THE RISE AND PROGRESS OF PALAEONTOLOGY

THIS IS ESSAY #2 FROM "SCIENCE AND HEBREW TRADITION"

By Thomas Henry Huxley



That application of the sciences of biology and geology, which is
commonly known as palaeontology, took its origin in the mind of the
first person who, finding something like a shell, or a bone, naturally
imbedded in gravel or rock, indulged in speculations upon the nature
of this thing which he had dug out--this "fossil"--and upon the causes
which had brought it into such a position. In this rudimentary form, a
high antiquity may safely be ascribed to palaeontology, inasmuch as we
know that, 500 years before the Christian era, the philosophic doctrines
of Xenophanes were influenced by his observations upon the fossil
remains exposed in the quarries of Syracuse. From this time forth not
only the philosophers, but the poets, the historians, the geographers
of antiquity occasionally refer to fossils; and, after the revival of
learning, lively controversies arose respecting their real nature.
But hardly more than two centuries have elapsed since this fundamental
problem was first exhaustively treated; it was only in the last century
that the archaeological value of fossils--their importance, I mean, as
records of the history of the earth--was fully recognised; the first
adequate investigation of the fossil remains of any large group of
vertebrated animals is to be found in Cuvier's "Recherches sur les
Ossemens Fossiles," completed in 1822; and, so modern is stratigraphical
palaeontology, that its founder, William Smith, lived to receive the
just recognition of his services by the award of the first Wollaston
Medal in 1831.

But, although palaeontology is a comparatively youthful scientific
speciality, the mass of materials with which it has to deal is already
prodigious. In the last fifty years the number of known fossil remains
of invertebrated animals has been trebled or quadrupled. The work of
interpretation of vertebrate fossils, the foundations of which were
so solidly laid by Cuvier, was carried on, with wonderful vigour and
success, by Agassiz in Switzerland, by Von Meyer in Germany, and
last, but not least, by Owen in this country, while, in later years, a
multitude of workers have laboured in the same field. In many groups of
the animal kingdom the number of fossil forms already known is as great
as that of the existing species. In some cases it is much greater; and
there are entire orders of animals of the existence of which we should
know nothing except for the evidence afforded by fossil remains. With
all this it may be safely assumed that, at the present moment, we are
not acquainted with a tittle of the fossils which will sooner or later
be discovered. If we may judge by the profusion yielded within the last
few years by the Tertiary formations of North America, there seems to be
no limit to the multitude of mammalian remains to be expected from that
continent; and analogy leads us to expect similar riches in Eastern
Asia, whenever the Tertiary formations of that region are as carefully
explored. Again, we have, as yet, almost everything to learn respecting
the terrestrial population of the Mesozoic epoch; and it seems as if
the Western territories of the United States were about to prove as
instructive in regard to this point as they have in respect of tertiary
life. My friend Professor Marsh informs me that, within two years,
remains of more than 160 distinct individuals of mammals, belonging to
twenty species and nine genera, have been found in a space not larger
than the floor of a good-sized room; while beds of the same age have
yielded 300 reptiles, varying in size from a length of 60 feet or 80
feet to the dimensions of a rabbit.

The task which I have set myself to-night is to endeavour to lay before
you, as briefly as possible, a sketch of the successive steps by
which our present knowledge of the facts of palaeontology and of those
conclusions from them which are indisputable, has been attained; and I
beg leave to remind you, at the outset, that in attempting to sketch
the progress of a branch of knowledge to which innumerable labours
have contributed, my business is rather with generalisations than with
details. It is my object to mark the epochs of palaeontology, not to
recount all the events of its history.

