Chapter V: Creatures of the Past

Beginning Palæontological Work--Fossil Amphibia and
Reptilia--Ancestry of Birds--Ancestry of the Horse--Imperfect
European Series Completed by Marsh's American Fossils--Meaning of
Geological Contemporaneity--Uniformitarianism and Catastrophism
Compared with Evolution in Geology--Age of the
Earth--Intermediate and Linear Types.

Although Huxley took a post connected with Geology only because it was the most convenient opening for him, it was not long before he became deeply interested not only in the fossils, which at first he despised, but in the general problems of geology. He began by co-operation with Mr. Salter in the determination of fossils for the Geological Survey. The mere work of defining genera and species and naming and describing new species appealed very little to him. He had none of the collector's passion for new species; his interest in a creature being not whether or no it was new to science, but what general problems of biology its structure helped to elucidate. While he assisted in the routine work of determining the zoölogical position of the fossils sent in to the museum by the Survey, he carried investigations much farther than the duties of the post required when interesting zoölogical problems arose. His earliest notes were written in association with his colleague, and consisted of technical descriptions of some small fossils from the Downton Sandstones which were supposed to be fish-shields. The peculiarities of structure presented by these aroused his interest, and he began an elaborate series of investigations upon palæozoic fishes in general. Earlier zoölogists, such as the great Agassiz, had devoted most of their attention to careful and exact description of the different fossil fishes with which they became acquainted. Huxley at once began to investigate the relations that existed among the different kinds of structure exhibited in the different fish. He laid down the lines upon which future work has been conducted, and, precisely as he did in the case of molluscs, he started future investigators upon lines of research the ends of which have not yet been reached. His work upon _Devonian Fishes_, published in 1861, threw an entirely new light upon the affinities of these creatures, and still remains a standard work.

He made a similar, although less important, series of investigations upon some of the great extinct Crustacea; but, perhaps, his most important palæontological work was done later, after he had been convinced by Darwin of the fact of evolution. In 1855 he had expressed the opinion that the study of fossils was hopeless if one sought in it confirmation of the doctrine of evolution; but five-and-twenty years' continuous work completely reversed his opinion, and in 1881, addressing the British Association at York he declared that "if zoölogists and embryologists had not put forward the theory, it would have been necessary for palæontologists to invent it." In three special groups of animals his study of fossils enabled him to assist in bridging over the gaps between surviving groups of creatures by study of creatures long extinct. He began to study the structure of the Labyrinthodonts, a group of extinct monsters which received their name from the peculiar structure of their teeth. He published elaborate descriptions of Anthracosaurus from the coal-measures of Northumberland, of Loxomma from the lower carboniferous of Scotland, and of several small forms from the coal-measures of Kilkenny, in Ireland, as well as describing skulls from Africa and a number of fragmentary bones from different localities. But in all this work it was the morphology of the creatures that interested him, and the light which their structure threw upon the structure of each other and of their nearest allies. He shewed that these monsters stood on the borderland between fishes, amphibia, and reptiles, and he added much to our knowledge of the true structure of these great groups. Next, he turned to the extinct reptiles of the Mesozoic age. It was generally believed that the Pterodactyls, or flying reptiles, were the nearest allies of birds, but Huxley insisted that the resemblances between the wings were simply such superficial resemblances as necessarily exist in organs adapted to the same purpose. About the same time, Cope in America, and Phillips and Huxley, in England, from study of the bones of the Dinosaurs, another great group of extinct reptiles, declared that these were the nearest in structure to birds. In association with the upright posture, the ilium or great haunch-bone of birds extends far forwards in front of the articulation of the thigh-bone, so that the pelvis in this region has a T-shape, the ilium forming the cross-bar of the T, and the femur or thigh-bone the downward limb. Huxley shewed that a large number of the Dinosaurs had this and other peculiarities of the bird's pelvis, and separated these into a group which he called the "Ornithoscelida," seeing in them the closest representatives of the probable reptilian ancestors of birds. While further work and the discovery of a still greater number of extinct reptiles has made it less probable that these were the actual ancestors of birds, Huxley's work in this, as in the many other cases we have shown, proved not only of great value in itself, but led to a continually increasing series of investigations by others. It is not always the pioneer that makes the greatest discoveries in a new country, but the work of the pioneer makes possible and easier the more assured discoveries of his followers.

