We live in and form part of a system of things of immense diversity
and perplexity, which we call Nature; and it is a matter of the deepest
interest to all of us that we should form just conceptions of the
constitution of that system and of its past history. With relation to
this universe, man is, in extent, little more than a mathematical point;
in duration but a fleeting shadow; he is a mere reed shaken in the winds
of force. But as Pascal long ago remarked, although a mere reed, he is
a thinking reed; and in virtue of that wonderful capacity of thought,
he has the power of framing for himself a symbolic conception of the
universe, which, although doubtless highly imperfect and inadequate as
a picture of the great whole, is yet sufficient to serve him as a chart
for the guidance of his practical affairs. It has taken long ages of
toilsome and often fruitless labour to enable man to look steadily at
the shifting scenes of the phantasmagoria of Nature, to notice what is
fixed among her fluctuations, and what is regular among her apparent
irregularities; and it is only comparatively lately, within the last few
centuries, that the conception of a universal order and of a definite
course of things, which we term the course of Nature, has emerged.
But, once originated, the conception of the constancy of the order of
Nature has become the dominant idea of modern thought. To any person who
is familiar with the facts upon which that conception is based, and
is competent to estimate their significance, it has ceased to be
conceivable that chance should have any place in the universe, or that
events should depend upon any but the natural sequence of cause and
effect. We have come to look upon the present as the child of the past
and as the parent of the future; and, as we have excluded chance from a
place in the universe, so we ignore, even as a possibility, the notion
of any interference with the order of Nature. Whatever may be men's
speculative doctrines, it is quite certain that every intelligent person
guides his life and risks his fortune upon the belief that the order of
Nature is constant, and that the chain of natural causation is never
broken.
In fact, no belief which we entertain has so complete a logical basis as
that to which I have just referred. It tacitly underlies every process
of reasoning; it is the foundation of every act of the will. It is based
upon the broadest induction, and it is verified by the most constant,
regular, and universal of deductive processes. But we must recollect
that any human belief, however broad its basis, however defensible it
may seem, is, after all, only a probable belief, and that our widest and
safest generalisations are simply statements of the highest degree of
probability. Though we are quite clear about the constancy of the order
of Nature, at the present time, and in the present state of things, it
by no means necessarily follows that we are justified in expanding this
generalisation into the infinite past, and in denying, absolutely, that
there may have been a time when Nature did not follow a fixed order,
when the relations of cause and effect were not definite, and when
extra-natural agencies interfered with the general course of Nature.
Cautious men will allow that a universe so different from that which we
know may have existed; just as a very candid thinker may admit that a
world in which two and two do not make four, and in which two straight
lines do inclose a space, may exist. But the same caution which forces
the admission of such possibilities demands a great deal of evidence
before it recognises them to be anything more substantial. And when
it is asserted that, so many thousand years ago, events occurred in a
manner utterly foreign to and inconsistent with the existing laws of
Nature, men, who without being particularly cautious, are simply honest
thinkers, unwilling to deceive themselves or delude others, ask for
trustworthy evidence of the fact.
Did things so happen or did they not? This is a historical question, and
one the answer to which must be sought in the same way as the solution
of any other historical problem.
So far as I know, there are only three hypotheses which ever have been
entertained, or which well can be entertained, respecting the past
history of Nature. I will, in the first place, state the hypotheses,
and then I will consider what evidence bearing upon them is in our
possession, and by what light of criticism that evidence is to be
interpreted.
Upon the first hypothesis, the assumption is, that phenomena of Nature
similar to those exhibited by the present world have always existed; in
other words, that the universe has existed, from all eternity, in what
may be broadly termed its present condition.
The second hypothesis is that the present state of things has had only
a limited duration; and that, at some period in the past, a condition
of the world, essentially similar to that which we now know, came into
existence, without any precedent condition from which it could have
naturally proceeded. The assumption that successive states of Nature
have arisen, each without any relation of natural causation to an
antecedent state, is a mere modification of this second hypothesis.
The third hypothesis also assumes that the present state of things has
had but a limited duration; but it supposes that this state has been
evolved by a natural process from an antecedent state, and that from
another, and so on; and, on this hypothesis, the attempt to assign any
limit to the series of past changes is, usually, given up.
It is so needful to form clear and distinct notions of what is really
meant by each of these hypotheses that I will ask you to imagine what,
according to each, would have been visible to a spectator of the events
which constitute the history of the earth. On the first hypothesis,
however far back in time that spectator might be placed, he would see
a world essentially, though perhaps not in all its details, similar to
that which now exists. The animals which existed would be the ancestors
of those which now live, and similar to them; the plants, in like
manner, would be such as we know; and the mountains, plains, and waters
would foreshadow the salient features of our present land and water.
This view was held more or less distinctly, sometimes combined with
the notion of recurrent cycles of change, in ancient times; and its
influence has been felt down to the present day. It is worthy of remark
that it is a hypothesis which is not inconsistent with the doctrine of
Uniformitarianism, with which geologists are familiar. That doctrine was
held by Hutton, and in his earlier days by Lyell. Hutton was struck by
the demonstration of astronomers that the perturbations of the planetary
bodies, however great they may be, yet sooner or later right themselves;
and that the solar system possesses a self-adjusting power by which
these aberrations are all brought back to a mean condition. Hutton
imagined that the like might be true of terrestrial changes; although no
one recognised more clearly than he the fact that the dry land is being
constantly washed down by rain and rivers and deposited in the sea; and
that thus, in a longer or shorter time, the inequalities of the earth's
surface must be levelled, and its high lands brought down to the ocean.
But, taking into account the internal forces of the earth, which,
upheaving the sea-bottom give rise to new land, he thought that these
operations of degradation and elevation might compensate each other; and
that thus, for any assignable time, the general features of our planet
might remain what they are. And inasmuch as, under these circumstances,
there need be no limit to the propagation of animals and plants, it is
clear that the consistent working out of the uniformitarian idea might
lead to the conception of the eternity of the world. Not that I mean
to say that either Hutton or Lyell held this conception--assuredly
not; they would have been the first to repudiate it. Nevertheless, the
logical development of some of their arguments tends directly towards
this hypothesis.
