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diff --git a/old/thx1610.txt b/old/thx1610.txt new file mode 100644 index 0000000..719743a --- /dev/null +++ b/old/thx1610.txt @@ -0,0 +1,1131 @@ +The Project Gutenberg Etext of Geological Contemporaneity and +Persistent Types of Life +#26 in our series by Thomas H. Huxley + +Copyright laws are changing all over the world, be sure to check +the laws for your country before redistributing these files!!! + +Please take a look at the important information in this header. +We encourage you to keep this file on your own disk, keeping an +electronic path open for the next readers. + +Please do not remove this. + +This should be the first thing seen when anyone opens the book. +Do not change or edit it without written permission. 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Huxley + + + + + [footnote] *The Anniversary Address to the Geological + Society for 1862. + +MERCHANTS occasionally go through a wholesome, though troublesome and +not always satisfactory, process which they term "taking stock." After +all the excitement of speculation, the pleasure of gain, and the pain +of loss, the trader makes up his mind to face facts and to learn the +exact quantity and quality of his solid and reliable possessions. + +The man of science does well sometimes to imitate this procedure; and, +forgetting for the time the importance of his own small winnings, to +re-examine the common stock in trade, so that he may make sure how far +the stock of bullion in the cellar--on the faith of whose existence so +much paper has been circulating--is really the solid gold of truth. + +The Anniversary Meeting of the Geological Society seems to be an +occasion well suited for an undertaking of this kind--for an inquiry, +in fact, into the nature and value of the present results of +paleontological investigation; and the more so, as all those who have +paid close attention to the late multitudinous discussions in which +paleontology is implicated, must have felt the urgent necessity of some +such scrutiny. + +First in order, as the most definite and unquestionable of all the +results of paleontology, must be mentioned the immense extension and +impulse given to botany, zoology, and comparative anatomy, by the +investigation of fossil remains. Indeed, the mass of biological facts +has been so greatly increased, and the range of biological speculation +has been so vastly widened, by the researches of the geologist and +paleontologist, that it is to be feared there are naturalists in +existence who look upon geology as Brindley regarded rivers. "Rivers," +said the great engineer, "were made to feed canals"; and geology, some +seem to think, was solely created to advance comparative anatomy. + +Were such a thought justifiable, it could hardly expect to be received +with favour by this assembly. But it is not justifiable. Your +favourite science has her own great aims independent of all others; and +if, notwithstanding her steady devotion to her own progress, she can +scatter such rich alms among her sisters, it should be remembered that +her charity is of the sort that does not impoverish, but "blesseth him +that gives and him that takes." + +Regard the matter as we will, however, the facts remain. Nearly 40,000 +species of animals and plants have been added to the Systema Naturae by +paleontologic research. This is a living population equivalent to that +of a new continent in mere number; equivalent to that of a new +hemisphere, if we take into account the small population of insects as +yet found fossil, and the large proportion and peculiar organization of +many of the Vertebrata. + +But, beyond this, it is perhaps not too much to say that, except for the +necessity of interpreting paleontologic facts, the laws of distribution +would have received less careful study; while few comparative +anatomists (and those not of the first order) would have been induced by +mere love of detail, as such, to study the minutiae of osteology, were +it not that in such minutiae lie the only keys to the most interesting +riddles offered by the extinct animal world. + +These assuredly are great and solid gains. Surely it is matter for no +small congratulation that in half a century (for paleontology, though +it dawned earlier, came into full day only with Cuvier) a subordinate +branch of biology should have doubled the value and the interest of the +whole group of sciences to which it belongs. + +But this is not all. Allied with geology, paleontology has established +two laws of inestimable importance: the first, that one and the same +area of the earth's surface has been successively occupied by very +different kinds of living beings; the second, that the order of +succession established in one locality holds good, approximately, in +all. + +The first of these laws is universal and irreversible; the second is an +induction from a vast number of observations, though it may possibly, +and even probably, have to admit of exceptions. As a consequence of +the second law, it follows that a peculiar relation frequently subsists +between series of strata, containing organic remains, in different +localities. The series resemble one another, not only in virtue of a +general resemblance of the organic remains in the two, but also in +virtue of a resemblance in the order and character of the serial +succession in each. There is a resemblance of arrangement; so that the +separate terms of each series, as well as the whole series, exhibit a +correspondence. + +Succession implies time; the lower members of a series of sedimentary +rocks are certainly older than the upper; and when the notion of age +was once introduced as the equivalent of succession, it was no wonder +that correspondence in succession came to be looked upon