Discourses: Biological and Geological — Geological Contemporaneity and Persistent Types of Life Pt 2

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 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.

[1] But, pending the construction of a surer foundation than palaeontology 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 palaeontological facts are justifiable.

[Footnote 1: "Le plus grand service qu'on puisse rendre à 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,[2] 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.

[Footnote 2: 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.

[3] [Footnote 3: 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 Palaeozoic 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 palaeontological evidence tending towards the same conclusion-- afforded by the existence of what he termed "persistent types" of vegetable and of animal life.

[4] 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 4: 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 Mollusca similar facts were adduced.

Let it be borne in mind that _Avicula, Mytilus, Chiton, Natica, Patella, Trochus, Discina, Orbicula, Lingula, Rhynchonclla_, 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 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_.

[5] [Footnote 5: "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 organisation 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 organisation 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 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 organisation, than the existing _Heliopora_.

As for the Aporose Corals, in what respect is the Silurian _Paloeocyclus_ less highly organised 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 specialised, than the palaeozoic _Spirifer_; or the existing _Rhynchonelloe, Cranioe, Discinoe, Linguloe_, than the Silurian species of the same genera?

In what sense can _Loligo_ or _Spirula_ be said to be more specialised, 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 specialised, 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 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 Liassic to the Eocene formations inclusively.

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 generalised type, than the modern Opossum; or a _Lophiodon_, or a _Paloeotherium_, 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 Siphonostone 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 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 passage from a more generalised to a more specialised 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 biconclave 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 Palaeozoic 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 organisation 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.

[6] [Footnote 6: As this 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 palaeontology testify in relation to the common doctrines of progressive modification, which suppose that modification to have taken place by a necessary progress from more to less embryonic forms, or from more to less generalised 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 generalised 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 coëval 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 Palaeozoic 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 palaeontology 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.

X GEOLOGICAL REFORM