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
[1869]
"A great reform in geological speculation seems now to have become
necessary."
"It is quite certain that a great mistake has been made--that British
popular geology at the present time is in direct opposition to the
principles of Natural Philosophy."[1]
[Footnote 1: On Geological Time. By Sir W. Thomson, LL.D. _Transactions
of the Geological Society of Glasgow_, vol. iii.]
In reviewing the course of geological thought during the past year, for
the purpose of discovering those matters to which I might most fitly
direct your attention in the Address which it now becomes my duty to
deliver from the Presidential Chair, the two somewhat alarming sentences
which I have just read, and which occur in an able and interesting essay
by an eminent natural philosopher, rose into such prominence before my
mind that they eclipsed everything else.
It surely is a matter of paramount importance for the British geologists
(some of them very popular geologists too) here in solemn annual session
assembled, to inquire whether the severe judgment thus passed upon them
by so high an authority as Sir William Thomson is one to which they must
plead guilty _sans phrase_, or whether they are prepared to say "not
guilty," and appeal for a reversal of the sentence to that higher court
of educated scientific opinion to which we are all amenable.
As your attorney-general for the time being, I thought I could not do
better than get up the case with a view of advising you. It is true that
the charges brought forward by the other side involve the consideration
of matters quite foreign to the pursuits with which I am ordinarily
occupied; but, in that respect, I am only in the position which is, nine
times out of ten, occupied by counsel, who nevertheless contrive to gain
their causes, mainly by force of mother-wit and common-sense, aided by
some training in other intellectual exercises.
Nerved by such precedents, I proceed to put my pleading before you.
And the first question with which I propose to deal is, What is it to
which Sir W. Thomson refers when he speaks of "geological speculation"
and "British popular geology"?
I find three, more or less contradictory, systems of geological thought,
each of which might fairly enough claim these appellations, standing side
by side in Britain. I shall call one of them CATASTROPHISM, another
UNIFORMITARIANISM, the third EVOLUTIONISM; and I shall try briefly to
sketch the characters of each, that you may say whether the
classification is, or is not, exhaustive.
By CATASTROPHISM, I mean any form of geological speculation which, in
order to account for the phenomena of geology, supposes the operation of
forces different in their nature, or immeasurably different in power,
from those which we at present see in action in the universe.
The Mosaic cosmogony is, in this sense, catastrophic, because it assumes
the operation of extra-natural power. The doctrine of violent upheavals,
_débâcles_, and cataclysms in general, is catastrophic, so far as it
assumes that these were brought about by causes which have now no
parallel. There was a time when catastrophism might, pre-eminently, have
claimed the title of "British popular geology"; and assuredly it has yet
many adherents, and reckons among its supporters some of the most
honoured members of this Society.
By UNIFORMITARIANISM, I mean especially, the teaching of Hutton and of
Lyell.
That great though incomplete work, "The Theory of the Earth," seems to me
to be one of the most remarkable contributions to geology which is
recorded in the annals of the science. So far as the not-living world is
concerned, uniformitarianism lies there, not only in germ, but in blossom
and fruit.
If one asks how it is that Hutton was led to entertain views so far in
advance of those prevalent in his time, in some respects; while, in
others, they seem almost curiously limited, the answer appears to me to
be plain.
Hutton was in advance of the geological speculation of his time, because,
in the first place, he had amassed a vast store of knowledge of the facts
of geology, gathered by personal observation in travels of considerable
extent; and because, in the second place, he was thoroughly trained in
the physical and chemical science of his day, and thus possessed, as much
as any one in his time could possess it, the knowledge which is requisite
for the just interpretation of geological phenomena, and the habit of
thought which fits a man for scientific inquiry.
It is to this thorough scientific training that I ascribe Hutton's steady
and persistent refusal to look to other causes than those now in
operation, for the explanation of geological phenomena.
Thus he writes:--"I do not pretend, as he [M. de Luc] does in his theory,
to describe the beginning of things. I take things such as I find them at
present; and from these I reason with regard to that which must have
been."[2]
[Footnote 2: _The Theory of the Earth_, vol. i. p. 173, note.]
And again:--"A theory of the earth, which has for object truth, can have
no retrospect to that which had preceded the present order of the world;
for this order alone is what we have to reason upon; and to reason
without data is nothing but delusion. A theory, therefore, which is
limited to the actual constitution of this earth cannot be allowed to
proceed one step beyond the present order of things."[3]
[Footnote 3: _Ibid._, vol. i. p. 281.]