That which I just now called the fundamental problem of palaeontology,
the question which has to be settled before any other can be profitably
discussed, is this, What is the nature of fossils? Are they, as the
healthy common sense of the ancient Greeks appears to have led them to
assume without hesitation, the remains of animals and plants? Or are
they, as was so generally maintained in the fifteenth, sixteenth, and
seventeenth centuries, mere figured stones, portions of mineral matter
which have assumed the forms of leaves and shells and bones, just as
those portions of mineral matter which we call crystals take on the form
of regular geometrical solids? Or, again, are they, as others thought,
the products of the germs of animals and of the seeds of plants which
have lost their way, as it were, in the bowels of the earth, and have
achieved only an imperfect and abortive development? It is easy to sneer
at our ancestors for being disposed to reject the first in favour of one
or other of the last two hypotheses; but it is much more profitable to
try to discover why they, who were really not one whit less sensible
persons than our excellent selves, should have been led to entertain
views which strike us as absurd, The belief in what is erroneously
called spontaneous generation, that is to say, in the development
of living matter out of mineral matter, apart from the agency of
pre-existing living matter, as an ordinary occurrence at the present
day--which is still held by some of us, was universally accepted as an
obvious truth by them. They could point to the arborescent forms
assumed by hoar-frost and by sundry metallic minerals as evidence of the
existence in nature of a "plastic force" competent to enable inorganic
matter to assume the form of organised bodies. Then, as every one who is
familiar with fossils knows, they present innumerable gradations, from
shells and bones which exactly resemble the recent objects, to masses of
mere stone which, however accurately they repeat the outward form of the
organic body, have nothing else in common with it; and, thence, to mere
traces and faint impressions in the continuous substance of the rock.
What we now know to be the results of the chemical changes which take
place in the course of fossilisation, by which mineral is substituted
for organic substance, might, in the absence of such knowledge, be
fairly interpreted as the expression of a process of development in the
opposite direction--from the mineral to the organic. Moreover, in an age
when it would have seemed the most absurd of paradoxes to suggest that
the general level of the sea is constant, while that of the solid land
fluctuates up and down through thousands of feet in a secular ground
swell, it may well have appeared far less hazardous to conceive that
fossils are sports of nature than to accept the necessary alternative,
that all the inland regions and highlands, in the rocks of which marine
shells had been found, had once been covered by the ocean. It is not so
surprising, therefore, as it may at first seem, that although such men
as Leonardo da Vinci and Bernard Palissy took just views of the nature
of fossils, the opinion of the majority of their contemporaries set
strongly the other way; nor even that error maintained itself long after
the scientific grounds of the true interpretation of fossils had been
stated, in a manner that left nothing to be desired, in the latter half
of the seventeenth century. The person who rendered this good service
to palaeontology was Nicolas Steno, professor of anatomy in Florence,
though a Dane by birth. Collectors of fossils at that day were familiar
with certain bodies termed "glossopetrae," and speculation was rife as
to their nature. In the first half of the seventeenth century, Fabio
Colonna had tried to convince his colleagues of the famous Accademia dei
Lincei that the glossopetrae were merely fossil sharks' teeth, but his
arguments made no impression. Fifty years later, Steno re-opened the
question, and, by dissecting the head of a shark and pointing out the
very exact correspondence of its teeth with the glossopetrae, left no
rational doubt as to the origin of the latter. Thus far, the work of
Steno went little further than that of Colonna, but it fortunately
occurred to him to think out the whole subject of the interpretation of
fossils, and the result of his meditations was the publication, in 1669,
of a little treatise with the very quaint title of "De Solido intra
Solidum naturaliter contento." The general course of Steno's argument
may be stated in a few words. Fossils are solid bodies which, by some
natural process, have come to be contained within other solid bodies,
namely, the rocks in which they are embedded; and the fundamental
problem of palaeontology, stated generally, is this: "Given a body
endowed with a certain shape and produced in accordance with natural