The second hypothesis supposes that the present order of things, at some
no very remote time, had a sudden origin, and that the world, such as
it now is, had chaos for its phenomenal antecedent. That is the doctrine
which you will find stated most fully and clearly in the immortal poem
of John Milton--the English _Divina Commedia--_ "Paradise Lost." I
believe it is largely to the influence of that remarkable work, combined
with the daily teachings to which we have all listened in our childhood,
that this hypothesis owes its general wide diffusion as one of the
current beliefs of English-speaking people. If you turn to the seventh
book of "Paradise Lost," you will find there stated the hypothesis to
which I refer, which is briefly this: That this visible universe of ours
came into existence at no great distance of time from the present;
and that the parts of which it is composed made their appearance, in
a certain definite order, in the space of six natural days, in such a
manner that, on the first of these days, light appeared; that, on the
second, the firmament, or sky, separated the waters above, from the
waters beneath the firmament; that, on the third day, the waters drew
away from the dry land, and upon it a varied vegetable life, similar
to that which now exists, made its appearance; that the fourth day was
signalised by the apparition of the sun, the stars, the moon, and the
planets; that, on the fifth day, aquatic animals originated within the
waters; that, on the sixth day, the earth gave rise to our four-footed
terrestrial creatures, and to all varieties of terrestrial animals
except birds, which had appeared on the preceding day; and, finally,
that man appeared upon the earth, and the emergence of the universe from
chaos was finished. Milton tells us, without the least ambiguity, what a
spectator of these marvellous occurrences would have witnessed. I doubt
not that his poem is familiar to all of you, but I should like to recall
one passage to your minds, in order that I may be justified in what I
have said regarding the perfectly concrete, definite, picture of the
origin of the animal world which Milton draws. He says:--
"The sixth, and of creation last, arose
With evening harp and matin, when God said,
'Let the earth bring forth soul living in her kind,
Cattle and creeping things, and beast of the earth.
Each in their kind!' The earth obeyed, and, straight
Opening her fertile womb, teemed at a birth
Innumerous living creatures, perfect forms,
Limbed and full-grown. Out of the ground uprose,
As from his lair, the wild beast, where he wons
In forest wild, in thicket, brake, or den;
Among the trees in pairs they rose, they walked;
The cattle in the fields and meadows green;
Those rare and solitary; these in flocks
Pasturing at once, and in broad herds upsprung.
The grassy clods now calved; now half appears
The tawny lion, pawing to get free
His hinder parts--then springs, as broke from bonds,
And rampant shakes his brinded mane; the ounce,
The libbard, and the tiger, as the mole
Rising, the crumbled earth above them threw
In hillocks; the swift stag from underground
Bore up his branching head; scarce from his mould
Behemoth, biggest born of earth, upheaved
His vastness; fleeced the flocks and bleating rose
As plants; ambiguous between sea and land,
The river-horse and scaly crocodile.
At once came forth whatever creeps the ground,
Insect or worm."
There is no doubt as to the meaning of this statement, nor as to what a
man of Milton's genius expected would have been actually visible to an
eye-witness of this mode of origination of living things.
The third hypothesis, or the hypothesis of evolution, supposes that,
at any comparatively late period of past time, our imaginary spectator
would meet with a state of things very similar to that which now
obtains; but that the likeness of the past to the present would
gradually become less and less, in proportion to the remoteness of
his period of observation from the present day; that the existing
distribution of mountains and plains, of rivers and seas, would show
itself to be the product of a slow process of natural change operating
upon more and more widely different antecedent conditions of the mineral
frame-work of the earth; until, at length, in place of that frame-work,
he would behold only a vast nebulous mass, representing the constituents
of the sun and of the planetary bodies. Preceding the forms of life
which now exist, our observer would see animals and plants, not
identical with them, but like them, increasing their differences with
their antiquity and, at the same time, becoming simpler and simpler;
until, finally, the world of life would present nothing but that
undifferentiated protoplasmic matter which, so far as our present
knowledge goes, is the common foundation of all vital activity.
The hypothesis of evolution supposes that in all this vast progression
there would be no breach of continuity, no point at which we could say
"This is a natural process," and "This is not a natural process;"
but that the whole might be compared to that wonderful operation of
development which may be seen going on every day under our eyes, in
virtue of which there arises, out of the semi-fluid comparatively
homogeneous substance which we call an egg, the complicated organisation
of one of the higher animals. That, in a few words, is what is meant by
the hypothesis of evolution.
I have already suggested that, in dealing with these three hypotheses,
in endeavouring to form a judgment as to which of them is the more
worthy of belief, or whether none is worthy of belief--in which case
our condition of mind should be that suspension of judgment which is so
difficult to all but trained intellects--we should be indifferent to
all _a priori_ considerations. The question is a question of historical
fact. The universe has come into existence somehow or other, and the
problem is, whether it came into existence in one fashion, or whether
it came into existence in another; and, as an essential preliminary to
further discussion, permit me to say two or three words as to the nature
and the kinds of historical evidence.
The evidence as to the occurrence of any event in past time may be
ranged under two heads which, for convenience' sake, I will speak of
as testimonial evidence and as circumstantial evidence. By testimonial
evidence I mean human testimony; and by circumstantial evidence I mean
evidence which is not human testimony. Let me illustrate by a familiar
example what I understand by these two kinds of evidence, and what is to
be said respecting their value.
Suppose that a man tells you that he saw a person strike another and
kill him; that is testimonial evidence of the fact of murder. But it is
possible to have circumstantial evidence of the fact of murder; that
is to say, you may find a man dying with a wound upon his head having
exactly the form and character of the wound which is made by an axe,
and, with due care in taking surrounding circumstances into account, you
may conclude with the utmost certainty that the man has been murdered;
that his death is the consequence of a blow inflicted by another man
with that implement. We are very much in the habit of considering
circumstantial evidence as of less value than testimonial evidence,
and it may be that, where the circumstances are not perfectly clear and
intelligible, it is a dangerous and unsafe kind of evidence; but it
must not be forgotten that, in many cases, circumstantial is quite as
conclusive as testimonial evidence, and that, not unfrequently, it is
a great deal weightier than testimonial evidence. For example, take the
case to which I referred just now. The circumstantial evidence may be
better and more convincing than the testimonial evidence; for it may be
impossible, under the conditions that I have defined, to suppose that
the man met his death from any cause but the violent blow of an axe
wielded by another man. The circumstantial evidence in favour of a
murder having been committed, in that case, is as complete and as
convincing as evidence can be. It is evidence which is open to no doubt
and to no falsification. But the testimony of a witness is open to
multitudinous doubts. He may have been mistaken. He may have been
actuated by malice. It has constantly happened that even an accurate man
has declared that a thing has happened in this, that, or the other way,
when a careful analysis of the circumstantial evidence has shown that it
did not happen in that way, but in some other way.
We may now consider the evidence in favour of or against the three
hypotheses. Let me first direct your attention to what is to be said
about the hypothesis of the eternity of the state of things in which we
now live. What will first strike you is, that it is a hypothesis which,
whether true or false, is not capable of verification by any evidence.