A third great piece of palæontological investigation with which the name of Huxley will always be associated, is the most familiar of all the instances taken from fossils in support of the evolution of animals. This famous case is the pedigree of the horse. In 1870, in an address delivered to the Geological Society of London, Huxley had shewn that there was a series of animals leading backwards from the modern horse to a more generalised creature called Anchitherium, and found in the rocks of the Miocene period. He suggested that there were, no doubt, similar fossils leading still further backwards towards the common mammalian type of animal, with five fingers and five toes, and went the length of suggesting one or two fossils which might stand in the direct line of ancestry. But in 1876 he visited America, and had the opportunity of consulting the marvellous series of fossils which Professor Marsh had collected from American Tertiary beds. Professor Marsh allowed him the freest use of his materials and of his conclusions, and the credit of the final result is to be shared at least equally between Marsh and Huxley. The final result was a demonstrative proof of the possible course of evolution of the horse, given in a lecture delivered by Huxley in New York on Sept. 22, 1876, and illustrated by drawings from specimens in Marsh's collection. The matter of the lecture has become so important a part of all descriptive writing on evolution, and the treatment is so characteristic of Huxley's brilliant exposition, that it is worth while to make some rather long quotations from it. The lecture was published in the New York papers, and afterwards with other matter formed a volume of _American Addresses_, published by Macmillan, in London.

"In most quadrupeds, as in ourselves, the forearm contains
distinct bones called the radius and the ulna. The corresponding
region in the horse seems at first to possess but one bone.
Careful observation, however, enables us to distinguish in this
bone a part which clearly answers to the upper end of the ulna.
This is closely united with the chief mass of the bone which
represents the radius, and runs out into a slender shaft which
may be traced for some distance downwards on the back of the
radius, and then in most cases thins out and vanishes. It takes
still more trouble to make sure of what is nevertheless the fact,
that a small part of the lower end of the bone of the horse's
forearm, which is only distinct in a very young foal, is really
the lower extremity of the ulna.

"What is commonly called the knee of a horse is its wrist. The
'cannon bone' answers to the middle bone of the five metacarpal
bones which support the palm of the hand in ourselves. The
'pastern,' 'coronary,' and 'coffin' bones of veterinarians answer
to the joints of our middle fingers, while the hoof is simply a
greatly enlarged and thickened nail. But, if what lies below the
horse's 'knee' thus corresponds to the middle finger in
ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two
slender splint-like bones, about two-thirds as long as the cannon
bone, which gradually taper to their lower ends and bear no
finger joints, or, as they are termed, phalanges. Sometimes small
bony or gristly nodules are to be found at the bases of these two
metacarpal splints, and it is probable that these represent
rudiments of the first and fifth digits. Thus the part of the
horse's skeleton which corresponds with that of the human hand
contains one overgrown middle digit, and at least two imperfect
lateral digits; and these answer, respectively, to the third, the
second, and the fourth digits in man.

"Corresponding modifications are found in the hind limb. In
ourselves, and in most quadrupeds, the leg contains two distinct
bones, a large bone, the tibia, and a smaller and more slender
bone, the fibula. But, in the horse, the fibula seems, at first,
to be reduced to its upper end; a short slender bone united with
the tibia and ending in a point below occupying its place.
Examination of the lower end of a young foal's shin-bone,
however, shews a distinct portion of osseous matter, which is the
lower end of the fibula; so that the apparently single lower end
of the shin-bone is really made up of the coalesced ends of the
tibia and fibula, just as the apparently single lower end of the
fore-arm bone is composed of the coalesced radius and ulna.

"The heel of the horse is the part commonly known as the hock;
the hinder cannon bone answers to the middle metatarsal bone of
the human foot, the pastern, coronary, and coffin bones, to the
middle-toe bones; the hind hoof to the nail, as in the fore foot.
And, as in the fore foot, there are merely two splints to
represent the second and fourth toes. Sometimes a rudiment of a
fifth toe appears to be traceable."