as a +correspondence in age, or "contemporaneity." And, indeed, so long as +relative age only is spoken of, correspondence in succession 'is' +correspondence in age; it is 'relative' contemporaneity. + +But it would have been very much better for geology if so loose and +ambiguous a word as "contemporaneous" had been excluded from her +terminology, and if, in its stead, some term expressing similarity of +serial relation, and excluding the notion of time altogether, had been +employed to denote correspondence in position in two or more series of +strata. + +In anatomy, where such correspondence of position has constantly to be +spoken of, it is denoted by the word "homology" and its derivatives; +and for Geology (which after all is only the anatomy and physiology of +the earth) it might be well to invent some single word, such as +"homotaxis" (similarity of order), in order to express an essentially +similar idea. This, however, has not been done, and most probably the +inquiry will at once be made--To what end burden science with a new and +strange term in place of one old, familiar, and part of our common +language? + +The reply to this question will become obvious as the inquiry into the +results of paleontology is pushed further. + +Those whose business it is to acquaint themselves specially with the +works of paleontologists, in fact, will be fully aware that very few, +if any, would rest satisfied with such a statement of the conclusions +of their branch of biology as that which has just been given. + +Our standard repertories of paleontology profess to teach us far higher +things--to disclose the entire succession of living forms upon the +surface of the globe; to tell us of a wholly different distribution of +climatic conditions in ancient times; to reveal the character of the +first of all living existences; and to trace out the law of progress +from them to us. + +It may not be unprofitable to bestow on these professions a somewhat +more critical examination than they have hitherto received, in order to +ascertain how far they rest on an irrefragable basis; or whether, after +all, it might not be well for paleontologists to learn a little more +carefully that scientific "ars artium," the art of saying "I don't +know." And to this end let us define somewhat more exactly the extent +of these pretensions of paleontology. + +Every one is aware that Professor Bronn's 'Untersuchungen' and Professor +Pictet's 'Traite de Paleontologie' are works of standard authority, +familiarly consulted by every working paleontologist. It is desirable +to speak of these excellent books, and of their distinguished authors, +with the utmost respect, and in a tone as far as possible removed from +carping criticism; indeed, if they are specially cited in this place, +it is merely in justification of the assertion that the following +propositions, which may be found implicitly, or explicitly, in the +works in question, are regarded by the mass of paleontologists and +geologists, not only on the Continent but in this country, as +expressing some of the best-established results of paleontology. +Thus:-- + +Animals and plants began their existence together, not long after the +commencement of the deposition of the sedimentary rocks; and then +succeeded one another, in such a manner, that totally distinct faunae +and florae occupied the whole surface of the earth, one after the +other, and during distinct epochs of time. + +A geological formation is the sum of all the strata deposited over the +whole surface of the earth during one of these epochs: a geological +fauna or flora is the sum of all the species of animals or plants which +occupied the whole surface of the globe, during one of these epochs. + +The population of the earth's surface was at first very similar in all +parts, and only from the middle of the Tertiary epoch onwards, began to +show a distinct distribution in zones. + +The constitution of the original population, as well as the numerical +proportions of its members, indicates a warmer and, on the whole, +somewhat tropical climate, which remained tolerably equable throughout +the year. The subsequent distribution of living beings in zones is the +result of a gradual lowering of the general temperature, which first +began to be felt at the poles. + +It is not now proposed to inquire whether these doctrines are true or +false; but to direct your attention to a much simpler though very +essential preliminary question--What is their logical basis? what are +the fundamental assumptions upon which they all logically depend? and +what is the evidence on which those fundamental propositions demand our +assent? + +These assumptions are two: the first, that the commencement of the +geological record is coeval with the commencement of life on the globe; +the second, that geological contemporaneity is the same thing as +chronological synchrony. Without the first of these assumptions there +would of course be no ground for any statement respecting the +commencement of life; without the second, all the other statements +cited, every one of which implies a knowledge of the state of different +parts of the earth at one and the same time, will be no less devoid of +demonstration. + +The first assumption obviously rests entirely on negative evidence. +This is, of course, the only evidence that ever can be available to +prove the commencement of any series of phenomena; but, at the same +time, it must be recollected that the value of negative evidence +depends entirely on the amount of positive corroboration it receives. +If A B wishes to prove an 'alibi', it is of no use for him to get a +thousand witnesses simply to swear that they did not see him in such +and such a place, unless the witnesses are prepared to prove that they +must have