For, in order to obtain either circumstantial or testimonial evidence
sufficient to prove the eternity of duration of the present state
of nature, you must have an eternity of witnesses or an infinity
of circumstances, and neither of these is attainable. It is utterly
impossible that such evidence should be carried beyond a certain point
of time; and all that could be said, at most, would be, that so far
as the evidence could be traced, there was nothing to contradict the
hypothesis. But when you look, not to the testimonial evidence--which,
considering the relative insignificance of the antiquity of human
records, might not be good for much in this case--but to the
circumstantial evidence, then you find that this hypothesis is
absolutely incompatible with such evidence as we have; which is of so
plain and so simple a character that it is impossible in any way to
escape from the conclusions which it forces upon us.
You are, doubtless, all aware that the outer substance of the earth,
which alone is accessible to direct observation, is not of a homogeneous
character, but that it is made up of a number of layers or strata, the
titles of the principal groups of which are placed upon the accompanying
diagram. Each of these groups represents a number of beds of sand, of
stone, of clay, of slate, and of various other materials.
On careful examination, it is found that the materials of which each of
these layers of more or less hard rock are composed are, for the most
part, of the same nature as those which are at present being formed
under known conditions on the surface of the earth. For example, the
chalk, which constitutes a great part of the Cretaceous formation in
some parts of the world, is practically identical in its physical and
chemical characters with a substance which is now being formed at the
bottom of the Atlantic Ocean, and covers an enormous area; other beds
of rock are comparable with the sands which are being formed upon
sea-shores, packed together, and so on. Thus, omitting rocks of igneous
origin, it is demonstrable that all these beds of stone, of which a
total of not less than seventy thousand feet is known, have been formed
by natural agencies, either out of the waste and washing of the dry
land, or else by the accumulation of the exuviae of plants and animals.
Many of these strata are full of such exuviae--the so-called "fossils."
Remains of thousands of species of animals and plants, as perfectly
recognisable as those of existing forms of life which you meet with in
museums, or as the shells which you pick up upon the sea-beach, have
been imbedded in the ancient sands, or muds, or limestones, just as they
are being imbedded now, in sandy, or clayey, or calcareous subaqueous
deposits. They furnish us with a record, the general nature of which
cannot be misinterpreted, of the kinds of things that have lived upon
the surface of the earth during the time that is registered by this
great thickness of stratified rocks. But even a superficial study of
these fossils shows us that the animals and plants which live at the
present time have had only a temporary duration; for the remains of
such modern forms of life are met with, for the most part, only in the
uppermost or latest tertiaries, and their number rapidly diminishes in
the lower deposits of that epoch. In the older tertiaries, the places
of existing animals and plants are taken by other forms, as numerous and
diversified as those which live now in the same localities, but more or
less different from them; in the mesozoic rocks, these are replaced
by others yet more divergent from modern types; and, in the paleozoic
formations, the contrast is still more marked. Thus the circumstantial
evidence absolutely negatives the conception of the eternity of the
present condition of things. We can say, with certainty, that the
present condition of things has existed for a comparatively short
period; and that, so far as animal and vegetable nature are concerned,
it has been preceded by a different condition. We can pursue this
evidence until we reach the lowest of the stratified rocks, in which we
lose the indications of life altogether. The hypothesis of the eternity
of the present state of nature may therefore be put out of court.
Fig. 1.--Ideal Section of the Crust of the Earth.
We now come to what I will term Milton's hypothesis--the hypothesis that
the present condition of things has endured for a comparatively short
time; and, at the commencement of that time, came into existence within
the course of six days. I doubt not that it may have excited some
surprise in your minds that I should have spoken of this as Milton's
hypothesis, rather than that I should have chosen the terms which are
more customary, such as "the doctrine of creation," or "the Biblical
doctrine," or "the doctrine of Moses," all of which denominations, as
applied to the hypothesis to which I have just referred, are certainly
much more familiar to you than the title of the Miltonic hypothesis. But
I have had what I cannot but think are very weighty reasons for taking
the course which I have pursued. In the first place, I have discarded
the title of the "doctrine of creation," because my present business is
not with the question why the objects which constitute Nature came into
existence, but when they came into existence, and in what order. This
is as strictly a historical question as the question when the Angles
and the Jutes invaded England, and whether they preceded or followed the
Romans. But the question about creation is a philosophical problem,
and one which cannot be solved, or even approached, by the historical
method. What we want to learn is, whether the facts, so far as they are
known, afford evidence that things arose in the way described by Milton,
or whether they do not; and, when that question is settled it will be
time enough to inquire into the causes of their origination.
In the second place, I have not spoken of this doctrine as the Biblical
doctrine. It is quite true that persons as diverse in their general
views as Milton the Protestant and the celebrated Jesuit Father Suarez,
each put upon the first chapter of Genesis the interpretation embodied
in Milton's poem. It is quite true that this interpretation is that
which has been instilled into every one of us in our childhood; but I
do not for one moment venture to say that it can properly be called the
Biblical doctrine. It is not my business, and does not lie within my
competency, to say what the Hebrew text does, and what it does not
signify; moreover, were I to affirm that this is the Biblical doctrine,
I should be met by the authority of many eminent scholars, to say
nothing of men of science, who, at various times, have absolutely denied
that any such doctrine is to be found in Genesis. If we are to listen to
many expositors of no mean authority, we must believe that what seems so
clearly defined in Genesis--as if very great pains had been taken that
there should be no possibility of mistake--is not the meaning of the
text at all. The account is divided into periods that we may make just
as long or as short as convenience requires. We are also to understand
that it is consistent with the original text to believe that the most
complex plants and animals may have been evolved by natural processes,
lasting for millions of years, out of structureless rudiments. A
person who is not a Hebrew scholar can only stand aside and admire
the marvellous flexibility of a language which admits of such diverse
interpretations. But assuredly, in the face of such contradictions of
authority upon matters respecting which he is incompetent to form any
judgment, he will abstain, as I do, from giving any opinion.
In the third place, I have carefully abstained from speaking of this as
the Mosaic doctrine, because we are now assured upon the authority of
the highest critics and even of dignitaries of the Church, that there is
no evidence that Moses wrote the Book of Genesis, or knew anything
about it. You will understand that I give no judgment--it would be an
impertinence upon my part to volunteer even a suggestion--upon such a
subject. But, that being the state of opinion among the scholars and the
clergy, it is well for the unlearned in Hebrew lore, and for the laity,
to avoid entangling themselves in such a vexed question. Happily, Milton
leaves us no excuse for doubting what he means, and I shall therefore be
safe in speaking of the opinion in question as the Miltonic hypothesis.