Having in the same fashion described the highly complicated and peculiar structure of the teeth of modern horses, Huxley proceeded:

"To anyone who is acquainted with the morphology of vertebrated
animals, these characteristic structures of the horse show that
it deviates widely from the general structure of mammals; and
that the horse type is, in many respects, an extreme modification
of the general mammalian plan. The least modified mammals, in
fact, have the radius and ulna, the tibia and fibula, distinct
and separate. They have five distinct and complete digits on each
foot, and no one of these digits is very much larger than the
rest. Moreover, in the least modified mammals, the total number
of the teeth is very generally forty-four, while in the horse the
usual number is forty, and, in the absence of the canines, it may
be reduced to thirty-six; the incisor teeth are devoid of the
fold seen in those of the horse; the grinders regularly diminish
in size from the middle of the series to its front end; while
their crowns are short, early attain their full length, and
exhibit simple ridges or tubercles, in place of the complex
foldings of the horse's grinders.

"Hence the general principles of the hypothesis of evolution
lead to the conclusion that the horse must have been derived from
some quadruped which possessed five complete digits on each foot;
which had the bones of the forearm and of the leg complete and
separate; and which possessed forty-four teeth, among which the
crown of the incisors and grinders had a simple structure; while
the latter gradually increased in size from before backwards, at
any rate in the anterior part of the series, and had short
crowns.

"And if the horse had been thus evolved, and the remains of the
different stages of its evolution have been preserved, they ought
to present us with a series of forms in which the number of the
digits becomes reduced; the bones of the forearm and leg
gradually take on the equine condition; and the form and
arrangement of the teeth successively approximate to those which
obtain in existing horses.

"Let us turn to the facts and see how far they fulfill these
requirements of the doctrine of evolution.

"In Europe abundant remains of horses are found in the
Quaternary and later Tertiary strata as far as the Pliocene
formation. But these horses, which are so common in the
cave-deposits and in the gravel of Europe, are in all essential
respects like existing horses, and that is true of all the horses
of the later part of the Pliocene epoch. But, in the deposits
which belong to the earlier Pliocene, and later Miocene epochs,
and which occur in Britain, in France, in Germany, in Greece, in
India, we find animals which are extremely like horses--which in
fact are so similar to horses, that you may follow descriptions
given in works upon the anatomy of the horse, upon the skeletons
of these animals--but which differ in some important particulars.
For example, the structure of their fore and hind limbs is
somewhat different. The bones, which, in the horse are
represented by two long splints, imperfect below, are as long as
the middle metacarpal and metatarsal bones; and, attached to the
extremity of each, is a digit with three joints of the same
general character as those of the middle digit, only very much
smaller. These small digits are so disposed that they could have
had but very little functional importance, and they must have
been rather of the nature of the dew-claws, such as are to be
found in many ruminant animals. The _Hipparion_, as the extinct
European three-toed horse is called, in fact presents a foot
similar to that of the American _Protohippus_ except that in
_Hipparion_ the smaller digits are situated further back, and are
of smaller proportional size than in the _Protohippus_.

"The ulna is slightly more distinct than in the horse; and the
whole length of it, as a very slender shaft, intimately united
with the radius, is completely traceable. The fibula appears to
be in the same condition as in the horse. The teeth of the
_Hipparion_ are essentially similar to those of the horse, but
the pattern of the grinders is in some respects a little more
complex, and there is a depression on the face of the skull in
front of the orbit, which is not seen in existing horses.

"In the earlier Miocene and perhaps in the Eocene deposits of
some parts of Europe, another distinct animal has been
discovered, which Cuvier, who first described some fragments of
it, considered to be a _Palæotherium_, but as further discoveries
threw new light on its structure, it was recognised as a distinct
genus, under the name of _Anchitherium_.

