals of mammals, belonging totwenty species and nine genera, have been found in a space not largerthan the floor of a good-sized room; while beds of the same age haveyielded 300 reptiles, varying in size from a length of 60 feet or 80feet to the dimensions of a rabbit. 

The task which I have set myself to-night is to endeavour to lay beforeyou, as briefly as possible, a sketch of the successive steps bywhich our present knowledge of the facts of palaeontology and of thoseconclusions from them which are indisputable, has been attained; and Ibeg leave to remind you, at the outset, that in attempting to sketchthe progress of a branch of knowledge to which innumerable labourshave contributed, my business is rather with generalisations than withdetails. It is my object to mark the epochs of palaeontology, not torecount all the events of its history. 

That which I just now called the fundamental problem of palaeontology, the question which has to be settled before any other can be profitablydiscussed, is this, What is the nature of fossils? Are they, as thehealthy common sense of the ancient Greeks appears to have led them toassume without hesitation, the remains of animals and plants? Or arethey, as was so generally maintained in the fifteenth, sixteenth, andseventeenth centuries, mere figured stones, portions of mineral matterwhich have assumed the forms of leaves and shells and bones, just asthose portions of mineral matter which we call crystals take on the formof regular geometrical solids? Or, again, are they, as others thought, the products of the germs of animals and of the seeds of plants whichhave lost their way, as it were, in the bowels of the earth, and haveachieved only an imperfect and abortive development? It is easy to sneerat our ancestors for being disposed to reject the first in favour of oneor other of the last two hypotheses; but it is much more profitable totry to discover why they, who were really not one whit less sensiblepersons than our excellent selves, should have been led to entertainviews which strike us as absurd, The belief in what is erroneouslycalled spontaneous generation, that is to say, in the developmentof living matter out of mineral matter, apart from the agency ofpre-existing living matter, as an ordinary occurrence at the presentday--which is still held by some of us, was universally accepted as anobvious truth by them. They could point to the arborescent formsassumed by hoar-frost and by sundry metallic minerals as evidence of theexistence in nature of a "plastic force" competent to enable inorganicmatter to assume the form of organised bodies. Then, as every one who isfamiliar with fossils knows, they present innumerable gradations, fromshells and bones which exactly resemble the recent objects, to masses ofmere stone which, however accurately they repeat the outward form of theorganic body, have nothing else in common with it; and, thence, to meretraces and faint impressions in the continuous substance of the rock. What we now know to be the results of the chemical changes which takeplace in the course of fossilisation, by which mineral is substitutedfor organic substance, might, in the absence of such knowledge, befairly interpreted as the expression of a process of development in theopposite direction--from the mineral to the organic. Moreover, in an agewhen it would have seemed the most absurd of paradoxes to suggest thatthe general level of the sea is constant, while that of the solid landfluctuates up and down through thousands of feet in a secular groundswell, it may well have appeared far less hazardous to conceive thatfossils are sports of nature than to accept the necessary alternative, that all the inland regions and highlands, in the rocks of which marineshells had been found, had once been covered by the ocean. It is not sosurprising, therefore, as it may at first seem, that although such menas Leonardo da Vinci and Bernard Palissy took just views of the natureof fossils, the opinion of the majority of their contemporaries setstrongly the other way; nor even that error maintained itself long afterthe scientific grounds of the true interpretation of fossils had beenstated, in a manner that left nothing to be desired, in the latter halfof the seventeenth century. The person who rendered this good serviceto palaeontology was Nicolas Steno, professor of anatomy in Florence, though a Dane by birth. Collectors of fossils at that day were familiarwith certain bodies termed "glossopetrae, " and speculation was rife asto their nature. In the first half of the seventeenth century, FabioColonna had tried to convince his colleagues of the famous Accademia deiLincei that the glossopetrae were merely fossil sharks' teeth, but hisarguments made no impression. Fifty years later, Steno re-opened thequestion, and, by dissecting the head of a shark and pointing out thevery exact correspondence of its teeth with the glossopetrae, left norational doubt as to the origin of the latter. Thus far, the work ofSteno went little further than that of Colonna, but it fortunatelyoccurred to him to think out the whole subject of the interpretation offossils, and the result of his meditations was the publication, in 1669, of a little treatise with the very quaint title of "De Solido intraSolidum