laws, to find in that body itself the evidence of the place and manner
of its production." [1] The only way of solving this problem is by the
application of the axiom that "like effects imply like causes," or as
Steno puts it, in reference to this particular case, that "bodies which
are altogether similar have been produced in the same way." [2] Hence,
since the glossopetrae are altogether similar to sharks' teeth, they
must have been produced by sharklike fishes; and since many fossil
shells correspond, down to the minutest details of structure, with the
shells of existing marine or freshwater animals, they must have been
produced by similar animals; and the like reasoning is applied by
Steno to the fossil bones of vertebrated animals, whether aquatic
or terrestrial. To the obvious objection that many fossils are not
altogether similar to their living analogues, differing in substance
while agreeing in form, or being mere hollows or impressions, the
surfaces of which are figured in the same way as those of animal or
vegetable organisms, Steno replies by pointing out the changes which
take place in organic remains embedded in the earth, and how their solid
substance may be dissolved away entirely, or replaced by mineral matter,
until nothing is left of the original but a cast, an impression, or
a mere trace of its contours. The principles of investigation thus
excellently stated and illustrated by Steno in 1669, are those
which have, consciously or unconsciously, guided the researches of
palaeontologists ever since. Even that feat of palaeontology which has
so powerfully impressed the popular imagination, the reconstruction of
an extinct animal from a tooth or a bone, is based upon the simplest
imaginable application of the logic of Steno. A moment's consideration
will show, in fact, that Steno's conclusion that the glossopetrae are
sharks' teeth implies the reconstruction of an animal from its tooth. It
is equivalent to the assertion that the animal of which the glossopetrae
are relics had the form and organisation of a shark; that it had
a skull, a vertebral column, and limbs similar to those which are
characteristic of this group of fishes; that its heart, gills, and
intestines presented the peculiarities which those of all sharks
exhibit; nay, even that any hard parts which its integument contained
were of a totally different character from the scales of ordinary
fishes. These conclusions are as certain as any based upon probable
reasonings can be. And they are so, simply because a very large
experience justifies us in believing that teeth of this particular form
and structure are invariably associated with the peculiar organisation
of sharks, and are never found in connection with other organisms. Why
this should be we are not at present in a position even to imagine; we
must take the fact as an empirical law of animal morphology, the reason
of which may possibly be one day found in the history of the evolution
of the shark tribe, but for which it is hopeless to seek for an
explanation in ordinary physiological reasonings. Every one practically
acquainted with palaeontology is aware that it is not every tooth, nor
every bone, which enables us to form a judgment of the character of the
animal to which it belonged; and that it is possible to possess many
teeth, and even a large portion of the skeleton of an extinct animal,
and yet be unable to reconstruct its skull or its limbs. It is only
when the tooth or bone presents peculiarities, which we know by previous
experience to be characteristic of a certain group, that we can safely
predict that the fossil belonged to an animal of the same group. Any one
who finds a cow's grinder may be perfectly sure that it belonged to an
animal which had two complete toes on each foot and ruminated; any oner may be as sure that it had one complete
toe on each foot and did not ruminate; but if ruminants and horses
were extinct animals of which nothing but the grinders had ever been
discovered, no amount of physiological reasoning could have enabled
us to reconstruct either animal, still less to have divined the
wide differences between the two. Cuvier, in the "Discours sur les
Revolutions de la Surface du Globe," strangely credits himself, and has
ever since been credited by others, with the invention of a new method
of palaeontological research. But if you will turn to the "Recherches
sur les Ossemens Fossiles" and watch Cuvier, not speculating, but
working, you will find that his method is neither more nor less than
that of Steno. If he was able to make his famous prophecy from the jaw
which lay upon the surface of a block of stone to the pelvis of the same
animal which lay hidden in it, it was not because either he, or any
one else, knew, or knows, why a certain form of jaw is, as a rule,
constantly accompanied by the presence of marsupial bones, but simply
because experience has shown that these two structures are co-ordinated.