"In its general characters the skeleton of _Anchitherium_ is
very similar to that of the horse, in fact Lartet and De
Blainville called it _Palæotherium equinum_ or _Hippoides_; and
De Cristol, in 1847, said that it differed from _Hipparion_ in
little more than the characters of the teeth, and gave it the
name of _Hipparitherium_. Each foot possesses three complete
toes: while the lateral toes are much larger in proportion to the
middle toe than in _Hipparion_, and doubtless rested on the
ground in ordinary locomotion. The ulna is complete and quite
distinct from the radius, although firmly united with the latter.
The fibula seems also to have been complete; its lower end,
though intimately united with that of the tibia, is clearly
united with that of the latter bone. There are forty-four teeth;
the incisors have no strong pit. The canines seem to have been
well developed in both sexes. The first of the seven grinders,
which, as I have said, is frequently absent, and, when it does
exist, is small in the horse, is a good-sized and permanent
tooth, while the grinder which follows it is but little larger
than the hinder ones. The crowns of the grinders are short, and,
although the fundamental pattern of the horse-tooth is
discernible, the front and back ridges are less curved, the
accessory pillars are wanting, and the valleys, much shallower,
are not filled up with cement."

Then, after describing his early efforts to trace the descent of the horse from European fossils, Huxley goes on to relate the new light thrown on the matter from the American discoveries of Professor Marsh:

"You are all aware that, when your country was first discovered
by Europeans, there were no traces of the existence of the horse
in any part of the American continent. The accounts of the
conquest of Mexico dwell on the astonishment of the natives of
that country when they first became acquainted with that
astounding phenomenon, a man seated upon a horse. Nevertheless,
the investigations of American geologists have proved that the
remains of horses occur in the most superficial deposits of both
North and South America, just as they do in Europe. Therefore,
for some reason or other,--no feasible suggestion on that
subject, so far as I know, has been made,--the horse must have
died out on this continent at some period preceding the discovery
of America. Of late years there has been discovered in your
Western territories that marvellous accumulation of deposits,
admirably adapted for the preservation of organic remains, to
which I referred the other evening, and which furnishes us with a
consecutive series of records of the fauna of the older half of
the Tertiary epoch, for which we have no parallel in Europe. The
researches of Leidy and others have shewn that forms allied to
the _Hipparion_ and the _Anchitherium_ are to be found among
these remains. Rut it is only recently that the admirably
conceived and most thoroughly and patiently worked-out
investigations of Professor Marsh have given us a just idea of
the vast fossil wealth and of the scientific importance of these
deposits. I have had the advantage of glancing over the
collections in Yale Museum; and I can truly say that, so far as
my knowledge extends, there is no collection from any one region
and series of strata comparable, for extent, or for care with
which the remains have been got together, or for their scientific
importance, to the series of fossils which he has deposited
there. This vast collection has yielded evidence bearing on the
question of the pedigree of the horse of the most striking
character. It tends to show that we must look to America rather
than to Europe for the original seat of the equine series; and
that the archaic forms and successive modifications of the
horse's ancestry are far better preserved here than in Europe.

"Professor Marsh's kindness has enabled me to put before you a
diagram, every figure of which is an actual representation of
some specimen which is to be seen at Yale at this present time.

"The succession of forms which he has brought together carries
us from the top to the bottom of the Tertiaries. Firstly, there
is the true horse. Next we have the American Pliocene form of the
horse (_Pliohippus_): in the conformation of its limbs it
presents some very slight deviations from the ordinary horse, and
the crowns of the grinding teeth are shorter. Then comes the
_Protohippus_, which represents the European _Hipparion_, having
one large digit and two small ones on each foot, and the general
characters of the forearm and leg to which I have referred. But
it is more valuable than the European _Hipparion_ for the reason
that it is devoid of some of the peculiarities of that
form--peculiarities which tend to show that the European
_Hipparion_ is rather a member of a collateral branch than a form
in the direct line of succession. Next, in the backward order in
time, is the _Miohippus_, which corresponds pretty nearly with
the _Anchitherium_ of Europe. It presents three complete
toes--one large median and two smaller lateral ones: and there
is a rudiment of that digit which answers to the little finger of
the human race.