Now we have to test that hypothesis. For my part, I have no prejudice
one way or the other. If there is evidence in favour of this view, I am
burdened by no theoretical difficulties in the way of accepting it; but
there must be evidence. Scientific men get an awkward habit--no, I won't
call it that, for it is a valuable habit--of believing nothing unless
there is evidence for it; and they have a way of looking upon belief
which is not based upon evidence, not only as illogical, but as immoral.
We will, if you please, test this view by the circumstantial evidence
alone; for, from what I have said, you will understand that I do not
propose to discuss the question of what testimonial evidence is to be
adduced in favour of it. If those whose business it is to judge are not
at one as to the authenticity of the only evidence of that kind which is
offered, nor as to the facts to which it bears witness, the discussion
of such evidence is superfluous.
But I may be permitted to regret this necessity of rejecting the
testimonial evidence the less, because the examination of the
circumstantial evidence leads to the conclusion, not only that it is
incompetent to justify the hypothesis, but that, so far as it goes, it
is contrary to the hypothesis.
The considerations upon which I base this conclusion are of the simplest
possible character. The Miltonic hypothesis contains assertions of a
very definite character relating to the succession of living forms. It
is stated that plants, for example, made their appearance upon the third
day, and not before. And you will understand that what the poet means by
plants are such plants as now live, the ancestors, in the ordinary way
of propagation of like by like, of the trees and shrubs which flourish
in the present world. It must needs be so; for, if they were different,
either the existing plants have been the result of a separate
origination since that described by Milton, of which we have no record,
nor any ground for supposition that such an occurrence has taken place;
or else they have arisen by a process of evolution from the original
stocks.
In the second place, it is clear that there was no animal life before
the fifth day, and that, on the fifth day, aquatic animals and birds
appeared. And it is further clear that terrestrial living things, other
than birds, made their appearance upon the sixth day and not before.
Hence, it follows that, if, in the large mass of circumstantial evidence
as to what really has happened in the past history of the globe we find
indications of the existence of terrestrial animals, other than birds,
at a certain period, it is perfectly certain that all that has taken
place, since that time, must be referred to the sixth day.
In the great Carboniferous formation, whence America derives so vast a
proportion of her actual and potential wealth, in the beds of coal which
have been formed from the vegetation of that period, we find abundant
evidence of the existence of terrestrial animals. They have been
described, not only by European but by your own naturalists. There are
to be found numerous insects allied to our cockroaches. There are to
be found spiders and scorpions of large size, the latter so similar to
existing scorpions that it requires the practised eye of the naturalist
to distinguish them. Inasmuch as these animals can be proved to have
been alive in the Carboniferous epoch, it is perfectly clear that, if
the Miltonic account is to be accepted, the huge mass of rocks extending
from the middle of the Palaeozoic formations to the uppermost members of
the series, must belong to the day which is termed by Milton the sixth.
But, further, it is expressly stated that aquatic animals took their
origin on the fifth day, and not before; hence, all formations in which
remains of aquatic animals can be proved to exist, and which therefore
testify that such animals lived at the time when these formations were
in course of deposition, must have been deposited during or since the
period which Milton speaks of as the fifth day. But there is absolutely
no fossiliferous formation in which the remains of aquatic animals are
absent. The oldest fossils in the Silurian rocks are exuviae of marine
animals; and if the view which is entertained by Principal Dawson and
Dr. Carpenter respecting the nature of the _Eozoon_ be well-founded,
aquatic animals existed at a period as far antecedent to the deposition
of the coal as the coal is from us; inasmuch as the _Eozoon_ is met
with in those Laurentian strata which lie at the bottom of the series
of stratified rocks. Hence it follows, plainly enough, that the whole
series of stratified rocks, if they are to be brought into harmony with
Milton, must be referred to the fifth and sixth days, and that we cannot
hope to find the slightest trace of the products of the earlier days in
the geological record. When we consider these simple facts, we see
how absolutely futile are the attempts that have been made to draw a
parallel between the story told by so much of the crust of the earth
as is known to us and the story which Milton tells. The whole series of
fossiliferous stratified rocks must be referred to the last two days;
and neither the Carboniferous, nor any other, formation can afford
evidence of the work of the third day.
Not only is there this objection to any attempt to establish a harmony
between the Miltonic account and the facts recorded in the fossiliferous
rocks, but there is a further difficulty. According to the Miltonic
account, the order in which animals should have made their appearance in
the stratified rocks would be thus: Fishes, including the great whales,
and birds; after them, all varieties of terrestrial animals except
birds. Nothing could be further from the facts as we find them; we
know of not the slightest evidence of the existence of birds before the
Jurassic, or perhaps the Triassic, formation; while terrestrial animals,
as we have just seen, occur in the Carboniferous rocks.
If there were any harmony between the Miltonic account and the
circumstantial evidence, we ought to have abundant evidence of the
existence of birds in the Carboniferous, the Devonian, and the Silurian
rocks. I need hardly say that this is not the case, and that not a trace
of birds makes its appearance until the far later period which I have
mentioned.
And again, if it be true that all varieties of fishes and the great
whales, and the like, made their appearance on the fifth day, we ought
to find the remains of these animals in the older rocks--in those which
were deposited before the Carboniferous epoch. Fishes we do find, in
considerable number and variety; but the great whales are absent, and
the fishes are not such as now live. Not one solitary species of fish
now in existence is to be found in the Devonian or Silurian formations.
Hence we are introduced afresh to the dilemma which I have already
placed before you: either the animals which came into existence on the
fifth day were not such as those which are found at present, are not the
direct and immediate ancestors of those which now exist; in which
case, either fresh creations of which nothing is said, or a process of
evolution, must have occurred; or else the whole story must be given up,
as not only devoid of any circumstantial evidence, but contrary to such
evidence as exists.
I placed before you in a few words, some little time ago, a statement of
the sum and substance of Milton's hypothesis. Let me now try to state as
briefly, the effect of the circumstantial evidence bearing upon the
past history of the earth which is furnished, without the possibility
of mistake, with no chance of error as to its chief features, by the
stratified rocks. What we find is, that the great series of formations
represents a period of time of which our human chronologies hardly
afford us a unit of measure. I will not pretend to say how we ought to
estimate this time, in millions or in billions of years. For my purpose,
the determination of its absolute duration is wholly unessential. But
that the time was enormous there can be no question.