The settlement of the nature of fossils led at once to the next advance
of palaeontology, viz. its application to the deciphering of the history
of the earth. When it was admitted that fossils are remains of animals
and plants, it followed that, in so far as they resemble terrestrial, or
freshwater, animals and plants, they are evidences of the existence of
land, or fresh water; and, in so far as they resemble marine organisms,
they are evidences of the existence of the sea at the time at which
they were parts of actually living animals and plants. Moreover, in
the absence of evidence to the contrary, it must be admitted that the
terrestrial or the marine organisms implied the existence of land or
sea at the place in which they were found while they were yet living.
In fact, such conclusions were immediately drawn by everybody, from
the time of Xenophanes downwards, who believed that fossils were really
organic remains. Steno discusses their value as evidence of repeated
alteration of marine and terrestrial conditions upon the soil of Tuscany
in a manner worthy of a modern geologist. The speculations of De Maillet
in the beginning of the eighteenth century turn upon fossils; and Buffon
follows him very closely in those two remarkable works, the "Theorie
de la Terre" and the "Epoques de la Nature" with which he commenced and
ended his career as a naturalist.

The opening sentences of the "Epoques de la Nature" show us how
fully Buffon recognised the analogy of geological with archaeological
inquiries. "As in civil history we consult deeds, seek for coins, or
decipher antique inscriptions in order to determine the epochs of human
revolutions and fix the date of moral events; so, in natural history,
we must search the archives of the world, recover old monuments from the
bowels of the earth, collect their fragmentary remains, and gather into
one body of evidence all the signs of physical change which may enable
us to look back upon the different ages of nature. It is our only
means of fixing some points in the immensity of space, and of setting a
certain number of waymarks along the eternal path of time."

Buffon enumerates five classes of these monuments of the past history of
the earth, and they are all facts of palaeontology. In the first place,
he says, shells and other marine productions are found all over the
surface and in the interior of the dry land; and all calcareous rocks
are made up of their remains. Secondly, a great many of these shells
which are found in Europe are not now to be met with in the adjacent
seas; and, in the slates and other deep-seated deposits, there are
remains of fishes and of plants of which no species now exist in our
latitudes, and which are either extinct, or exist only in more northern
climates. Thirdly, in Siberia and in other northern regions of
Europe and of Asia, bones and teeth of elephants, rhinoceroses, and
hippopotamuses occur in such numbers that these animals must once have
lived and multiplied in those regions, although at the present day they
are confined to southern climates. The deposits in which these remains
are found are superficial, while those which contain shells and other
marine remains lie much deeper. Fourthly, tusks and bones of elephants
and hippopotamuses are found not only in the northern regions of the old
world, but also in those of the new world, although, at present, neither
elephants nor hippopotamuses occur in America. Fifthly, in the middle of
the continents, in regions most remote from the sea, we find an infinite
number of shells, of which the most part belong to animals of those
kinds which still exist in southern seas, but of which many others have
no living analogues; so that these species appear to be lost, destroyed
by some unknown cause. It is needless to inquire how far these
statements are strictly accurate; they are sufficiently so to justify
Buffon's conclusions that the dry land was once beneath the sea; that
the formation of the fossiliferous rocks must have occupied a vastly
greater lapse of time than that traditionally ascribed to the age of
the earth; that fossil remains indicate different climatal conditions
to have obtained in former times, and especially that the polar regions
were once warmer; that many species of animals and plants have become
extinct; and that geological change has had something to do with
geographical distribution.

But these propositions almost constitute the frame-work of
palaeontology. In order to complete it but one addition was needed, and
that was made, in the last years of the eighteenth century, by William
Smith, whose work comes so near our own times that many living men may
have been personally acquainted with him. This modest land-surveyor,
whose business took him into many parts of England, profited by the
peculiarly favourable conditions offered by the arrangement of our
secondary strata to make a careful examination and comparison of their
fossil contents at different points of the large area over which they
extend. The result of his accurate and widely-extended observations
was to establish the important truth that each stratum contains certain
fossils which are peculiar to it; and that the order in which the
strata, characterised by these fossils, are super-imposed one upon the
other is always the same. This most important generalisation was
rapidly verified and extended to all parts of the world accessible to
geologists; and now it rests upon such an immense mass of observations
as to be one of the best established truths of natural science. To the
geologist the discovery was of infinite importance as it enabled him to
identify rocks of the same relative age, however their continuity might
be interrupted or their composition altered. But to the biologist it
had a still deeper meaning, for it demonstrated that, throughout the
prodigious duration of time registered by the fossiliferous rocks, the
living population of the earth had undergone continual changes, not
merely by the extinction of a certain number of the species which had at
first existed, but by the continual generation of new species, and the
no less constant extinction of old ones.

