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palæozoic Spirifer; or the existing Rhynchonellæ, Craniæ, Discina, Lingulæ, 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 Calacanthini (comprising the genera Cælacanthus, 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 Cælacanthus 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
In these groups
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 embryonie, or of a more generalized type, than the modern Opossum ; or a Lophiodon, or a Palæotherium, 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. 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
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 palæozoic 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 clongated Spatangoids do ; and that the peculiar dental apparatus and the pedicellariæ of the former are marks of at least as great differentiation as the petaloid ambulacra and semitæ 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
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 vertebræ, are, for the most part, wholly devoid of ossified vertebral centra. The Mesozoic Lepidosteidæ, again, have, at most, biconcave vertebræ, while the existing Lepidosteus has Salamandroid, opisthocælous, vertebra. So, none of the Palæozoic Sharks have shown themselves to be possessed of ossified vertebræ, while the majority of modern Sharks possess such vertebræ. Again, the more ancient
Crocodilia and Lacertilia have vertebræ with the articular facets of their centra flattened or biconcave, while the modern members of the same group have them proceelous. 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 vertebræ 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.)
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
1 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.