It results from the simplest methods of interpretation, that leaving
out of view certain patches of metamorphosed rocks, and certain volcanic
products, all that is now dry land has once been at the bottom of the
waters. It is perfectly certain that, at a comparatively recent period
of the world's history--the Cretaceous epoch--none of the great physical
features which at present mark the surface of the globe existed. It
is certain that the Rocky Mountains were not. It is certain that
the Himalaya Mountains were not. It is certain that the Alps and the
Pyrenees had no existence. The evidence is of the plainest possible
character and is simply this:--We find raised up on the flanks of these
mountains, elevated by the forces of upheaval which have given rise
to them, masses of Cretaceous rock which formed the bottom of the sea
before those mountains existed. It is therefore clear that the elevatory
forces which gave rise to the mountains operated subsequently to the
Cretaceous epoch; and that the mountains themselves are largely made up
of the materials deposited in the sea which once occupied their place.
As we go back in time, we meet with constant alternations of sea and
land, of estuary and open ocean; and, in correspondence with these
alternations, we observe the changes in the fauna and flora to which I
have referred.
But the inspection of these changes gives us no right to believe that
there has been any discontinuity in natural processes. There is no trace
of general cataclysms, of universal deluges, or sudden destructions of a
whole fauna or flora. The appearances which were formerly interpreted
in that way have all been shown to be delusive, as our knowledge has
increased and as the blanks which formerly appeared to exist between
the different formations have been filled up. That there is no absolute
break between formation and formation, that there has been no sudden
disappearance of all the forms of life and replacement of them by
others, but that changes have gone on slowly and gradually, that one
type has died out and another has taken its place, and that thus,
by insensible degrees, one fauna has been replaced by another, are
conclusions strengthened by constantly increasing evidence. So that
within the whole of the immense period indicated by the fossiliferous
stratified rocks, there is assuredly not the slightest proof of any
break in the uniformity of Nature's operations, no indication that
events have followed other than a clear and orderly sequence.
That, I say, is the natural and obvious teaching of the circumstantial
evidence contained in the stratified rocks. I leave you to consider
how far, by any ingenuity of interpretation, by any stretching of the
meaning of language, it can be brought into harmony with the Miltonic
hypothesis.
There remains the third hypothesis, that of which I have spoken as the
hypothesis of evolution; and I purpose that, in lectures to come, we
should discuss it as carefully as we have considered the other two
hypotheses. I need not say that it is quite hopeless to look for
testimonial evidence of evolution. The very nature of the case precludes
the possibility of such evidence, for the human race can no more be
expected to testify to its own origin, than a child can be tendered as
a witness of its own birth. Our sole inquiry is, what foundation
circumstantial evidence lends to the hypothesis, or whether it lends
none, or whether it controverts the hypothesis. I shall deal with the
matter entirely as a question of history. I shall not indulge in the
discussion of any speculative probabilities. I shall not attempt to show
that Nature is unintelligible unless we adopt some such hypothesis.
For anything I know about the matter, it may be the way of Nature to be
unintelligible; she is often puzzling, and I have no reason to suppose
that she is bound to fit herself to our notions.
I shall place before you three kinds of evidence entirely based upon
what is known of the forms of animal life which are contained in the
series of stratified rocks. I shall endeavour to show you that there is
one kind of evidence which is neutral, which neither helps evolution
nor is inconsistent with it. I shall then bring forward a second kind
of evidence which indicates a strong probability in favour of evolution,
but does not prove it; and, lastly, I shall adduce a third kind of
evidence which, being as complete as any evidence which we can hope to
obtain upon such a subject, and being wholly and strikingly in favour
of evolution, may fairly be called demonstrative evidence of its
occurrence.
II. THE HYPOTHESIS OF EVOLUTION. THE NEUTRAL AND THE FAVOURABLE
EVIDENCE.
In the preceding lecture I pointed out that there are three hypotheses
which may be entertained, and which have been entertained, respecting
the past history of life upon the globe. According to the first of these
hypotheses, living beings, such as now exist, have existed from
all eternity upon this earth. We tested that hypothesis by the
circumstantial evidence, as I called it, which is furnished by the
fossil remains contained in the earth's crust, and we found that it was
obviously untenable. I then proceeded to consider the second hypothesis,
which I termed the Miltonic hypothesis, not because it is of any
particular consequence whether John Milton seriously entertained it or
not, but because it is stated in a clear and unmistakable manner in his
great poem. I pointed out to you that the evidence at our command as
completely and fully negatives that hypothesis as it did the preceding
one. And I confess that I had too much respect for your intelligence
to think it necessary to add that the negation was equally clear and
equally valid, whatever the source from which that hypothesis might be
derived, or whatever the authority by which it might be supported.
I further stated that, according to the third hypothesis, or that of
evolution, the existing state of things is the last term of a long
series of states, which, when traced back, would be found to show no
interruption and no breach in the continuity of natural causation.
I propose, in the present and the following lecture, to test this
hypothesis rigorously by the evidence at command, and to inquire how
far that evidence can be said to be indifferent to it, how far it can be
said to be favourable to it, and, finally, how far it can be said to be
demonstrative.
From almost the origin of the discussions about the existing condition
of the animal and vegetable worlds and the causes which have determined
that condition, an argument has been put forward as an objection to
evolution, which we shall have to consider very seriously. It is an
argument which was first clearly stated by Cuvier in his criticism of
the doctrines propounded by his great contemporary, Lamarck. The French
expedition to Egypt had called the attention of learned men to the
wonderful store of antiquities in that country, and there had been
brought back to France numerous mummified corpses of the animals which
the ancient Egyptians revered and preserved, and which, at a reasonable
computation, must have lived not less than three or four thousand
years before the time at which they were thus brought to light. Cuvier
endeavoured to test the hypothesis that animals have undergone gradual
and progressive modifications of structure, by comparing the skeletons
and such other parts of the mummies as were in a fitting state of
preservation, with the corresponding parts of the representatives of the
same species now living in Egypt. He arrived at the conviction that no
appreciable change had taken place in these animals in the course of
this considerable lapse of time, and the justice of his conclusion is
not disputed.
It is obvious that, if it can be proved that animals have endured,
without undergoing any demonstrable change of structure, for so long a
period as four thousand years, no form of the hypothesis of evolution
which assumes that animals undergo a constant and necessary progressive
change can be tenable; unless, indeed, it be further assumed that
four thousand years is too short a time for the production of a change
sufficiently great to be detected.
But it is no less plain that if the process of evolution of animals is
not independent of surrounding conditions; if it may be indefinitely
hastened or retarded by variations in these conditions; or if evolution
is simply a process of accommodation to varying conditions; the argument
against the hypothesis of evolution based on the unchanged character of
the Egyptian fauna is worthless. For the monuments which are coeval with
the mummies testify as strongly to the absence of change in the physical
geography and the general conditions of the land of Egypt, for the time
in question, as the mummies do to the unvarying characters of its living
population.
