Part 28

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 ordinarily 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 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. 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 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.

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 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 CĆLENTERATA.--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 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 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 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, 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 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 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 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 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 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.