naturaliter contento. " The general course of Steno's argumentmay be stated in a few words. Fossils are solid bodies which, by somenatural process, have come to be contained within other solid bodies, namely, the rocks in which they are embedded; and the fundamentalproblem of palaeontology, stated generally, is this: "Given a bodyendowed with a certain shape and produced in accordance with naturallaws, to find in that body itself the evidence of the place and mannerof its production. " [1] The only way of solving this problem is by theapplication of the axiom that "like effects imply like causes, " or asSteno puts it, in reference to this particular case, that "bodies whichare altogether similar have been produced in the same way. " [2] Hence, since the glossopetrae are altogether similar to sharks' teeth, theymust have been produced by sharklike fishes; and since many fossilshells correspond, down to the minutest details of structure, with theshells of existing marine or freshwater animals, they must have beenproduced by similar animals; and the like reasoning is applied bySteno to the fossil bones of vertebrated animals, whether aquaticor terrestrial. To the obvious objection that many fossils are notaltogether similar to their living analogues, differing in substancewhile agreeing in form, or being mere hollows or impressions, thesurfaces of which are figured in the same way as those of animal orvegetable organisms, Steno replies by pointing out the changes whichtake place in organic remains embedded in the earth, and how their solidsubstance may be dissolved away entirely, or replaced by mineral matter, until nothing is left of the original but a cast, an impression, ora mere trace of its contours. The principles of investigation thusexcellently stated and illustrated by Steno in 1669, are thosewhich have, consciously or unconsciously, guided the researches ofpalaeontologists ever since. Even that feat of palaeontology which hasso powerfully impressed the popular imagination, the reconstruction ofan extinct animal from a tooth or a bone, is based upon the simplestimaginable application of the logic of Steno. A moment's considerationwill show, in fact, that Steno's conclusion that the glossopetrae aresharks' teeth implies the reconstruction of an animal from its tooth. Itis equivalent to the assertion that the animal of which the glossopetraeare relics had the form and organisation of a shark; that it hada skull, a vertebral column, and limbs similar to those which arecharacteristic of this group of fishes; that its heart, gills, andintestines presented the peculiarities which those of all sharksexhibit; nay, even that any hard parts which its integument containedwere of a totally different character from the scales of ordinaryfishes. These conclusions are as certain as any based upon probablereasonings can be. And they are so, simply because a very largeexperience justifies us in believing that teeth of this particular formand structure are invariably associated with the peculiar organisationof sharks, and are never found in connection with other organisms. Whythis should be we are not at present in a position even to imagine; wemust take the fact as an empirical law of animal morphology, the reasonof which may possibly be one day found in the history of the evolutionof the shark tribe, but for which it is hopeless to seek for anexplanation in ordinary physiological reasonings. Every one practicallyacquainted with palaeontology is aware that it is not every tooth, norevery bone, which enables us to form a judgment of the character of theanimal to which it belonged; and that it is possible to possess manyteeth, and even a large portion of the skeleton of an extinct animal, and yet be unable to reconstruct its skull or its limbs. It is onlywhen the tooth or bone presents peculiarities, which we know by previousexperience to be characteristic of a certain group, that we can safelypredict that the fossil belonged to an animal of the same group. Any onewho finds a cow's grinder may be perfectly sure that it belonged to ananimal which had two complete toes on each foot and ruminated; any onewho finds a horse's grinder may be as sure that it had one completetoe on each foot and did not ruminate; but if ruminants and horseswere extinct animals of which nothing but the grinders had ever beendiscovered, no amount of physiological reasoning could have enabledus to reconstruct either animal, still less to have divined thewide differences between the two. Cuvier, in the "Discours sur lesRevolutions de la Surface du Globe, " strangely credits himself, and hasever since been credited by others, with the invention of a new methodof palaeontological research. But if you will turn to the "Recherchessur les Ossemens Fossiles" and watch Cuvier, not speculating, butworking, you will find that his method is neither more nor less thanthat of Steno. If he was able to make his famous prophecy from the jawwhich lay upon the surface of a block of stone to the pelvis of the sameanimal which lay hidden in it, it was not because either he, or anyone else, knew, or knows, why a certain form of jaw is, as a rule, constantly accompanied by the presence of marsupial bones, but simplybecause experience has shown that these two structures are co-ordinated. 