Thus the broad outlines of palaeontology, in so far as it is the common
property of both the geologist and the biologist, were marked out at
the close of the last century. In tracing its subsequent progress I must
confine myself to the province of biology, and, indeed, to the
influence of palaeontology upon zoological morphology. And I accept
this limitation the more willingly as the no less important topic of
the bearing of geology and of palaeontology upon distribution has been
luminously treated in the address of the President of the Geographical
Section. [3]

The succession of the species of animals and plants in time being
established, the first question which the zoologist or the botanist had
to ask himself was, What is the relation of these successive species
one to another? And it is a curious circumstance that the most important
event in the history of palaeontology which immediately succeeded
William Smith's generalisation was a discovery which, could it have been
rightly appreciated at the time, would have gone far towards suggesting
the answer, which was in fact delayed for more than half a century. I
refer to Cuvier's investigation of the mammalian fossils yielded by
the quarries in the older tertiary rocks of Montmartre, among the chief
results of which was the bringing to light of two genera of extinct
hoofed quadrupeds, the _Anoplotherium_ and the _Palaeotherium._ The rich
materials at Cuvier's disposition enabled him to obtain a full
knowledge of the osteology and of the dentition of these two forms, and
consequently to compare their structure critically with that of existing
hoofed animals. The effect of this comparison was to prove that the
_Anoplotherium,_ though it presented many points of resemblance with the
pigs on the one hand and with the ruminants on the other, differed from
both to such an extent that it could find a place in neither group. In
fact, it held, in some respects, an intermediate position, tending to
bridge over the interval between these two groups, which in the existing
fauna are so distinct. In the same way, the _Palaeotherium_ tended to
connect forms so different as the tapir, the rhinoceros, and the horse.
Subsequent investigations have brought to light a variety of facts of
the same order, the most curious and striking of which are those
which prove the existence, in the mesozoic epoch, of a series of forms
intermediate between birds and reptiles--two classes of vertebrate
animals which at present appear to be more widely separated than any
others. Yet the interval between them is completely filled, in the
mesozoic fauna, by birds which have reptilian characters, on the one
side, and reptiles which have ornithic characters, on the other. So
again, while the group of fishes, termed ganoids, is, at the present
time, so distinct from that of the dipnoi, or mudfishes, that they have
been reckoned as distinct orders, the Devonian strata present us with
forms of which it is impossible to say with certainty whether they are
dipnoi or whether they are ganoids.

Agassiz's long and elaborate researches upon fossil fishes, published
between 1833 and 1842, led him to suggest the existence of another kind
of relation between ancient and modern forms of life. He observed that
the oldest fishes present many characters which recall the embryonic
conditions of existing fishes; and that, not only among fishes, but in
several groups of the invertebrata which have a long palaeontological
history, the latest forms are more modified, more specialised, than the
earlier. The fact that the dentition of the older tertiary ungulate
and carnivorous mammals is always complete, noticed by Professor Owen,
illustrated the same generalisation.

Another no less suggestive observation was made by Mr. Darwin, whose
personal investigations during the voyage of the _Beagle_ led him
to remark upon the singular fact, that the fauna, which immediately
precedes that at present existing in any geographical province of
distribution, presents the same peculiarities as its successor. Thus, in
South America and in Australia, the later tertiary or quaternary fossils
show that the fauna which immediately preceded that of the present day
was, in the one case, as much characterised by edentates and, in the
other, by marsupials as it is now, although the species of the older are
largely different from those of the newer fauna.

However clearly these indications might point in one direction, the
question of the exact relation of the successive forms of animal and
vegetable life could be satisfactorily settled only in one way; namely,
by comparing, stage by stage, the series of forms presented by one and
the same type throughout a long space of time. Within the last few years
this has been done fully in the case of the horse, less completely
in the case of the other principal types of the ungulata and of the
carnivora; and all these investigations tend to one general result,
namely, that, in any given series, the successive members of that series
present a gradually increasing specialisation of structure. That is to
say, if any such mammal at present existing has specially modified and
reduced limbs or dentition and complicated brain, its predecessors in
time show less and less modification and reduction in limbs and teeth
and a less highly developed brain. The labours of Gaudry, Marsh, and
Cope furnish abundant illustrations of this law from the marvellous
fossil wealth of Pikermi and the vast uninterrupted series of tertiary
rocks in the territories of North America.