"The European pedigree of the horse stops here; in the America
Tertiaries, on the contrary, the series of ancestral equine forms
is continued into the Eocene formations. An older Miocene form,
called _Mesohippus_, has three toes in front, with a large
splint-like rudiment representing the little finger; and three
toes behind. The radius and ulna, the tibia and fibula, are
distinct, and the short crowned molar teeth are _Anchitherioid_
in pattern.

"But the most important discovery of all is the _Orohippus_
which comes from the Eocene formation, and is the oldest member
of the equine series yet known. Here we find four complete toes
on the front limb, three toes on the hind limb, a well-developed
ulna, a well-developed fibula, and short-crowned grinders of a
simple pattern.

"Thus, thanks to these important researches, it has become
evident that, so far as our present knowledge extends, the
history of the horse type is exactly and precisely that which
could have been predicted from a knowledge of the principles of
evolution; and the knowledge we now possess justifies us
completely in the anticipation that, when the still lower Eocene
deposits, and those which belong to the Cretaceous period have
yielded up their remains of ancestral equine animals, we shall
find, first, a form with four complete toes and a rudiment of the
innermost or first digit in front, with probably a rudiment of
the fifth digit in the hind foot; while, in the older forms, the
series of digits will be more and more complete until we come to
the five-toed animals, in which, if the doctrine of evolution is
well founded, the whole series must have taken its origin."

Just as Huxley was successful, when only the ancestry to Miocene times was known, in predicting the discovery of older forms in the older Miocene and upper Eocene, so his prediction of older Eocene forms carrying the chain back to five-toed creatures proved correct. One of the new links was indeed discovered before his lecture had passed through the press, and he was able to add in a footnote some details of the structure of the four-toed Eohippus from the lower Eocene beds. Further discoveries have connected these with the five-toed ancestors of the Tapirs, and there is the strongest reason to suppose that we now know as nearly as possible the line of ancestry of the horse back to the primitive forms common to all the higher mammals. It would, of course, be beyond possibility of proof that the exact fossils described were the actual ancestors of the horse; but that they are exceedingly close allies of these, and that among them some actual ancestors exist cannot reasonably be doubted.

Although he had embarked upon geological work with some distaste, Huxley became very closely associated with it as years went on, and indeed, about the seventies, had abandoned his intention to devote himself specially to physiology, and declared himself to be in the first place a palæontologist. In 1876 he had accomplished so much that the Geological Society gave him its chief distinction, awarding him the Wollaston Medal in recognition of his services to geological science. He acted as Secretary to the Geological Society from 1859 to 1862, and he was President from 1868 to 1870. In 1862, the President being incapacitated, Huxley delivered as Deputy-President the Presidential Address. This address is famous in the history of geology, because for the first time it stated clearly and in permanent form a doctrine now taken as a first principle in all geological text-books. A large part of geology is the attempt to read the past history of the earth from the evidence given by the successive strata of rocks that form its crust.

"It is mathematically certain that, in any given vertical linear
section of an undisturbed series of sedimentary deposits, the bed
which lies lowest is the oldest. In many other vertical linear
sections of the same series, of course corresponding beds will
occur in a similar order."

It is of the utmost importance to determine whether or no the same series occurring vertically in the same order in different parts of the earth were deposited at the same time. To explain the problem, Huxley took the following concrete example:

"The Lias of England and the Lias of Germany, the Cretaceous
rocks of Britain and the Cretaceous rocks of Southern India, are
termed by geologists 'Contemporaneous' formations; but whenever
any thoughtful geologist is asked whether he means to say that
they were deposited at the same time, he says, 'No, only within
the same great epoch.' And if, in pursuing the enquiry, he is
asked what may be the approximate value in time of a 'great
epoch'--whether it means a hundred years, or a thousand, or a
million, or ten million years--his reply is, 'I cannot tell.'"