The settlement of the nature of fossils led at once to the next advanceof palaeontology, viz. Its application to the deciphering of the historyof the earth. When it was admitted that fossils are remains of animalsand plants, it followed that, in so far as they resemble terrestrial, orfreshwater, animals and plants, they are evidences of the existence ofland, or fresh water; and, in so far as they resemble marine organisms, they are evidences of the existence of the sea at the time at whichthey were parts of actually living animals and plants. Moreover, inthe absence of evidence to the contrary, it must be admitted that theterrestrial or the marine organisms implied the existence of land orsea at the place in which they were found while they were yet living. In fact, such conclusions were immediately drawn by everybody, fromthe time of Xenophanes downwards, who believed that fossils were reallyorganic remains. Steno discusses their value as evidence of repeatedalteration of marine and terrestrial conditions upon the soil of Tuscanyin a manner worthy of a modern geologist. The speculations of De Mailletin the beginning of the eighteenth century turn upon fossils; and Buffonfollows him very closely in those two remarkable works, the "Theoriede la Terre" and the "Epoques de la Nature" with which he commenced andended his career as a naturalist. 

The opening sentences of the "Epoques de la Nature" show us howfully Buffon recognised the analogy of geological with archaeologicalinquiries. "As in civil history we consult deeds, seek for coins, ordecipher antique inscriptions in order to determine the epochs of humanrevolutions and fix the date of moral events; so, in natural history, we must search the archives of the world, recover old monuments from thebowels of the earth, collect their fragmentary remains, and gather intoone body of evidence all the signs of physical change which may enableus to look back upon the different ages of nature. It is our onlymeans of fixing some points in the immensity of space, and of setting acertain number of waymarks along the eternal path of time. "

Buffon enumerates five classes of these monuments of the past history ofthe earth, and they are all facts of palaeontology. In the first place, he says, shells and other marine productions are found all over thesurface and in the interior of the dry land; and all calcareous rocksare made up of their remains. Secondly, a great many of these shellswhich are found in Europe are not now to be met with in the adjacentseas; and, in the slates and other deep-seated deposits, there areremains of fishes and of plants of which no species now exist in ourlatitudes, and which are either extinct, or exist only in more northernclimates. Thirdly, in Siberia and in other northern regions ofEurope and of Asia, bones and teeth of elephants, rhinoceroses, andhippopotamuses occur in such numbers that these animals must once havelived and multiplied in those regions, although at the present day theyare confined to southern climates. The deposits in which these remainsare found are superficial, while those which contain shells and othermarine remains lie much deeper. Fourthly, tusks and bones of elephantsand hippopotamuses are found not only in the northern regions of the oldworld, but also in those of the new world, although, at present, neitherelephants nor hippopotamuses occur in America. Fifthly, in the middle ofthe continents, in regions most remote from the sea, we find an infinitenumber of shells, of which the most part belong to animals of thosekinds which still exist in southern seas, but of which many others haveno living analogues; so that these species appear to be lost, destroyedby some unknown cause. It is needless to inquire how far thesestatements are strictly accurate; they are sufficiently so to justifyBuffon's conclusions that the dry land was once beneath the sea; thatthe formation of the fossiliferous rocks must have occupied a vastlygreater lapse of time than that traditionally ascribed to the age ofthe earth; that fossil remains indicate different climatal conditionsto have obtained in former times, and especially that the polar regionswere once warmer; that many species of animals and plants have becomeextinct; and that geological change has had something to do withgeographical distribution. 

But these propositions almost constitute the frame-work ofpalaeontology. In order to complete it but one addition was needed, andthat was made, in the last years of the eighteenth century, by WilliamSmith, whose work comes so near our own times that many living men mayhave been personally acquainted with him. This modest land-surveyor, whose business took him into many parts of England, profited by thepeculiarly favourable conditions offered by the arrangement of oursecondary strata to make a careful examination and comparison of theirfossil contents at different points of the large area over which theyextend. The result of his accurate and widely-extended observationswas to establish the important truth that each stratum contains certainfossils which are peculiar to it; and that the order in which thestrata, characterised by these fossils, are super-imposed one upon theother is always the same. This most important generalisation wasrapidly verified and extended to all parts of the world accessible togeologists; and now it rests upon such an immense mass of observationsas to be one of the best established truths of natural science. To thegeologist the discovery was of infinite importance as it enabled him toidentify rocks of the same relative age, however their continuity mightbe interrupted or their composition altered. But to the biologist ithad a still deeper meaning, for it demonstrated that, throughout theprodigious duration of time registered by the fossiliferous rocks, theliving population of the earth had undergone continual changes, notmerely by the extinction of a certain number of the species which had atfirst existed, but by the continual generation of new species, and theno less constant extinction of old ones. 