The progress of research since Cuvier's time has supplied far more
striking examples of the long duration of specific forms of life than
those which are furnished by the mummified Ibises and Crocodiles of
Egypt. A remarkable case is to be found in your own country, in the
neighbourhood of the falls of Niagara. In the immediate vicinity of the
whirlpool, and again upon Goat Island, in the superficial deposits which
cover the surface of the rocky subsoil in those regions, there are
found remains of animals in perfect preservation, and among them, shells
belonging to exactly the same species as those which at present inhabit
the still waters of Lake Erie. It is evident, from the structure of the
country, that these animal remains were deposited in the beds in which
they occur at a time when the lake extended over the region in which
they are found. This involves the conclusion that they lived and died
before the falls had cut their way back through the gorge of Niagara;
and, indeed, it has been determined that, when these animals lived,
the falls of Niagara must have been at least six miles further down the
river than they are at present. Many computations have been made of
the rate at which the falls are thus cutting their way back. Those
computations have varied greatly, but I believe I am speaking within
the bounds of prudence, if I assume that the falls of Niagara have
not retreated at a greater pace than about a foot a year. Six miles,
speaking roughly, are 30,000 feet; 30,000 feet, at a foot a year, gives
30,000 years; and thus we are fairly justified in concluding that no
less a period than this has passed since the shell-fish, whose remains
are left in the beds to which I have referred, were living creatures.
But there is still stronger evidence of the long duration of certain
types. I have already stated that, as we work our way through the great
series of the Tertiary formations, we find many species of animals
identical with those which live at the present day, diminishing in
numbers, it is true, but still existing, in a certain proportion, in the
oldest of the Tertiary rocks. Furthermore, when we examine the rocks
of the Cretaceous epoch, we find the remains of some animals which the
closest scrutiny cannot show to be, in any important respect, different
from those which live at the present time. That is the case with one of
the cretaceous lamp-shells (_Terebratula_), which has continued to exist
unchanged, or with insignificant variations, down to the present
day. Such is the case with the _Globigerinæ,_ the skeletons of which,
aggregated together, form a large proportion of our English chalk. Those
_Globigerinae_ can be traced down to the _Globigerinae_ which live
at the surface of the present great oceans, and the remains of which,
falling to the bottom of the sea, give rise to a chalky mud. Hence
it must be admitted that certain existing species of animals show no
distinct sign of modification, or transformation, in the course of a
lapse of time as great as that which carries us back to the Cretaceous
period; and which, whatever its absolute measure, is certainly vastly
greater than thirty thousand years.
There are groups of species so closely allied together, that it needs
the eye of a naturalist to distinguish them one from another. If we
disregard the small differences which separate these forms, and consider
all the species of such groups as modifications of one type, we
shall find that, even among the higher animals, some types have had a
marvellous duration. In the chalk, for example, there is found a fish
belonging to the highest and the most differentiated group of osseous
fishes, which goes by the name of _Beryx._ The remains of that fish are
among the most beautiful and well-preserved of the fossils found in our
English chalk. It can be studied anatomically, so far as the hard parts
are concerned, almost as well as if it were a recent fish. But the
genus _Beryx_ is represented, at the present day, by very closely allied
species which are living in the Pacific and Atlantic Oceans. We may
go still farther back. I have already referred to the fact that the
Carboniferous formations, in Europe and in America, contain the remains
of scorpions in an admirable state of preservation, and that those
scorpions are hardly distinguishable from such as now live. I do not
mean to say that they are not different, but close scrutiny is needed in
order to distinguish them from modern scorpions.
More than this. At the very bottom of the Silurian series, in beds which
are by some authorities referred to the Cambrian formation, where the
signs of life begin to fail us--even there, among the few and scanty
animal remains which are discoverable, we find species of molluscous
animals which are so closely allied to existing forms that, at one time,
they were grouped under the same generic name. I refer to the well-known
_Lingula_ of the _Lingula_ flags, lately, in consequence of some slight
differences, placed in the new genus _Lingulella._ Practically, it
belongs to the same great generic group as the _Lingula,_ which is to
be found at the present day upon your own shores and those of many other
parts of the world.
The same truth is exemplified if we turn to certain great periods of the
earth's history--as, for example, the Mesozoic epoch. There are groups
of reptiles, such as the _Ichthyosauria_ and the _Plesiosauria,_ which
appear shortly after the commencement of this epoch, and they occur in
vast numbers. They disappear with the chalk and, throughout the whole of
the great series of Mesozoic rocks, they present no such modifications
as can safely be considered evidence of progressive modification.
Facts of this kind are undoubtedly fatal to any form of the doctrine of
evolution which postulates the supposition that there is an intrinsic
necessity, on the part of animal forms which have once come into
existence, to undergo continual modification; and they are as distinctly
opposed to any view which involves the belief, that such modification
may occur, must take place, at the same rate, in all the different types
of animal or vegetable life. The facts, as I have placed them before
you, obviously directly contradict any form of the hypothesis of
evolution which stands in need of these two postulates.
But, one great service that has been rendered by Mr. Darwin to the
doctrine of evolution in general is this: he has shown that there
are two chief factors in the process of evolution: one of them is the
tendency to vary, the existence of which in all living forms may
be proved by observation; the other is the influence of surrounding
conditions upon what I may call the parent form and the variations which
are thus evolved from it. The cause of the production of variations is
a matter not at all properly understood at present. Whether variation
depends upon some intricate machinery--if I may use the phrase--of the
living organism itself, or whether it arises through the influence of
conditions upon that form, is not certain, and the question may, for
the present, be left open. But the important point is that, granting the
existence of the tendency to the production of variations; then, whether
the variations which are produced shall survive and supplant the parent,
or whether the parent form shall survive and supplant the variations, is
a matter which depends entirely on those conditions which give rise to
the struggle for existence. If the surrounding conditions are such that
the parent form is more competent to deal with them, and flourish in
them than the derived forms, then, in the struggle for existence,
the parent form will maintain itself and the derived forms will be
exterminated. But if, on the contrary, the conditions are such as to be
more favourable to a derived than to the parent form, the parent form
will be extirpated and the derived form will take its place. In the
first case, there will be no progression, no change of structure,
through any imaginable series of ages; in the second place there will be
modification of change and form.