Most of the standard writers on palæontology had assumed that the presence in two beds at different parts of the world of the same fossils implied that the beds were contemporaneous, that they had been formed at the same time. Huxley pointed out that the fact of identical fossils being present was, on the whole, evidence against the beds having been formed at the same time. Even some of the older writers who believed in species having been created at definite places at definite times had seen that time must have been required for sets of animals to wander from the places in which they had come into existence. The newer theory of evolution was equally opposed to the notion of the appearance of similar animals at the same time on far-distant parts of the earth. For such reasons he proposed to reject the use of the word _Contemporaneous_ as applied to rockbeds in different localities which contained the same fossils, and to replace it by the word _Homotaxial_, which meant no more than that the beds occupied corresponding places in the geological history of the earth. Huxley did not pretend that these arguments were entirely original: they represented the drift of the best geological opinion, and he seized hold of them and set them down as permanent geological truths.

In 1869, in a Presidential Address to the Geological Society, Huxley took up one of the burning questions of the day. In the early part of the century, the discoveries of geologists had been the occasion of great distress to those good people who clung to a literal interpretation of everything in the Bible. Long before the doctrine of evolution and the descent of man from lower animals had taken practical shape, there had been a battle royal between geologists who declared that the earth was many million years old, and had been inhabited at least by animals and plants for enormous periods, and those who clung to the traditional chronology which placed the date of creation only a few thousand years from now. The continued progress of geology, and the sturdy championship of it by men like Sedgwick, Chalmers, and Buckland, who were at the same time reputable theologians and distinguished men of science, had decided the battle in favour of the conclusions of science, and it was accepted generally that the earth was almost indefinitely old. At the same time, another and more strictly scientific dispute had been in progress. The older school of geologists, looking on the face of the world, and seeing it scarred by mighty fissures, displaying huge distortions of the beds in the crust, had argued that geological change had taken place by a series of mighty catastrophes. The tremendous results which they saw seemed to them only possible on the theory that unusual and gigantic displays of force had caused them. On the other hand, Hutton and Lyell attempted to find adequate explanation of the greatest changes in the slow forces which may be seen in operation at the present time. Slow movements of upheaval and depression, amounting at most to an inch or two in a century, may be shown to be actually in existence now, and such slow changes acting for very many centuries would account for the raising of continents above the sea, so that old sea-bottoms became the surface of the land, and for the depression of land areas so that new sedimentary rocks might be deposited upon them. They shewed how air and water slowly crumbled away the hardest rocks, and how rivers deepened their beds steadily but excessively slowly; and they held that while great catastrophic changes might occasionally have occurred, there was ample evidence of the present operation of forces which, granted sufficient time for their operation, would have made the crust of the earth such as it is. This doctrine of _Uniformitarianism_, of the action of similar forces in the past and present history of the earth, had almost completely triumphed over the older catastrophic views. As Huxley put it, the school of catastrophe put no limit to the violence of forces which had operated; the uniformitarians put no limit to the length of time during which forces had operated.

"Catastrophism has insisted upon the existence of a practically
unlimited bank of force, on which the theorist might draw; and it
has cherished the idea of development of the earth from a state
in which its form, and the forces which it exerted, were very
different from those which we now know.

"Uniformitarianism, on the other hand, has with equal justice
insisted upon a practically unlimited bank of time, ready to
discount any quantity of hypothetical paper. It has kept before
our eyes the power of the infinitely little, time being granted,
and has compelled us to exhaust known causes before flying to the
unknown."

But there was a third influence at work in geology, an influence which may best be described in Huxley's own words:

"I shall not make what I have to say on this head clear unless I
diverge, or seem to diverge, for a while, from the direct path of
my discourse so far as to explain what I take to be the scope of
geology itself. I conceive geology to be the history of the
earth, in precisely the same sense as biology is the history of
living beings; and I trust you will not think that I am
overpowered by the influence of a dominant pursuit if I say that
I trace a close analogy between these two histories.

"If I study a living being, under what heads does the knowledge
I obtain fall? I can learn its structure, or what we call its
Anatomy; and its development, or the series of changes it passes
through to acquire its complete structure. Then I find that the
living being has certain powers resulting from its own
activities, and the interaction of these with the activities of
other things--the knowledge of which is Physiology. Beyond this,
the living being has a position in space and time, which is its
Distribution. All these form the body of ascertainable facts
which constitute the _status quo_ of the living creature. But
these facts have their causes; and the ascertainment of these
causes is the doctrine of Ætiology.

