beg to thank you, and those societies which have joined together in
presenting this address, for the kind terms in which it is expressed and
has been delivered to me. It is most gratifying to us--who believe in
the beneficial results upon human life conferred by the diffusion of
scientific knowledge, as all members of this great association must
do--to find that our work, and the objects of our meeting are so highly
appreciated by such a wide circle as that represented by the signatories
to this address. It is, moreover, particularly satisfactory to find such
a recognition--where, perhaps, at first sight, we might hardly have
expected it--of the advantages of pure or abstract science apart from
its practical application to the material welfare of mankind. You have
recognised what is certainly known to those who have followed most
closely the history of science, but what is not so generally known or
appreciated by others, that nearly all the marvellous benefits which
have been conferred on man by the application of scientific knowledge
have been the results of the discoveries of philosophers who are
pursuing knowledge solely for its own sake--without any hope of reward,
without any hope of benefit to themselves or to others, and, very often,
amid the indifference, the neglect, and even the scorn of their
contemporaries. The particular branch of science which it is my lot to
represent is, at first sight, very little specially connected with the
general welfare of man, and is looked upon by many as little more than
idle speculation or mere curiosity. I well remember--though it is
certainly many years ago now--one who, more than any one else living in
this country, has advanced that branch of science--Professor Huxley--in
a lecture delivered at the Royal Institution, saying that the common
idea of a naturalist was “a wet, dirty man poking about the sea-shore
with a net in one hand and a bottle in the other--an innocent and
perhaps harmless individual, but a very useless one.” Well, I may say
that the description was made some thirty years ago, before Professor
Huxley himself had done so much to raise the character of naturalists
and natural history in this country, yet it still holds good in the
opinion of many at the present time. You must recollect, however, that
the researches of naturalists of that class, men who have occupied
themselves in closely observing the ways and habits, and studying the
structure of animals of a low type of organisation, have produced
already marvellous results upon the happiness and welfare of mankind.
Through researches of this kind we are obtaining knowledge of the causes
and prevention of disease, which, when further advanced, for they are
only beginning now, will, I have no doubt, lead to an enormous saving of
health and of life. Moreover, through the researches of such naturalists
greater results still have been produced. They have produced effects
upon our mode of thinking on many subjects--on our relations to each
other and to the universe--effects the end of which we hardly see at
present. And they have taught us one great lesson, one that I alluded to
in the address which I had the honour of giving here last
Wednesday--namely, that progress in living nature has been due in great
measure to the principle which Darwin most popularised, if he did not
first enunciate, which he, at all events, brought into the condition in
which we now know it--that of the survival of the fittest in the
struggle for existence. Now, it is a law in nature that there should be
a certain amount of individual differences or variations in the
different animals and plants inhabiting the earth, and that the progress
from the lower to the higher forms of animals and plants has been due
to the opportunity of those individuals who are a little superior in
some respects to their fellows of asserting that superiority, of
continuing to live, and of propagating that superiority as an
inheritance to their descendants. That law, established in nature, is, I
believe, equally applicable to ourselves; and this is the message which
pure and abstract biological research has sent to help us on in some of
the commonest problems of human life. The lesson is this--that there is
always a certain amount of variability, that there is no such thing as
equality--equality in powers of work, equality in powers of endurance,
or equality in the powers of men for doing great things in the world;
and that progress depends on giving full liberty to that inequality,
wherever it asserts itself, having full play. Now, supposing this law
did not exist in the animal kingdom, instead of the world being filled
with all the diversity and beauty which it now possesses, every living
thing would have been in the condition of slimy polyps at the bottom of
the sea, and if it did not exist in mankind, we should all still be in
the condition of flint-chipping savages. I will leave it to you to apply
to yourselves, in your own social condition and social life, the
application of that law. I will not press it any further at present, but
leave you to think it out afterwards. You will, however, easily see that
it means that, as there is no such thing as equality among yourselves,
if you are to have progress there must be no attempt whatever to keep
down the capacities of the superior to the level of the inferior. Any
man who gets a little rise above his fellows helps on the progress of
the world, and brings all the others on with him. Now, having merely
mentioned an application of abstract science from a quarter where,
perhaps, you little expect it, I will conclude by once more thanking
you, Mr. Girling, and those who have forwarded the address in the name
of the Association. I am sure it will give the Association great
pleasure to hear what you have done, and how you have sympathised with
their work, and to assure you that the hopes and wishes expressed at the
end of the address are, as far as possible, most fully realised.
XI
ON PALÆONTOLOGICAL EVIDENCE OF GRADUAL MODIFICATION OF ANIMAL FORMS[30]
I need scarcely say that one of the greatest, if not absolutely the
greatest problem which has ever exercised the minds of naturalists is
that of the fixity or the mutability of species.