Thus the existence of these persistent types, as I have termed them, is
no real obstacle in the way of the theory of evolution. Take the case
of the scorpions to which I have just referred. No doubt, since the
Carboniferous epoch, conditions have always obtained, such as existed
when the scorpions of that epoch flourished; conditions in which
scorpions find themselves better off, more competent to deal with the
difficulties in their way, than any variation from the scorpion type
which they may have produced; and, for that reason, the scorpion type
has persisted, and has not been supplanted by any other form. And
there is no reason, in the nature of things, why, as long as this world
exists, if there be conditions more favourable to scorpions than to
any variation which may arise from them, these forms of life should not
persist.
Therefore, the stock objection to the hypothesis of evolution, based on
the long duration of certain animal and vegetable types, is no objection
at all. The facts of this character--and they are numerous--belong to
that class of evidence which I have called indifferent. That is to say,
they may afford no direct support to the doctrine of evolution, but they
are capable of being interpreted in perfect consistency with it.
There is another order of facts belonging to the class of negative or
indifferent evidence. The great group of Lizards, which abound in the
present world, extends through the whole series of formations as far
back as the Permian, or latest Palaeozoic, epoch. These Permian lizards
differ astonishingly little from the lizards which exist at the present
day. Comparing the amount of the differences between them and modern
lizards, with the prodigious lapse of time between the Permian epoch
and the present day, it may be said that the amount of change is
insignificant. But, when we carry our researches farther back in time,
we find no trace of lizards, nor of any true reptile whatever, in the
whole mass of formations beneath the Permian.
Now, it is perfectly clear that if our palaeontological collections are
to be taken, even approximately, as an adequate representation of all
the forms of animals and plants that have ever lived; and if the record
furnished by the known series of beds of stratified rock covers the
whole series of events which constitute the history of life on the
globe, such a fact as this directly contravenes the hypothesis of
evolution; because this hypothesis postulates that the existence of
every form must have been preceded by that of some form little different
from it. Here, however, we have to take into consideration that
important truth so well insisted upon by Lyell and by Darwin--the
imperfection of the geological record. It can be demonstrated that the
geological record must be incomplete, that it can only preserve remains
found in certain favourable localities and under particular conditions;
that it must be destroyed by processes of denudation, and obliterated by
processes of metamorphosis. Beds of rock of any thickness crammed full
of organic remains, may yet, either by the percolation of water through
them, or by the influence of subterranean heat, lose all trace of
these remains, and present the appearance of beds of rock formed under
conditions in which living forms were absent. Such metamorphic rocks
occur in formations of all ages; and, in various cases, there are very
good grounds for the belief that they have contained organic remains,
and that those remains have been absolutely obliterated.
I insist upon the defects of the geological record the more because
those who have not attended to these matters are apt to say, "It is
all very well, but, when you get into a difficulty with your theory of
evolution, you appeal to the incompleteness and the imperfection of the
geological record;" and I want to make it perfectly clear to you that
this imperfection is a great fact, which must be taken into account in
all our speculations, or we shall constantly be going wrong.
You see the singular series of footmarks, drawn of its natural size in
the large diagram hanging up here (Fig. 2), which I owe to the kindness
of my friend Professor Marsh, with whom I had the opportunity recently
of visiting the precise locality in Massachusetts in which these tracks
occur. I am, therefore, able to give you my own testimony, if needed,
that the diagram accurately represents what we saw. The valley of the
Connecticut is classical ground for the geologist. It contains great
beds of sandstone, covering many square miles, which have evidently
formed a part of an ancient sea-shore, or, it may be, lake-shore. For a
certain period of time after their deposition, these beds have remained
sufficiently soft to receive the impressions of the feet of whatever
animals walked over them, and to preserve them afterwards, in exactly
the same way as such impressions are at this hour preserved on the
shores of the Bay of Fundy and elsewhere. The diagram represents the
track of some gigantic animal, which walked on its hind legs. You see
the series of marks made alternately by the right and by the left foot;
so that, from one impression to the other of the three-toed foot on the
same side, is one stride, and that stride, as we measured it, is six
feet nine inches. I leave you, therefore, to form an impression of the
magnitude of the creature which, as it walked along the ancient shore,
made these impressions.
Fig. 2.--Tracks of Brontozoum.
Of such impressions there are untold thousands upon these sandstones.
Fifty or sixty different kinds have been discovered, and they cover vast
areas. But, up to this present time, not a bone, not a fragment, of any
one of the animals which left these great footmarks has been found;
in fact, the only animal remains which have been met with in all these
deposits, from the time of their discovery to the present day--though
they have been carefully hunted over--is a fragmentary skeleton of one
of the smaller forms. What has become of the bones of all these animals?
You see we are not dealing with little creatures, but with animals that
make a step of six feet nine inches; and their remains must have been
left somewhere. The probability is, that they have been dissolved away,
and completely lost.
I have had occasion to work out the nature of fossil remains, of which
there was nothing left except casts of the bones, the solid material of
the skeleton having been dissolved out by percolating water. It was
a chance, in this case, that the sandstone happened to be of such a
constitution as to set, and to allow the bones to be afterward dissolved
out, leaving cavities of the exact shape of the bones. Had that
constitution been other than what it was, the bones would have been
dissolved, the layers of sandstone would have fallen together into one
mass, and not the slightest indication that the animal had existed would
have been discoverable.
I know of no more striking evidence than these facts afford, of the
caution which should be used in drawing the conclusion, from the absence
of organic remains in a deposit, that animals or plants did not exist
at the time it was formed. I believe that, with a right understanding of
the doctrine of evolution on the one hand, and a just estimation of the
importance of the imperfection of the geological record on the other,
all difficulty is removed from the kind of evidence to which I have
adverted; and that we are justified in believing that all such cases
are examples of what I have designated negative or indifferent
evidence--that is to say, they in no way directly advance the hypothesis
of evolution, but they are not to be regarded as obstacles in the way of
our belief in that doctrine.
I now pass on to the consideration of those cases which, for reasons
which I will point out to you by and by, are not to be regarded as
demonstrative of the truth of evolution, but which are such as must
exist if evolution be true, and which therefore are, upon the whole,
evidence in favour of the doctrine. If the doctrine of evolution be
true, it follows, that, however diverse the different groups of animals
and of plants may be, they must all, at one time or other, have been
connected by gradational forms; so that, from the highest animals,
whatever they may be, down to the lowest speck of protoplasmic matter in
which life can be manifested, a series of gradations, leading from
one end of the series to the other, either exists or has existed.
Undoubtedly that is a necessary postulate of the doctrine of evolution.
But when we look upon living Nature as it is, we find a totally
different state of things. We find that animals and plants fall into
groups, the different members of which are pretty closely allied
together, but which are separated by definite, larger or smaller,
breaks, from other groups. In other words, no intermediate forms which
bridge over these gaps or intervals are, at present, to be met with.