"If we consider what is knowable about the earth, we shall find
that such earth-knowledge--if I may so translate the word
geology--falls into the same categories.

"What is termed stratigraphical geology is neither more nor less
than the anatomy of the earth; and the history of the succession
of the formations is a history of the succession of such
anatomies, or corresponds with development, as distinct from
generation.

"The internal heat of the earth, the elevation and depression of
its crust, its belching forth of vapours, ashes, and lava, are
its activities, in as strict a sense as are warmth and the
movements and products of respiration the activities of an
animal. The phenomena of the seasons, of the trade-winds, of the
Gulf Stream, are as much the results of the reaction between
these inner activities and outward forces, as are the budding of
the leaves in spring, and their falling in autumn the effects of
the interaction between the organisation of a plant and the solar
light and heat. And, as the study of the activities of the living
being is called its physiology, so are these phenomena the
subject matter of an analogous telluric physiology, to which we
sometimes give the name of meteorology; sometimes of physical
geography, sometimes that of geology. Again, the earth has a
place in space and time, and relations to other bodies in both
these respects, which constitute its distribution. This subject
is usually left to the astronomer; but a knowledge of its broad
outlines seems to me to be an essential constituent of the stock
of geological ideas.

"All that can be ascertained concerning the structure,
succession of conditions, actions, and position in space of the
earth, is the matter of its natural history. But, as in Biology,
there remains the matter of reasoning from these facts to their
causes, which is just as much science as the other, and indeed
more; and this constitutes geological ætiology.

"Having regard to this general scheme of geological knowledge
and thought, it is obvious that geological speculation may be, so
to speak, anatomical and developmental speculation, so far as it
relates to points of stratigraphical arrangement which are out of
reach of direct observation; or, it may be physiological
speculation so far as it relates to undetermined problems
relative to the activities of the earth; or, it may be
distributional speculation, if it deals with modifications of the
earth's place in space; or, finally, it will be ætiological
speculation if it attempts to deduce the history of the world, as
a whole, from the known properties of the matter of the earth, in
the conditions in which the earth has been placed."

Huxley then proceeded to shew that uniformitarianism and catastrophism had neglected this last and most important branch of geology, the attempt to trace the interaction of causes which had brought the world into its present condition. He gave a striking display of the wide knowledge of his reading by going back to the foundation of this branch of modern science, and giving a masterly account of the then little-known treatise of Immanuel Kant, who in 1775 had written _An Attempt to Account for the Constitutional and Mechanical Origin of the Universe upon Newtonian Principles_. Next he declared that evolution embraced all that was sound in both catastrophism and uniformitarianism while rejecting the arbitrary limits and assumptions of both.

Finally he came to the great question to which these observations upon the existing schools of geology had led. The most distinguished physicist of the age, then Sir William Thomson, now Lord Kelvin, and Huxley's immediate successor in the Presidential Chair of the Royal Society, had stated that the English school of geology had assumed an impossible age for the earth. By physical reasonings, Thomson stated that he was able to prove "That the existing state of things on the earth--all geological history showing continuity of life--must be limited within some such period of time as one hundred million years." This pronouncement had been received with acclamation by those who feared the geological and biological sciences, as a sign of internal dissensions within the house of science. Huxley, then, as all through the latter part of his life, at once constituted himself the champion of science, and, taking Thomson's arguments one by one, shewed by a series of masterly deductions from known facts that there was a great deal to be said for the other side, and that physicists were as little certain as geologists could be of the exact duration of time that had elapsed since the dawn of life. His plea for more time since the cooling of the globe than physicists were willing to allow remains one of the classics of geological literature. But he carried the question much farther. The inference which was widely drawn by the enemies of evolution from the arguments of Sir William Thomson was that if geologists had overestimated the age of the cooled earth there was not time for the evolution of animals and plants to have taken place. Huxley pointed out a fact which should be quite obvious, but which even yet is frequently neglected. The evidence for the gradual appearance of life in the past history of the earth depends simply on the fact that the successive forms of life appear in successive strata, and the length of time taken for these changes simply depends upon the length of time which was taken up by the formation of the strata. Our only reason for supposing the evolution of life, made plain by fossil records, to have taken place very slowly is that geologists have stated that the deposition of the strata took place very slowly. Whether these strata were deposited slowly or less slowly, we know that the forms of life changed at the same rate.