Are the various specific forms under which animal and vegetable life
exist upon earth, now and in all times past, fixed within certain narrow
limits of variation, and did each originally appear upon the earth
without genetic connection with any previously existing forms, having
been created _de novo_ in fact? or have these different species been
produced by gradual modification from pre-existing living forms, under
the influence of certain laws, at present very imperfectly understood,
acting through vast and indefinite periods of time?
It is clear that these two views are strongly opposed to each other.
Both have been held and still are held by men who are justly considered
masters in the branch of knowledge to which they relate; and the
solution of the question will exercise so important an influence on the
progress of zoology that any real contribution towards it should be one
of the most welcome additions to science that a naturalist of the
present day can make.
The question is, indeed, so far-reaching, so all-pervading, that it
meets us everywhere in the study of every group of animal or vegetable
life, and in almost every aspect in which the study can be carried out.
It bears largely upon, and is greatly illustrated by, descriptive
zoology or botany. It adds vastly to the interest of the pursuit of
anatomy, by calling out the meaning of rudimentary structures and
so-called typical resemblances; it elucidates obscure questions relating
to the habits and instincts of animals; it brings into prominence the
signification of various facts of geographical distribution, and the
life it throws into the study of palæontology is too obvious to need
remark.
Evidences bearing upon, either for or against, the theory of _evolution_
or _descent_ can be collected from all these sources. I need only refer
to Mr. Darwin’s works, which must be familiar to you all, in
illustration of the great variety and number of the branches of science
which can be brought to throw light upon it. Indeed, in a subject like
this, where direct observation can count for little, in consequence of
the extreme shortness of the observing time of any individual compared
with the enormous period required for the assumed changes, it is only by
the accumulation of a vast number of facts from various sources, and
observing the direction in which they all point, that anything like
proof can be obtained.
Leaving aside, for the present occasion, all other sources of evidence
in favour of either of these views, I propose this evening to enter only
upon one which is in some respects, as all must admit, the most
important, as it comes nearer than any other to show what actually has
been the history of our existing species in times past; for as the most
natural and conclusive way of ascertaining the method by which a nation
has arrived at its present condition of society, customs, laws, etc.,
would certainly be to examine into the preserved records of its past
history, so it must be with the present condition of animal and
vegetable life.
We all know that such records have been preserved, that the solid rocks
beneath our feet in many places teem with the actual remains of
creatures which lived and died thousands or millions of years ago.
Why should they not yield to us the knowledge we are all so eager to
acquire?
If species are and ever have been immutable, shall we not find the same
hard and fast lines surrounding each as we do now? Shall we not find
long series of similar forms following without change on an abrupt
commencement? If the other alternative be correct, ought we not to find
specimens of all the various stages through which the wonderful variety
we meet with now has been brought about? Every gap which now so widely
separates group from group ought to be filled up, and the various phases
of modification should follow through the successive eras of geological
time.
Now, there can be no hesitation in saying that the evidence of
palæontology, in the present state of the science, does not reveal the
last-described condition of things. Notwithstanding the vast increase of
our knowledge in recent years, very many large groups of animals stand
completely isolated, and the more nearly allied forms are mostly
separated from each other by tolerably definite intervals.
Is, then, the question decisively answered against evolution or
derivation by palæontology?
We must pause before we can join in the assertion that it is. The
subject is far more complex than it may seem at first.
Before going further, a proper estimate must be arrived at of the nature
and value of our evidence, and in doing this we must give full weight to
the considerations derived from the “imperfection of the geological
record” so strikingly elucidated in Mr. Darwin’s chapter on the subject.
To those who have not fully considered this question, it is difficult to
conceive how immense is the interval between our excessively fragmentary
knowledge of extinct animals, and that perfect palæontological record
which would imply evidence, first, of every form of life that has ever
existed, and, secondly, of the period at which it existed.
If there were time, I might dwell long upon this part of the subject,
but I must leave you to imagine (1) What the chances are against the
fossilisation of any animal that dies. (2) What the chances of the
stratum in which some fossil
[Illustration: PROVISIONAL CLASSIFICATION OF UNGULATA.
PERISSODACTYLA ARTIODACTYLA
]
remains have been embedded being itself preserved during the constant
changes going on on the earth’s surface, and ultimately appearing in a
situation accessible to man’s research. (3) What the further chances
against their being so found, even if they should have been preserved in
an accessible locality.
I might refer you to the exceedingly minute portion of the earth’s
surface which has yet been really explored palæontologically; to the
cases that are occurring every day of new and most unexpected forms and
of whole species or orders, known only by an isolated individual, as the
_Archæopteryx_ of the Solenhofen oolite; to say nothing of more
recondite speculations in the work above referred to, on the
improbability of preservation of intermediate forms, owing to variation
having usually been most rife during periods of elevation, when
fossilisation is less likely to occur.