seen him had he been there. But the evidence that animal +life commenced with the Lingula-flags, 'e.g.', would seem to be exactly +of this unsatisfactory uncorroborated sort. The Cambrian witnesses +simply swear they "haven't seen anybody their way"; upon which the +counsel for the other side immediately puts in ten or twelve thousand +feet of Devonian sandstones to make oath they never saw a fish or a +mollusk, though all the world knows there were plenty in their time. + +But then it is urged that, though the Devonian rocks in one part of the +world exhibit no fossils, in another they do, while the lower Cambrian +rocks nowhere exhibit fossils, and hence no living being could have +existed in their epoch. + +To this there are two replies: the first, that the observational basis +of the assertion that the lowest rocks are nowhere fossiliferous is an +amazingly small one, seeing how very small an area, in comparison to +that of the whole world, has yet been fully searched; the second, that +the argument is good for nothing unless the unfossiliferous rocks in +question were not only 'contemporaneous' in the geological sense, but +'synchronous' in the chronological sense. To use the 'alibi' +illustration again. If a man wishes to prove he was in neither of two +places, A and B, on a given day, his witnesses for each place must be +prepared to answer for the whole day. If they can only prove that he +was not at A in the morning, and not at B in the afternoon, the +evidence of his absence from both is 'nil', because he might have been +at B in the morning and at A in the afternoon. + +Thus everything depends upon the validity of the second assumption. And +we must proceed to inquire what is the real meaning of the word +"contemporaneous" as employed by geologists. To this end a concrete +example may be taken. + +The Lias of England and the Lias of Germany, the Cretaceous rocks of +Britain and the Cretaceous rocks of Southern India, are termed by +geologists "contemporaneous" formations; but whenever any thoughtful +geologist is asked whether he means to say that they were deposited +synchronously, he says, "No,--only within the same great epoch." And +if, in pursuing the inquiry, he is asked what may be the approximate +value in time of a "great epoch"--whether it means a hundred years, or +a thousand, or a million, or ten million years--his reply is, "I cannot +tell." + +If the further question be put, whether physical geology is in +possession of any method by which the actual synchrony (or the reverse) +of any two distant deposits can be ascertained, no such method can be +heard of; it being admitted by all the best authorities that neither +similarity of mineral composition, nor of physical character, nor even +direct continuity of stratum, are 'absolute' proofs of the synchronism +of even approximated sedimentary strata: while, for distant deposits, +there seems to be no kind of physical evidence attainable of a nature +competent to decide whether such deposits were formed simultaneously, or +whether they possess any given difference of antiquity. To return to +an example already given: All competent authorities will probably +assent to the proposition that physical geology does not enable us in +any way to reply to this question--Were the British Cretaceous rocks +deposited at the same time as those of India, or are they a million of +years younger or a million of years older? + +Is paleontology able to succeed where physical geology fails? Standard +writers on paleontology, as has been seen, assume that she can. They +take it for granted, that deposits containing similar organic remains +are synchronous--at any rate in a broad sense; and yet, those who will +study the eleventh and twelfth chapters of Sir Henry De La Beche's +remarkable 'Researches in Theoretical Geology', published now nearly +thirty years ago, and will carry out the arguments there most +luminously stated, to their logical consequences, may very easily +convince themselves that even absolute identity of organic contents is +no proof of the synchrony of deposits, while absolute diversity is no +proof of difference of date. Sir Henry De La Beche goes even further, +and adduces conclusive evidence to show that the different parts of one +and the same stratum, having a similar composition throughout, +containing the same organic remains, and having similar beds above and +below it, may yet differ to any conceivable extent in age. + +Edward Forbes was in the habit of asserting that the similarity of the +organic contents of distant formations was 'prima facie' evidence, not +of their similarity, but of their difference of age; and holding as he +did the doctrine of single specific centres, the conclusion was as +legitimate as any other; for the two districts must have been occupied +by migration from one of the two, or from an intermediate spot, and the +chances against exact coincidence of migration and of imbedding are +infinite. + +In point of fact, however, whether the hypothesis of single or of +multiple specific centres be adopted, similarity of organic contents +cannot possibly afford any proof of the synchrony of the deposits which +contain them; on the contrary, it is demonstrably compatible with the +lapse of the most prodigious intervals of time, and with the +interposition of vast changes in the organic and inorganic worlds, +between the epochs in which such deposits were formed. + +On what amount of similarity of their faunae is the doctrine of the +contemporaneity of the European and of the North American Silurians +based? In the last edition of Sir Charles Lyell's 'Elementary Geology' +it is stated, on the authority of a former President of this Society, +the late Daniel Sharpe, that between 30 and 40 per cent. of the species +of Silurian