Thus the broad outlines of palaeontology, in so far as it is the commonproperty of both the geologist and the biologist, were marked out atthe close of the last century. In tracing its subsequent progress I mustconfine myself to the province of biology, and, indeed, to theinfluence of palaeontology upon zoological morphology. And I acceptthis limitation the more willingly as the no less important topic ofthe bearing of geology and of palaeontology upon distribution has beenluminously treated in the address of the President of the GeographicalSection. [3]

The succession of the species of animals and plants in time beingestablished, the first question which the zoologist or the botanist hadto ask himself was, What is the relation of these successive speciesone to another? And it is a curious circumstance that the most importantevent in the history of palaeontology which immediately succeededWilliam Smith's generalisation was a discovery which, could it have beenrightly appreciated at the time, would have gone far towards suggestingthe answer, which was in fact delayed for more than half a century. Irefer to Cuvier's investigation of the mammalian fossils yielded bythe quarries in the older tertiary rocks of Montmartre, among the chiefresults of which was the bringing to light of two genera of extincthoofed quadrupeds, the _Anoplotherium_ and the _Palaeotherium. _ The richmaterials at Cuvier's disposition enabled him to obtain a fullknowledge of the osteology and of the dentition of these two forms, andconsequently to compare their structure critically with that of existinghoofed animals. The effect of this comparison was to prove that the_Anoplotherium, _ though it presented many points of resemblance with thepigs on the one hand and with the ruminants on the other, differed fromboth to such an extent that it could find a place in neither group. Infact, it held, in some respects, an intermediate position, tending tobridge over the interval between these two groups, which in the existingfauna are so distinct. In the same way, the _Palaeotherium_ tended toconnect forms so different as the tapir, the rhinoceros, and the horse. Subsequent investigations have brought to light a variety of facts ofthe same order, the most curious and striking of which are thosewhich prove the existence, in the mesozoic epoch, of a series of formsintermediate between birds and reptiles--two classes of vertebrateanimals which at present appear to be more widely separated than anyothers. Yet the interval between them is completely filled, in themesozoic fauna, by birds which have reptilian characters, on the oneside, and reptiles which have ornithic characters, on the other. Soagain, while the group of fishes, termed ganoids, is, at the presenttime, so distinct from that of the dipnoi, or mudfishes, that they havebeen reckoned as distinct orders, the Devonian strata present us withforms of which it is impossible to say with certainty whether they aredipnoi or whether they are ganoids. 

Agassiz's long and elaborate researches upon fossil fishes, publishedbetween 1833 and 1842, led him to suggest the existence of another kindof relation between ancient and modern forms of life. He observed thatthe oldest fishes present many characters which recall the embryonicconditions of existing fishes; and that, not only among fishes, but inseveral groups of the invertebrata which have a long palaeontologicalhistory, the latest forms are more modified, more specialised, than theearlier. The fact that the dentition of the older tertiary ungulateand carnivorous mammals is always complete, noticed by Professor Owen, illustrated the same generalisation. 

Another no less suggestive observation was made by Mr. Darwin, whosepersonal investigations during the voyage of the _Beagle_ led himto remark upon the singular fact, that the fauna, which immediatelyprecedes that at present existing in any geographical province ofdistribution, presents the same peculiarities as its successor. Thus, inSouth America and in Australia, the later tertiary or quaternary fossilsshow that the fauna which immediately preceded that of the present daywas, in the one case, as much characterised by edentates and, in theother, by marsupials as it is now, although the species of the older arelargely different from those of the newer fauna. 

However clearly these indications might point in one direction, thequestion of the exact relation of the successive forms of animal andvegetable life could be satisfactorily settled only in one way; namely, by comparing, stage by stage, the series of forms presented by one andthe same type throughout a long space of time. Within the last few yearsthis has been done fully in the case of the horse, less completelyin the case of the other principal types of the ungulata and of thecarnivora; and all these investigations tend to one general result, namely, that, in any given series, the successive members of that seriespresent a gradually increasing specialisation of structure. That is tosay, if any such mammal at present existing has specially modified andreduced limbs or dentition and complicated brain, its predecessors intime show less and less modification and reduction in limbs and teethand a less highly developed brain. The labours of Gaudry, Marsh, andCope furnish abundant illustrations of this law from the marvellousfossil wealth of Pikermi and the vast uninterrupted series of tertiaryrocks in the territories of North America. 