All these show in such a striking manner the extremely small value of
negative evidence in palæontology, that I am quite justified in asking
you to leave it altogether out of consideration in thinking of, or
reasoning on, what is to follow.
Such being the material with which we have to deal, it will be seen that
we must go to work upon it in a most careful and circumspect manner. We
cannot rush at conclusions, but must be content cautiously, and often
with much labour and anxiety, to piece together our facts, scrupulously
observing the minutest hints, and following out the direction indicated
by often very obscure signs, before we can reconstruct even an outline
of the fabric from which we hope to gain an idea of the past history of
the beings of which we treat.
I have selected for illustration of the subject this evening the
division or order of Mammals called by naturalists UNGULATA, or hoofed
animals, chiefly because it is the one of which the palæontological
history--at least in the tertiary period (for beyond that we cannot
trace it)--is better known than any other, and as that of which the
classification,--that is, the relations of its various sub-groups to
each other,--is on the whole better understood than in most other
zoological divisions.
The order includes the most familiar of our domestic animals, and with
the general appearance of the rest we are most of us well acquainted,
thanks to the Zoological Gardens. They are the various forms of horses,
asses, and zebras, the rhinoceroses and the tapirs, the pigs,
hippopotamus, camels, deer, antelopes, sheep, oxen, and goats.
They are essentially herbivorous (though some few may be more or less
omnivorous), and their teeth are modified accordingly. Their limbs are
adapted for carrying the body in ordinary terrestrial progression, and
are of very little use for any other purpose, such as climbing, seizing
prey, or carrying food to the mouth. They never have clavicles or
collar-bones, and their toes never exceed four in number (the digit
which corresponds to the first of the complete pentadactyle foot being
always wanting), and have the ends encased in hoofs instead of nails or
claws. The species at present existing are very numerous, and widely
diffused over the earth’s surface, being wanting only in the Australian
province. These Ungulate animals are divided into two natural groups,
each having very many characters in common, the establishment of which,
though contrary to the views of the great naturalists of the beginning
of this century, has been a great gain to zoological science, especially
as this division pervades all the known extinct as well as recent forms;
and although some forms of either group may present some partial
approximation to the other, no directly intermediate species are known.
It is important, therefore, to apprehend thoroughly the distinction
between these groups, which have received from Professor Owen the names
of _Perissodactyle_, or odd-toed, and _Artiodactyle_, or even-toed, from
one of their most striking external characteristics. The first have the
toes of both feet arranged symmetrically to a line drawn through the
middle of what would be the third toe of the typical pentadactyle foot,
which toe is always the largest, and in some cases the only one fully
developed. In the second, the toes are arranged symmetrically to a line
drawn between the third and fourth toes, so that these two toes are
equally developed, and may be alone present, or may be supplemented by
an outer pair (the second and fifth), often in a more or less
rudimentary condition. Besides these distinctions in the limbs, there
are so many others correlated with them in the number of the vertebræ,
the structure of the cranial bones, of the teeth, of the digestive
organs, etc., that there can be no question about their forming natural
divisions, very important to palæontologists, as it often happens that
the position of an extinct and little-known form can be determined from
a very small fragment of bone.
Each of these groups is further divided into genera, the names of which
in what appears, in the present state of knowledge, to be their natural
position and relation to each other are indicated on the diagram. From
this it will be seen that the existing Perissodactyles (excluding Hyrax,
the position of which is doubtful, though I am inclined to consider it
as an aberrant member of this group) consist of three groups, the
tapirs, the rhinoceroses, and the horses, each represented by but few
species, and (except in the case of the horse, through the agency of
man) of rather restricted geographical distribution.
These groups at present are separated by very decided intervals, so much
so, that one of them, containing the horses, has been considered by many
naturalists as forming an order apart, the _Solidungula_.
The existing Artiodactyles range themselves around two principal types,
the tubercular-toothed, or bunodont, and the crescentic-toothed, or
selenodont. To the former belong the pigs in all their modifications,
including the babirussa and wart-hogs, and the hippopotamus and peccary.
To the latter the ruminants, _i.e._ vast numbers of species of animals
included under the general designation of sheep, oxen, goats, antelopes,
deer, musks, giraffes, and the two allied though aberrant forms, the
camels, and the _Tragulidæ_ or chevrotains, an interesting little group
long confounded with the musk deer.
The two extremes of this division, represented by the pigs and the
hollow-horned ruminants, seem to have very little in common at first
sight, and if we were acquainted with the organisation only of the
existing species, we might be justified in treating them as belonging to
very distinct groups. But even among existing forms there are some
examples, which may almost be called intercalary types, so widely do
they depart from the group to which they are most nearly relative in
the direction of the other.
These are among the bunodonts, the little South American peccaries
(_Dicotyles_), and among the selenodonts, in a far greater degree, the
chevrotains (_Tragulus_). The latter in many remarkable characters
deviate strongly from the ruminants, and approach the pigs, or rather,
as will be shown presently, to the generalised type of the entire group.