Mollusca are common to both sides of the Atlantic. By way +of due allowance for further discovery, let us double the lesser number +and suppose that 60 per cent. of the species are common to the North +American and the British Silurians. Sixty per cent. of species in +common is, then, proof of contemporaneity. + +Now suppose that, a million or two of years hence, when Britain has made +another dip beneath the sea and has come up again, some geologist +applies this doctrine, in comparing the strata laid bare by the +upheaval of the bottom, say, of St. George's Channel with what may then +remain of the Suffolk Crag. Reasoning in the same way, he will at once +decide the Suffolk Crag and the St. George's Channel beds to be +contemporaneous; although we happen to know that a vast period (even in +the geological sense) of time, and physical changes of almost +unprecedented extent, separate the two. + +But if it be a demonstrable fact that strata containing more than 60 or +70 per cent. of species of Mollusca in common, and comparatively close +together, may yet be separated by an amount of geological time +sufficient to allow of some of the greatest physical changes the world +has seen, what becomes of that sort of contemporaneity the sole +evidence of which is a similarity of facies, or the identity of half a +dozen species, or of a good many genera? + +And yet there is no better evidence for the contemporaneity assumed by +all who adopt the hypothesis of universal faunae and florae, of a +universally uniform climate, and of a sensible cooling of the globe +during geological time. + +There seems, then, no escape from the admission that neither physical +geology, nor paleontology, possesses any method by which the absolute +synchronism of two strata can be demonstrated. All that geology can +prove is local order of succession. It is mathematically certain that, +in any given vertical linear section of an undisturbed series of +sedimentary deposits, the bed which lies lowest is the oldest. In many +other vertical linear sections of the same series, of course, +corresponding beds will occur in a similar order; but, however great +may be the probability, no man can say with absolute certainty that the +beds in the two sections were synchronously deposited. For areas of +moderate extent, it is doubtless true that no practical evil is likely +to result from assuming the corresponding beds to be synchronous or +strictly contemporaneous; and there are multitudes of accessory +circumstances which may fully justify the assumption of such +synchrony. But the moment the geologist has to deal with large areas, +or with completely separated deposits, the mischief of confounding that +"homotaxis" or "similarity of arrangement," which 'can' be +demonstrated, with "synchrony" or "identity of date," for which there +is not a shadow of proof, under the one common term of +"contemporaneity" becomes incalculable, and proves the constant source +of gratuitous speculations. + +For anything that geology or paleontology are able to show to the +contrary, a Devonian fauna and flora in the British Islands may have +been contemporaneous with Silurian life in North America, and with a +Carboniferous fauna and flora in Africa. Geographical provinces and +zones may have been as distinctly marked in the Paleozoic 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. + +It may be so; it may be otherwise. In the present condition of our +knowledge and of our methods, one verdict--"not proven, and not +provable"--must be recorded against all the grand hypotheses of the +paleontologist respecting the general succession of life on the globe. +The order and nature of terrestrial life, as a whole, are open +questions. Geology at present provides us with most valuable +topographical records, but she has not the means of working them into a +universal history. Is such a universal history, then, to be regarded as +unattainable? Are all the grandest and most interesting problems which +offer themselves to the geological student essentially insoluble? Is +he in the position of a scientific Tantalus--doomed always to thirst +for a knowledge which he cannot obtain? The reverse is to be hoped; +nay, it may not be impossible to indicate the source whence help will +come. + +In commencing these remarks, mention was made of the great obligations +under which the naturalist lies to the geologist and paleontologist. +Assuredly the time will come when these obligations will be repaid +tenfold, and when the maze of the world's past history, through which +the pure geologist and the pure paleontologist find no guidance, will be +securely threaded by the clue furnished by the naturalist. + +All who are competent to express an opinion on the subject are, at +present, agreed that the manifold varieties of animal and vegetable +form have not either come into existence by chance, nor result from +capricious exertions of creative power; but that they have taken place +in a definite order, the statement of which order is what men of +science term a natural law. Whether such a law is to be regarded as an +expression of the mode of operation of natural forces, or whether it is +simply a statement of the manner in which a supernatural power has +thought fit to act, is a secondary question, so long as the existence of +the law and the possibility of its discovery by the human intellect are +granted. But he must be a half-hearted philosopher who, believing in +that possibility, and having watched the gigantic strides of the +biological sciences during the last twenty years, doubts that science +will sooner or later make this further step, so as to become possessed +of the law of evolution of organic forms--of the unvarying order of +that great chain of causes and effects of which all organic forms, +ancient and modern, are the links. And then, if ever, we shall be able +to begin to discuss, with profit, the questions respecting the +commencement of life, and the nature of the successive populations of +the globe, which so many seem to think are already answered. + +The preceding arguments make no particular claim to novelty; indeed they +have been floating more or less distinctly before the minds of +geologists for the last thirty years; and if, at the present time, it +has seemed desirable to give them more definite and systematic +expression, it is because paleontology is every day assuming a greater +importance, and now requires to rest on a basis the firmness of which +is thoroughly well assured. Among its fundamental conceptions, there +must be no confusion between what is certain and what is more or less +probable.