"Biology takes her time from geology. The only reason we have for
believing in the slow rate of change in living forms is the fact
that they persist through a series of deposits which, geology
informs us, have taken a long while to make. If the geological
clock is wrong, all the naturalist will have to do is to modify
his notion of the rapidity of change accordingly; and I venture
to point out that, when we are told that the limitation of the
period during which living beings have inhabited this planet to
one, two, or three hundred million years requires a complete
revolution in geological speculation, the _onus probandi_ rests
on the maker of the assertion, who brings forward not a shadow of
evidence in its support."

Perhaps, although this is now an old controversy, it is worth while to recall that the keenness of Huxley's language was not directed against Sir William Thomson, between whom and Huxley there was no more than the desire to argue out an interesting scientific question upon which their conclusions differed, but between Huxley and those outsiders who were always ready to turn any dubious question in science into an argument discrediting the general conclusions of science.

The last time that Huxley occupied the Presidential Chair of the Geological Society was in 1870, and he occupied his Presidential address by a review of the "old judgments" which he had given in the course of his first address in 1862. The address was entitled "Palæontology and Evolution," and the most important part of it was a complete withdrawal of the fears he had expressed that geology would not supply definite evidence of the transformation of species. Important discoveries had come thick and fast; and, at least in the case of the higher vertebrates, he declared that, however one might "sift and criticise them," they left a clear balance in favour of the doctrine of the evolution of living forms one from another. But, with his usual critical spirit, examining arguments that bore against a conclusion for which he hoped almost more stringently than arguments apparently favourable to what he expected to be true, Huxley made an important distinction, the value of which becomes more and more apparent as time goes on. In the first flush of enthusiasm for Darwinism, zoölogists and palæontologists allowed their zeal to outrun discretion in the formation of family trees. They examined large series of living or extinct creatures, and so soon as they found gradations of structure present, they arranged their specimens in a linear series, from the simplest to the most complex, and declared that the arrangement was a representation of the family tree. The fact that the line of descent apparently could have followed along the direction they suggested they were inclined to take as evidence that it had so followed. Huxley made the most careful distinction between what he called intermediate types and types with a right to be placed in linear order,

Every fossil which takes an intermediate place between forms of
life already known may be said, so far as it is intermediate, to
be evidence in favour of evolution, inasmuch as it shews a
possible road by which evolution may have taken place. But the
mere discovery of such a form does not, in itself, prove that
evolution took place by and through it, nor does it constitute
more than a presumptive evidence in favour of evolution in
general. The fact that _Anoplotheridæ_ are intermediate between
pigs and ruminants does not tell us whether the ruminants have
come from the pigs or the pigs from the ruminants, or both from
_Anoplotheridæ_, or whether pigs, ruminants, and _Anoplotheridæ_;
alike may not have diverged from some common stock.

A familiar instance will make the point at issue plain. Everyone knows that in many respects, in the structure of the skeleton, and the curve of the backbone, and in the development of the brain, the man-like monkeys, the gorilla and its allies, are intermediate between man and the lower monkeys. In the early days of evolution it was assumed frequently that the gorilla, etc., were therefore to be regarded as ancestors of man, and they appear as such in more than one well-known treatise on evolutionary biology. We now know that it is exceedingly probable that the gorilla and its allies, although truly intermediate types, and truly shewing a possible path of evolution from the brute to man, are not the actual ancestors of man, but cousins, descendants like man from some more or less remote common ancestor. And the tendency of recent advances in knowledge is more and more to throw stress on the value of Huxley's distinction, and to minimise confusion between "intermediate" and truly ancestral types.