I will now sum up the results of this sketch of the rise and progress
of palaeontology. The whole fabric of palaeontology is based upon two
propositions: the first is, that fossils are the remains of animals and
plants; and the second is, that the stratified rocks in which they
are found are sedimentary deposits; and each of these propositions is
founded upon the same axiom, that like effects imply like causes. If
there is any cause competent to produce a fossil stem, or shell, or
bone, except a living being, then palaeontology has no foundation; if
the stratification of the rocks is not the effect of such causes as
at present produce stratification, we have no means of judging of the
duration of past time, or of the order in which the forms of life have
succeeded one another. But if these two propositions are granted,
there is no escape, as it appears to me, from three very important
conclusions. The first is that living matter has existed upon the earth
for a vast length of time, certainly for millions of years. The second
is that, during this lapse of time, the forms of living matter have
undergone repeated changes, the effect of which has been that the animal
and vegetable population, at any period of the earth's history, contains
certain species which did not exist at some antecedent period, and
others which ceased to exist at some subsequent period. The third is
that, in the case of many groups of mammals and some of reptiles,
in which one type can be followed through a considerable extent of
geological time, the series of different forms by which the type is
represented, at successive intervals of this time, is exactly such as it
would be, if they had been produced by the gradual modification of the
earliest forms of the series. These are facts of the history of the
earth guaranteed by as good evidence as any facts in civil history.

Hitherto I have kept carefully clear of all the hypotheses to which men
have at various times endeavoured to fit the facts of palaeontology, or
by which they have endeavoured to connect as many of these facts as they
happened to be acquainted with. I do not think it would be a profitable
employment of our time to discuss conceptions which doubtless have had
their justification and even their use, but which are now obviously
incompatible with the well-ascertained truths of palaeontology. At
present these truths leave room for only two hypotheses. The first is
that, in the course of the history of the earth, innumerable species
of animals and plants have come into existence, independently of
one another, innumerable times. This, of course, implies either that
spontaneous generation on the most astounding scale, and of animals
such as horses and elephants, has been going on, as a natural process,
through all the time recorded by the fossiliferous rocks; or it
necessitates the belief in innumerable acts of creation repeated
innumerable times. The other hypothesis is, that the successive species
of animals and plants have arisen, the later by the gradual modification
of the earlier. This is the hypothesis of evolution; and the
palaeontological discoveries of the last decade are so completely in
accordance with the requirements of this hypothesis that, if it had not
existed, the palaeontologist would have had to invent it.ould have had to invent it.

I have always had a certain horror of presuming to set a limit upon the
possibilities of things. Therefore I will not venture to say that it is
impossible that the multitudinous species of animals and plants may have
been produced, one separately from the other, by spontaneous generation;
nor that it is impossible that they should have been independently
originated by an endless succession of miraculous creative acts. But
I must confess that both these hypotheses strike me as so astoundingly
improbable, so devoid of a shred of either scientific or traditional
support, that even if there were no other evidence than that of
palaeontology in its favour, I should feel compelled to adopt the
hypothesis of evolution. Happily, the future of palaeontology is
independent of all hypothetical considerations. Fifty years hence,
whoever undertakes to record the progress of palaeontology will note the
present time as the epoch in which the law of succession of the forms of
the higher animals was determined by the observation of palaeontological
facts. He will point out that, just as Steno and as Cuvier were enabled
from their knowledge of the empirical laws of co-existence of the parts
of animals to conclude from a part to the whole, so the knowledge of the
law of succession of forms empowered their successors to conclude, from
one or two terms of such a succession, to the whole series; and thus
to divine the existence of forms of life, of which, perhaps, no trace
remains, at epochs of inconceivable remoteness in the past.




FOOTNOTES:

[Footnote 1: _De Solidoiintra Solidum,_ p.5--"Dato corpore certa figura
praedito et juxta leges naturae producto, in ipso corpore argumenta
invenire locum et modum productionis detegentia."]

[Footnote 2: "Corpora sibi invicem omnino similia simili etiam modo
producta sunt."]

[Footnote 3: Sir J. D. Hooker.]