* But, pending the construction of a surer foundation than +paleontology now possesses, it may be instructive, assuming for the +nonce the general correctness of the ordinary hypothesis of geological +contemporaneity, to consider whether the deductions which are ordinarily +drawn from the whole body of paleontologic facts are justifiable. + + [footnote] *"le plus grand service qu'on puisse rendre a la + science est d'y faire place nette avant d'y rien + construire."--CUVIER + +The evidence on which such conclusions are based is of two kinds, +negative and positive. The value of negative evidence, in connection +with this inquiry, has been so fully and clearly discussed in an +address from the chair of this Society*, which none of us have +forgotten, that nothing need at present be said about it; the more, as +the considerations which have been laid before you have certainly not +tended to increase your estimation of such evidence. It will be +preferable to turn to the positive facts of paleontology, and to inquire +what they tell us. + + [footnote] *Anniversary Address for 1851, 'Quart. Journ. + Geol. Soc.' vol. vii. + +We are all accustomed to speak of the number and the extent of the +changes in the living population of the globe during geological time as +something enormous: and indeed they are so, if we regard only the +negative differences which separate the older rocks from the more +modern, and if we look upon specific and generic changes as great +changes, which from one point of view, they truly are. But leaving the +negative differences out of consideration, and looking only at the +positive data furnished by the fossil world from a broader point of +view--from that of the comparative anatomist who has made the study of +the greater modifications of animal form his chief business--a surprise +of another kind dawns upon the mind; and under 'this' aspect the +smallness of the total change becomes as astonishing as was its +greatness under the other. + +There are two hundred known orders of plants; of these not one is +certainly known to exist exclusively in the fossil state. The whole +lapse of geological time has as yet yielded not a single new ordinal +type of vegetable structure.* + + [footnote] *See Hooker's 'Introductory Essay to the Flora of + Tasmania', p. xxiii. + +The positive change in passing from the recent to the ancient animal +world is greater, but still singularly small. No fossil animal is so +distinct from those now living as to require to be arranged even in a +separate class from those which contain existing forms. It is only when +we come to the orders, which may be roughly estimated at about a +hundred and thirty, that we meet with fossil animals so distinct from +those now living as to require orders for themselves; and these do not +amount, on the most liberal estimate, to more than about 10 per cent. of +the whole. + +There is no certainly known extinct order of Protozoa; there is but one +among the Coelenterata--that of the rugose corals; there is none among +the Mollusca; there are three, the Cystidea, Blastoidea, and +Edrioasterida, among the Echinoderms; and two, the Trilobita and +Eurypterida, among the Crustacea; making altogether five for the great +sub-kingdom of Annulosa. Among Vertebrates there is no ordinally +distinct fossil fish: there is only one extinct order of Amphibia--the +Labyrinthodonts; but there are at least four distinct orders of +Reptilia, viz. the Ichthyosauria, Plesiosauria, Pterosauria, +Dinosauria, and perhaps another or two. There is no known extinct order +of Birds, and no certainly known extinct order of Mammals, the ordinal +distinctness of the "Toxodontia" being doubtful. + +The objection that broad statements of this kind, after all, rest +largely on negative evidence is obvious, but it has less force than may +at first be supposed; for, as might be expected from the circumstances +of the case, we possess more abundant positive evidence regarding Fishes +and marine Mollusks than respecting any other forms of animal life; and +yet these offer us, through the whole range of geological time, no +species ordinally distinct from those now living; while the far less +numerous class of Echinoderms presents three; and the Crustacea two, +such orders, though none of these come down later than the Paleozoic +age. Lastly, the Reptilia present the extraordinary and exceptional +phenomenon of as many extinct as existing orders, if not more; the four +mentioned maintaining their existence from the Lias to the Chalk +inclusive. + +Some years ago one of your Secretaries pointed out another kind of +positive paleontologic evidence tending towards the same +conclusion--afforded by the existence of what he termed "persistent +types" of vegetable and of animal life.