To illustrate what I mean: Let me call your attention to those
vertebrate animals which are most familiar to you, such as mammals,
birds, and reptiles. At the present day, these groups of animals are
perfectly well-defined from one another. We know of no animal now living
which, in any sense, is intermediate between the mammal and the bird, or
between the bird and the reptile; but, on the contrary, there are many
very distinct anatomical peculiarities, well-defined marks, by which the
mammal is separated from the bird, and the bird from the reptile. The
distinctions are obvious and striking if you compare the definitions of
these great groups as they now exist.
The same may be said of many of the subordinate groups, or orders, into
which these great classes are divided. At the present time, for example,
there are numerous forms of non-ruminant pachyderms, or what we may call
broadly, the pig tribe, and many varieties of ruminants. These
latter have their definite characteristics, and the former have their
distinguishing peculiarities. But there is nothing that fills up the gap
between the ruminants and the pig tribe. The two are distinct. Such also
is the case in respect of the minor groups of the class of reptiles. The
existing fauna shows us crocodiles, lizards, snakes, and tortoises; but
no connecting link between the crocodile and lizard, nor between the
lizard and snake, nor between the snake and the crocodile, nor between
any two of these groups. They are separated by absolute breaks. If,
then, it could be shown that this state of things had always
existed, the fact would be fatal to the doctrine of evolution. If the
intermediate gradations, which the doctrine of evolution requires to
have existed between these groups, are not to be found anywhere in the
records of the past history of the globe, their absence is a strong and
weighty negative argument against evolution; while, on the other hand,
if such intermediate forms are to be found, that is so much to the good
of evolution; although, for reasons which I will lay before you by and
by, we must be cautious in our estimate of the evidential cogency of
facts of this kind.
It is a very remarkable circumstance that, from the commencement of
the serious study of fossil remains, in fact, from the time when Cuvier
began his brilliant researches upon those found in the quarries of
Montmartre, palaeontology has shown what she was going to do in this
matter, and what kind of evidence it lay in her power to produce.
I said just now that, in the existing Fauna, the group of pig-like
animals and the group of ruminants are entirely distinct; but one of
the first of Cuvier's discoveries was an animal which he called the
_Anoplotherium,_ and which proved to be, in a great many important
respects, intermediate in character between the pigs, on the one hand,
and the ruminants on the other. Thus, research into the history of the
past did, to a certain extent, tend to fill up the breach between the
group of ruminants and the group of pigs. Another remarkable animal
restored by the great French palaeontologist, the _Palaeotherium,_
similarly tended to connect together animals to all appearance so
different as the rhinoceros, the horse, and the tapir. Subsequent
research has brought to light multitudes of facts of the same order; and
at the present day, the investigations of such anatomists as Rutimeyer
and Gaudry have tended to fill up, more and more, the gaps in our
existing series of mammals, and to connect groups formerly thought to be
distinct.
But I think it may have an especial interest if, instead of dealing with
these examples, which would require a great deal of tedious osteological
detail, I take the case of birds and reptiles; groups which, at the
present day, are so clearly distinguished from one another that there
are perhaps no classes of animals which, in popular apprehension, are
more completely separated. Existing birds, as you are aware, are covered
with feathers; their anterior extremities, specially and peculiarly
modified, are converted into wings by the aid of which most of them are
able to fly; they walk upright upon two legs; and these limbs, when
they are considered anatomically, present a great number of exceedingly
remarkable peculiarities, to which I may have occasion to advert
incidentally as I go on, and which are not met with, even approximately,
in any existing forms of reptiles. On the other hand, existing reptiles
have no feathers. They may have naked skins, or be covered with horny
scales, or bony plates, or with both. They possess no wings; they
neither fly by means of their fore-limbs, nor habitually walk upright
upon their hind-limbs; and the bones of their legs present no such
modifications as we find in birds. It is impossible to imagine any two
groups more definitely and distinctly separated, notwithstanding certain
characters which they possess in common.
As we trace the history of birds back in time, we find their remains,
sometimes in great abundance, throughout the whole extent of the
tertiary rocks; but, so far as our present knowledge goes, the birds of
the tertiary rocks retain the same essential characters as the birds
of the present day. In other words, the tertiary birds come within the
definition of the class constituted by existing birds, and are as much
separated from reptiles as existing birds are. Not very long ago no
remains of birds had been found below the tertiary rocks, and I am not
sure but that some persons were prepared to demonstrate that they could
not have existed at an earlier period. But, in the course of the
last few years, such remains have been discovered in England; though,
unfortunately, in so imperfect and fragmentary a condition, that it
is impossible to say whether they differed from existing birds in any
essential character or not. In your country the development of the
cretaceous series of rocks is enormous; the conditions under which the
later cretaceous strata have been deposited are highly favourable to the
preservation of organic remains; and the researches, full of labour and
risk, which have been carried on by Professor Marsh in these cretaceous
rocks of Western America, have rewarded him with the discovery of forms
of birds of which we had hitherto no conception. By his kindness, I am
enabled to place before you a restoration of one of these extraordinary
birds, every part of which can be thoroughly justified by the more or
less complete skeletons, in a very perfect state of preservation, which
he has discovered. This _Hesperornis_ (Fig. 3), which measured between
five and six feet in length, is astonishingly like our existing divers
or grebes in a great many respects; so like them indeed that, had the
skeleton of _Hesperornis_ been found in a museum without its skull, it
probably would have been placed in the same group of birds as the divers
and grebes of the present day. [1] But _Hesperornis_ differs from
all existing birds, and so far resembles reptiles, in one important
particular--it is provided with teeth. The long jaws are armed with
teeth which have curved crowns and thick roots (Fig. 4), and are not
set in distinct sockets, but are lodged in a groove. In possessing true
teeth, the _Hesperornis_ differs from every existing bird, and from
every bird yet discovered in the tertiary formations, the tooth-like
serrations of the jaws in the _Odontopteryx_ of the London clay being
mere processes of the bony substance of the jaws, and not teeth in the
proper sense of the word. In view of the characteristics of this bird we
are therefore obliged to modify the definitions of the classes of birds
and reptiles. Before the discovery of _Hesperornis,_ the definition of
the class Aves based upon our knowledge of existing birds might have
been extended to all birds; it might have been said that the absence of
teeth was characteristic of the class of birds; but the discovery of
an animal which, in every part of its skeleton, closely agrees with
existing birds, and yet possesses teeth, shows that there were ancient
birds which, in respect of possessing teeth, approached reptiles more
nearly than any existing bird does, and, to that extent, diminishes the
_hiatus_ between the two classes.