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 a.s.sertion 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_ ill.u.s.tration 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 a.s.sumption. 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 a.s.sent 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 palaeontology able to succeed where physical geology fails? Standard writers on palaeontology, as has been seen, a.s.sume 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 ident.i.ty 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 a.s.serting 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 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 ident.i.ty of half a dozen species, or of a good many genera?
And yet there is no better evidence for the contemporaneity a.s.sumed by all who adopt the hypotheses 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 palaeontology, 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 any other vertical linear section 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 a.s.suming the corresponding beds to be synchronous or strictly contemporaneous; and there are mult.i.tudes of accessory circ.u.mstances which may fully justify the a.s.sumption of such synchrony. But the moment the geologist has to deal with large areas, or with completely separated deposits, the mischief of confounding that "h.o.m.otaxis" or "similarity of arrangement," which _can_ be demonstrated, with "synchrony" or "ident.i.ty 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 palaeontology 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 Palaeozoic epoch as at present, and those seemingly sudden appearances of new genera and species, which we ascribe to new creation, may be simple results of migration.
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 proveable"--must be recorded against all the grand hypotheses of the palaeontologist 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 up 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 palaeontologist.
a.s.suredly 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 palaeontologist 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 palaeontology is every day a.s.suming a greater importance, and now requires to rest on a basis the firmness of which is thoroughly well a.s.sured. Among its fundamental conceptions, there must be no confusion between what is certain and what is more or less probable.[33] But, pending the construction of a surer foundation than palaeontology now possesses, it may be instructive, a.s.suming 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 palaeontological facts are justifiable.
The evidence on which such conclusions are based is of two kinds, negative and positive. The value of negative evidence, in connexion with this inquiry, has been so fully and clearly discussed in an address from the chair of this Society,[34] 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 palaeontology, and to inquire what they tell us.
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.[35]
The positive change in pa.s.sing 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 cla.s.s 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 circ.u.mstances 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 ordinarily distinct from those now living; while the far less numerous cla.s.s of Echinoderms presents three, and the Crustacea two, such orders, though none of these come down later than the Palaeozoic age. Lastly, the Reptilia present the extraordinary and exceptional phaenomenon 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 palaeontological evidence tending towards the same conclusion--afforded by the existence of what he termed "persistent types" of vegetable and of animal life.[36] 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.
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 Mollusca similar facts were adduced. Let it be borne in mind that _Avicula_, _Mytilus_, _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 palaeozoic 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 ma.s.s 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_.[37]
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 pa.s.sages 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 Tria.s.sic 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 acc.u.mulation of fossiliferous strata, the only circ.u.mstance 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 many 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 _Paloeocydus_ less highly organized or more embryonic than the modern _Fungia_, or the Lia.s.sic 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 palaeozoic _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 those that now live, nor are the Lia.s.sic 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 VERTEBRATA.--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 Lia.s.sic 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 Lia.s.sic 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 circ.u.mstance is the _Phascolotherium_ more embryonic, or of a more generalized type, than the modern Opossum; or a _Lophiodon_, or a _Palaeotherium_, 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 generalized, 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, coexisting with them, under the same conditions, in which more or less distinct indications of such a process seem 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 sh.e.l.ls 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 palaeozoic 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 pa.s.sage 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.