Geological Contemporaneity and Persistent Types of Life
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Thomas H. Huxley >> Geological Contemporaneity and Persistent Types of Life
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.