Such being the present condition of the order, what does palæontology
reveal of its past history?
In the first place, it is most necessary to bear in mind the provisional
character of all classifications of extinct animals, because of our
imperfect knowledge of their structure; but endeavouring to make the
best use of what little we possess, I have added in the scheme opposite
all the best-known extinct forms somewhere near the position, in
relation to the existing forms and each other, in which their affinities
would place them, and have shown by the different shading their relation
as regards time.
The tertiary period, with which we are now alone concerned, has here
been divided for convenience into six epochs. Of course, it were
possible to have gone into minute details and made many more divisions,
but it would have made the diagram less clear, and it is best, perhaps,
not to attempt to refine too much in this somewhat tentative exposition
of a biological history, especially as there is still much uncertainty
as to the exact relative age of many of our fossiliferous strata.
The epochs chosen are the recent (including the pleistocene), the
pliocene, late and early miocene, and late and early eocene, each
represented by a different shading. It is not meant that if a genus or
group is here assigned to one of these epochs, that some of its members
may not have extended in some degree beyond its limits (as it must be
always remembered that the boundaries of these epochs are quite
artificial), either before or after, but that the period assigned to it
was that in which it most chiefly flourished. When two shadings are
represented, one within another, it signifies that the group existed in
both, and of course in all intervening periods.
To begin with the Perissodactyles. The earliest known forms constitute a
family called _Lophiodontidæ_, composed of the genera _Lophiodon_,
_Coryphodon_, and _Hyracotherium_. Of these animals little is known
except the teeth, which, however, indicate rather a primitive or root
form, from which, by modification, all the other teeth of
Perissodactyles can be derived. The elevations and depressions of the
molar teeth of _Lophiodon_, for instance, are arranged on a pattern
which is the best key to that of all others of the sub-order; and it is
by going back, as it were, to it that we can understand and compare all
the other variously modified, and often more complicated, forms.
Moreover, these Lophiodonts possess a dental character which
distinguishes them from all other _Perissodactyles_, and brings them
into a more generalised ungulate type, for which reason I place them
nearest to the earlier forms of artiodactyles--that is, that all the
premolars are smaller and of a simpler form than the true molars.
Whether they possessed any modification of the limbs or other structures
which bear them out in this position, we unfortunately cannot say.
At a somewhat later epoch in the earth’s history appeared on the scene
the _Palæotheriidæ_, an important group, containing animals the osseous
structure and dentition of which are completely known, chiefly through
the famous researches of Cuvier into the fossils found in the gypsum
quarries at Montmartre. These were animals something like existing
tapirs, with three toes on each foot, complete and distinct radius and
ulna and tibia and fibula, complete typical number of teeth, _i.e._ _i_
3/3 _c_ 1/1 _p_ 4/4 _m_ 3/3 = 44; but the molar teeth modified in
pattern from that of the Lophiodonts. They flourished in the later
eocene, after which period they are no longer met with. They have been
divided into several genera, but Gaudry has shown that these are united
by transitional forms, and present a gradual series of modifications,
corresponding with successive geological epochs. Another offset, from
the ancient Lophiodont stock (with which it appears to be connected
through the American eocene _Hyracahyus_), constitutes the family
_Tapiridæ_, first known in the miocene and continued with scarcely any
modification to our own day, and therefore a most interesting form to
contemplate in its living state, as it brings back, in the most
striking way, the general facies of the fauna of those ancient times. In
one respect the tapir is remarkable among Perissodactyles, as it has on
its fore-feet as many as four toes, thus retaining a primitive or
generalised character. The other two existing forms, the rhinoceros and
the horse, appear to be more direct modifications of the Palæotherium
type, though in different directions. The existing rhinoceros closely
resembles the Palæotherium in the general structure of its skeleton,
limbs, number of toes, etc., and in the general pattern of the molar
teeth; it differs, however, in the greatly reduced number of front
teeth, incisors and canines, which in the African two-horned species are
often absolutely wanting; and also in the possession of those singular
epidermal appendages to the face, the well-known horns, either one or
two in number. Now palæontology points out with tolerable precision the
intermediate steps by which these modifications have been brought about.
A small ancient rhinoceros has been found in the early miocene of North
America, to which Leidy has given the name of _Hyracodon_, which had no
horn, and had the complete number of incisor and canine teeth, and was
in many ways, at least as far as the skull and teeth are concerned,
intermediate between _Palæotherium_ and _Rhinoceros_ proper. The earlier
known European rhinoceroses have had the name _Aceratherium_ given to
them, the small size of the nasal bones being apparently quite unfitted
to support such a weapon as a horn. The resemblance of their skull to
_Palæotherium_ has been pointed out by H. v. Meyer.