I will now sum up the results of this sketch of the rise and progressof palaeontology. The whole fabric of palaeontology is based upon twopropositions: the first is, that fossils are the remains of animals andplants; and the second is, that the stratified rocks in which theyare found are sedimentary deposits; and each of these propositions isfounded upon the same axiom, that like effects imply like causes. Ifthere is any cause competent to produce a fossil stem, or shell, orbone, except a living being, then palaeontology has no foundation; ifthe stratification of the rocks is not the effect of such causes asat present produce stratification, we have no means of judging of theduration of past time, or of the order in which the forms of life havesucceeded one another. But if these two propositions are granted, there is no escape, as it appears to me, from three very importantconclusions. The first is that living matter has existed upon the earthfor a vast length of time, certainly for millions of years. The secondis that, during this lapse of time, the forms of living matter haveundergone repeated changes, the effect of which has been that the animaland vegetable population, at any period of the earth's history, containscertain species which did not exist at some antecedent period, andothers which ceased to exist at some subsequent period. The third isthat, in the case of many groups of mammals and some of reptiles, in which one type can be followed through a considerable extent ofgeological time, the series of different forms by which the type isrepresented, at successive intervals of this time, is exactly such as itwould be, if they had been produced by the gradual modification of theearliest forms of the series. These are facts of the history of theearth guaranteed by as good evidence as any facts in civil history. 

Hitherto I have kept carefully clear of all the hypotheses to which menhave at various times endeavoured to fit the facts of palaeontology, orby which they have endeavoured to connect as many of these facts as theyhappened to be acquainted with. I do not think it would be a profitableemployment of our time to discuss conceptions which doubtless have hadtheir justification and even their use, but which are now obviouslyincompatible with the well-ascertained truths of palaeontology. Atpresent these truths leave room for only two hypotheses. The first isthat, in the course of the history of the earth, innumerable speciesof animals and plants have come into existence, independently ofone another, innumerable times. This, of course, implies either thatspontaneous generation on the most astounding scale, and of animalssuch as horses and elephants, has been going on, as a natural process, through all the time recorded by the fossiliferous rocks; or itnecessitates the belief in innumerable acts of creation repeatedinnumerable times. The other hypothesis is, that the successive speciesof animals and plants have arisen, the later by the gradual modificationof the earlier. This is the hypothesis of evolution; and thepalaeontological discoveries of the last decade are so completely inaccordance with the requirements of this hypothesis that, if it had notexisted, the palaeontologist would have had to invent it. 

I have always had a certain horror of presuming to set a limit upon thepossibilities of things. Therefore I will not venture to say that it isimpossible that the multitudinous species of animals and plants may havebeen produced, one separately from the other, by spontaneous generation;nor that it is impossible that they should have been independentlyoriginated by an endless succession of miraculous creative acts. ButI must confess that both these hypotheses strike me as so astoundinglyimprobable, so devoid of a shred of either scientific or traditionalsupport, that even if there were no other evidence than that ofpalaeontology in its favour, I should feel compelled to adopt thehypothesis of evolution. Happily, the future of palaeontology isindependent of all hypothetical considerations. Fifty years hence, whoever undertakes to record the progress of palaeontology will note thepresent time as the epoch in which the law of succession of the forms ofthe higher animals was determined by the observation of palaeontologicalfacts. He will point out that, just as Steno and as Cuvier were enabledfrom their knowledge of the empirical laws of co-existence of the partsof animals to conclude from a part to the whole, so the knowledge of thelaw of succession of forms empowered their successors to conclude, fromone or two terms of such a succession, to the whole series; and thusto divine the existence of forms of life, of which, perhaps, no traceremains, at epochs of inconceivable remoteness in the past. 

FOOTNOTES:

[Footnote 1: _De Solidoiintra Solidum, _ p. 5--"Dato corpore certa figurapraedito et juxta leges naturae producto, in ipso corpore argumentainvenire locum et modum productionis detegentia. "]

[Footnote 2: "Corpora sibi invicem omnino similia simili etiam modoproducta sunt. "]

[Footnote 3: Sir J. D. Hooker. ]