* He stated, on the authority of +Dr. Hooker, that there are Carboniferous plants which appear to be +generically identical with some now living; that the cone of the +Oolitic 'Araucaria' is hardly distinguishable from that of an existing +species; that a true 'Pinus' appears in the Purbecks, and a 'Juglans' in +the Chalk; while, from the Bagshot Sands, a 'Banksia', the wood of +which is not distinguishable from that of species now living in +Australia, had been obtained. + + [footnote] *See the abstract of a Lecture "On the Persistent + Types of Animal Life," in the 'Notices of the Meetings of + the Royal Institution of Great Britain'.--June 3, 1859, + vol. iii. p. 151. + +Turning to the animal kingdom, he affirmed the tabulate corals of the +Silurian rocks to be wonderfully like those which now exist; while even +the families of the Aporosa were all represented in the older Mesozoic +rocks. + +Among the Molluska similar facts were adduced. Let it be borne in mind +that 'Avicula', 'Mytails', 'Chiton', 'Natica', 'Patella', 'Trochus', +'Discina', 'Orbicula', 'Lingula', 'Rhynchonella', and 'Nautilus', all +of which are existing 'genera', are given without a doubt as Silurian in +the last edition of 'Siluria'; while the highest forms of the highest +Cephalopods are represented in the Lias by a genus, 'Belemnoteuthis', +which presents the closest relation to the existing 'Loligo'. + +The two highest groups of the Annulosa, the Insecta and the Arachnida, +are represented in the Coal, either by existing genera, or by forms +differing from existing genera in quite minor peculiarities. + +Turning to the Vertebrata, the only Paleozoic Elasmobranch Fish of which +we have any complete knowledge is the Devonian and Carboniferous +'Pleuracanthus', which differs no more from existing Sharks than these +do from one another. + +Again, vast as is the number of undoubtedly Ganoid fossil Fishes, and +great as is their range in time, a large mass of evidence has recently +been adduced to show that almost all those respecting which we possess +sufficient information, are referable to the same sub-ordinal groups as +the existing 'Lepidosteus', 'Polypterus', and Sturgeon; and that a +singular relation obtains between the older and the younger Fishes; the +former, the Devonian Ganoids, being almost all members of the same +sub-order as 'Polypterus', while the Mesozoic Ganoids are almost all +similarly allied to 'Lepidosteus'.* + + [footnote] *"Memoirs of the Geological Survey of the United + Kingdom.--Decade x. Preliminary Essay upon the Systematic + Arrangement of the Fishes of the Devonian Epoch." + +Again, what can be more remarkable than the singular constancy of +structure preserved throughout a vast period of time by the family of +the Pycnodonts and by that of the true Coelacanths; the former +persisting, with but insignificant modifications, from the +Carboniferous to the Tertiary rocks, inclusive; the latter existing, +with still less change, from the Carboniferous rocks to the Chalk, +inclusive? + +Among Reptiles, the highest living group, that of the Crocodilia, is +represented, at the early part of the Mesozoic epoch, by species +identical in the essential characters of their organization with those +now living, and differing from the latter only in such matters as the +form of the articular facets of the vertebral centra, in the extent to +which the nasal passages are separated from the cavity of the mouth by +bone, and in the proportions of the limbs. + +And even as regards the Mammalia, the scanty remains of Triassic and +Oolitic species afford no foundation for the supposition that the +organization of the oldest forms differed nearly so much from some of +those which now live as these differ from one another. + +It is needless to multiply these instances; enough has been said to +justify the statement that, in view of the immense diversity of known +animal and vegetable forms, and the enormous lapse of time indicated by +the accumulation of fossiliferous strata, the only circumstance to be +wondered at is, not that the changes of life, as exhibited by positive +evidence, have been so great, but that they have been so small. + +Be they great or small, however, it is desirable to attempt to estimate +them. Let us, therefore, take each great division of the animal world +in succession, and, whenever an order or a family can be shown to have +had a prolonged existence, let us endeavour to ascertain how far the +later members of the group differ from the earlier ones. If these later +members, in all or in many cases, exhibit a certain amount of +modification, the fact is, so far, evidence in favour of a general law +of change; and, in a rough way, the rapidity of that change will be +measured by the demonstrable amount of modification. On the other +hand, it must be recollected that the absence of any modification, +while it may leave the doctrine of the existence of a law of change +without positive support, cannot possibly disprove all forms of that +doctrine, though it may afford a sufficient refutation of any of them. + +The PROTOZOA.--The Protozoa are represented throughout the whole range +of geological series, from the Lower Silurian formation to the present +day. The most ancient forms recently made known by Ehrenberg are +exceedingly like those which now exist: no one has ever pretended that +the difference between any ancient and any modern Foraminifera is of +more than generic value, nor are the oldest Foraminifera either +simpler, more embryonic, or less differentiated, than the existing +forms. + +The COELENTERATA.--The Tabulate Corals have existed from the Silurian +epoch to the present day, but I am not aware that the ancient +'Heliolites' possesses a single mark of a more embryonic or less +differentiated character, or less high organization, than the existing +'Heliopora'. As for the Aporose Corals, in what respect is the Silurian +'Paleocyclus' less highly organized or more embryonic than the modern +'Fungia', or the Liassic Aporosa than the existing members of the same +families? + +The 'Mollusca'.--In what sense is the living 'Waldheimia' less +embryonic, or more specialized; than the paleozoic 'Spirifer'; or the +existing 'Rhynchonellae', 'Craniae', 'Discinae', 'Lingulae', than the +Silurian species of the same genera? In what sense can 'Loligo' or +'Spirula' be said to be more specialized, or less embryonic, than +'Belemnites'; or the modern species of Lamellibranch and Gasteropod +genera, than the Silurian species of the same genera? + +The ANNULOSA.--The Carboniferous Insecta and Arachnida are neither less +specialized, nor more embryonic, than these that now live, nor are the +Liassic Cirripedia and Macrura; while several of the Brachyura, which +appear in the Chalk, belong to existing genera; and none exhibit either +an intermediate, or an embryonic, character. + +The VERTEBRARA.--Among fishes I have referred to the Coelacanthini +(comprising the genera 'Coelacanthus', 'Holophagus', 'Undina', and +'Macropoma') as affording an example of a persistent type; and it is +most remarkable to note the smallness of the differences between any of +these fishes (affecting at most the proportions of the body and fins, +and the character and sculpture of the scales), notwithstanding their +enormous range in time. In all the essentials of its very peculiar +structure, the 'Macropoma' of the Chalk is identical with the +'Coelacanthus' of the Coal. Look at the genus 'Lepidotus', again, +persisting without a modification of importance from the Liassic to the +Eocene formations inclusive. + +Or among the Teleostei--in what respect is the 'Beryx' of the Chalk more +embryonic, or less differentiated, than 'Beryx lineatus' of King +George's Sound? + +Or to turn to the higher Vertebrata--in what sense are the Liassic +Chelonia inferior to those which now exist? How are the Cretaceous +Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more +differentiated, species than those of the Lias? + +Or lastly, in what circumstance is the 'Phascolotherium' more embryonic, +or of a more generalized type, than the modern Opossum; or a +'Lophiodon', or a 'Paleotherium', than a modern 'Tapirus' or 'Hyrax'? + +These examples might be almost indefinitely multiplied, but surely they +are sufficient to prove that the only safe and unquestionable testimony +we can procure--positive evidence--fails to demonstrate any sort of +progressive modification towards a less embryonic, or less generalised, +type in a great many groups of animals of long-continued geological +existence. In these groups there is abundant evidence of +variation--none of what is ordinarily understood as progression; and, +if the known geological record is to be regarded as even any +considerable fragment of the whole, it is inconceivable that any theory +of a necessarily progressive development can stand, for the numerous +orders and families cited afford no trace of such a process. + +But it is a most remarkable fact, that, while the groups which have been +mentioned, and many besides, exhibit no sign of progressive +modification, there are others, co-existing with them, under the same +conditions, in which more or less distinct indications of such a process +seems to be traceable. Among such indications I may remind you of the +predominance of Holostome Gasteropoda in the older rocks as compared +with that of Siphonostome Gasteropoda in the later. A case less open +to the objection of negative evidence, however, is that afforded by the +Tetrabranchiate Cephalopoda, the forms of the shells and of the septal +sutures exhibiting a certain increase of complexity in the newer +genera. Here, however, one is met at once with the occurrence of +'Orthoceras' and 'Baculites' at the two ends of the series, and of the +fact that one of the simplest Genera, 'Nautilus', is that which now +exists. + +The Crinoidea, in the abundance of stalked forms in the ancient +formations as compared with their present rarity, seem to present us +with a fair case of modification from a more embryonic towards a less +embryonic condition. But then, on careful consideration of the facts, +the objection arises that the stalk, calyx, and arms of the paleozoic +Crinoid are exceedingly different from the corresponding organs of a +larval 'Comatula'; and it might with perfect justice be argued that +'Actinocrinus' and 'Eucalyptocrinus', for example, depart to the full as +widely, in one direction, from the stalked embryo of 'Comatula', as +'Comatula' itself does in the other. + +The Echinidea, again, are frequently quoted as exhibiting a gradual +passage from a more generalized to a more specialized type, seeing that +the elongated, or oval, Spatangoids appear after the spheroidal +Echinoids. But here it might be argued, on the other hand, that the +spheroidal Echinoids, in reality, depart further from the general plan +and from the embryonic form than the elongated Spatangoids do; and that +the peculiar dental apparatus and the pedicellariae of the former are +marks of at least as great differentiation as the petaloid ambulacra +and semitae of the latter. + +Once more, the prevalence of Macrurous before Brachyurous Podophthalmia +is, apparently, a fair piece of evidence in favour of progressive +modification in the same order of Crustacea; and yet the case will not +stand much sifting, seeing that the Macrurous Podophthalmia depart as +far in one direction from the common type of Podophthalmia, or from any +embryonic condition of the Brachyura, as the Brachyura do in the other; +and that the middle terms between Macrura and Brachyura--the +Anomura--are little better represented in the older Mesozoic rocks than +the Brachyura are. + +None of the cases of progressive modification which are cited from among +the Invertebrata appear to me to have a foundation less open to +criticism than these; and if this be so, no careful reasoner would, I +think, be inclined to lay very great stress upon them. Among the +Vertebrata, however, there are a few examples which appear to be far +less open to objection. + +It is, in fact, true of several groups of Vertebrata which have lived +through a considerable range of time, that the endoskeleton (more +particularly the spinal column) of the older genera presents a less +ossified, and, so far, less differentiated, condition than that of the +younger genera. Thus the Devonian Ganoids, though almost all members +of the same sub-order as 'Polypterus', and presenting numerous +important resemblances to the existing genus, which possesses biconcave +vertebrae, are, for the most part, wholly devoid of ossified vertebral +centra. The Mesozoic Lepidosteidae, again, have, at most, biconcave +vertebrae, while the existing 'Lepidosteus' has Salamandroid, +opisthocoelous, vertebrae. So, none of the Paleozoic Sharks have shown +themselves to be possessed of ossified vertebrae, while the majority of +modern Sharks possess such vertebrae. Again, the more ancient +Crocodilia and Lacertilia have vertebrae with the articular facets of +their centra flattened or biconcave, while the modern members of the +same group have them procoelous. But the most remarkable examples of +progressive modification of the vertebral column, in correspondence +with geological age, are those afforded by the Pycnodonts among fish, +and the Labyrinthodonts among Amphibia. + +The late able ichthyologist Heckel pointed out the fact, that, while the +Pycnodonts never possess true vertebral centra, they differ in the +degree of expansion and extension of the ends of the bony arches of the +vertebrae upon the sheath of the notochord; the Carboniferous forms +exhibiting hardly any such expansion, while the Mesozoic genera present +a greater and greater development, until, in the Tertiary forms, the +expanded ends become suturally united so as to form a sort of false +vertebra. Hermann von Meyer, again, to whose luminous researches we +are indebted for our present large knowledge of the organization of the +older Labyrinthodonts, has proved that the Carboniferous +'Archegosaurus' had very imperfectly developed vertebral centra, while +the Triassic 'Mastodonsaurus' had the same parts completely ossified.* + + [footnote] *As the Address is passing through the press + (March 7, 1862), evidence lies before me of the existence + of a new Labyrinthodont ('Pholidogaster'), from the + Edinburgh coal-field, with well-ossified vertebral centra. + +The regularity and evenness of the dentition of the 'Anoplotherium', as +contrasted with that of existing Artiodactyles, and the assumed nearer +approach of the dentition of certain ancient Carnivores to the typical +arrangement, have also been cited as exemplifications of a law of +progressive development, but I know of no other cases based on positive +evidence which are worthy of particular notice. + +What, then, does an impartial survey of the positively ascertained +truths of paleontology 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, or +from more to less generalized types, within the limits of the period +represented by the fossiliferous rocks? + +It negatives those doctrines; for it either shows us no evidence of any +such modification, or demonstrates it 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 +generalized in structure than the later ones. To a certain extent, +indeed, it may be said that imperfect ossification of the vertebral +column is an embryonic character; but, on the other hand, it would be +extremely incorrect to suppose that the vertebral columns of the older +Vertebrata are in any sense embryonic in their whole structure. + +Obviously, if the earliest fossiliferous rocks now known are coeval with +the commencement of life, and if their contents give us any just +conception of the nature and the extent of the earliest fauna and +flora, the insignificant amount of modification which can be +demonstrated to have taken place in any one group of animals, or +plants, is quite incompatible with the hypothesis that all living forms +are the results of a necessary process of progressive development, +entirely comprised within the time represented by the fossiliferous +rocks. + +Contrariwise, any admissible hypothesis of progressive modification must +be compatible with persistence without progression, through indefinite +periods. And should such an hypothesis eventually be proved to be +true, in the only way in which it can be demonstrated, viz. by +observation and experiment upon the existing forms of life, the +conclusion will inevitably present itself, that the Paleozoic, +Mesozoic, and Cainozoic faunae and florae, taken together, bear +somewhat the same proportion to the whole series of living beings which +have occupied this globe, as the existing fauna and flora do to them. + +Such are the results of paleontology as they appear, and have for some +years appeared, to the mind of an inquirer who regards that study +simply as one of the applications of the great biological sciences, and +who desires to see it placed upon the same sound basis as other +branches of physical inquiry. If the arguments which have been brought +forward are valid, probably no one, in view of the present state of +opinion, will be inclined to think the time wasted which has been spent +upon their elaboration. + + + + + +End of The Project Gutenberg Etext of Geological Contemporaneity +and Persistent Types of Life by Thomas H. Huxley + diff --git a/old/thx1610.zip b/old/thx1610.zip Binary files differnew file mode 100644 index 0000000..bc40793 --- /dev/null +++ b/old/thx1610.zip |