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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 Palæozoic 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 exuviæ 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 this:
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 give 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
_Globigerinæ_ can be traced down to the _Globigerinæ_ 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 as
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 Palæozoic, 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 age, 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 palæontological 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.
[Illustration: 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,
palæontology 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 palæontologist, the
_Palæotherium_,
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,
improbably 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.
[Illustration: FIG.
3--HESPERORNIS REGALIS (Marsh).]
The same formation has yielded another
bird _Ichthyornis_ (Fig. 5),
which also possesses teeth; but the teeth are
situated in distinct
sockets, while those of _Hesperornis_ are not so lodged.
The latter also
has such very small, almost rudimentary wings, that it must
have been
chiefly a swimmer and a diver like a Penguin; while _Ichthyornis_
has
strong wings and no doubt possessed corresponding powers of
flight.
_Ichthyornis_ also differed in the fact that its vertebræ have not
the
peculiar characters of the vertebræ of existing and of all
known
tertiary birds, but were concave at each end. This discovery leads us
to
make a further modification in the definition of the group of birds,
and
to part with another of the characters by which almost all
existing
birds are distinguished from reptiles.
[Illustration: FIG.
4.--HESPERORNIS REGALIS (Marsh).
Side and upper views of half the lower
jaw; side and end views of a
vertebra and a separate tooth.]
Apart
from the few fragmentary remains from the English greensand, to
which I have
referred, the Mesozoic rocks, older than those in which
_Hesperornis_ and
_Ichthyornis_ have been discovered have afforded no
certain evidence of
birds, with the remarkable exception of the
Solenhofen slates. These
so-called slates are composed of a fine grained
calcareous mud which has
hardened into lithographic stone, and in which
organic remains are almost as
well preserved as they would be if they
had been imbedded in so much plaster
of Paris. They have yielded the
_Archæopteryx_, the existence of which was
first made known by the
finding of a fossil feather, or rather of the
impression of one. It is
wonderful enough that such a perishable thing as a
feather, and nothing
more, should be discovered; yet for a long time, nothing
was known of
this bird except its feather. But by and by a solitary skeleton
was
discovered which is now in the British Museum. The skull of
this
solitary specimen is unfortunately wanting, and it is
therefore
uncertain whether the _Archæopteryx_ possessed teeth or not.[2] But
the
remainder of the skeleton is so well preserved as to leave no
doubt
respecting the main features of the animal, which are very singular.
The
feet are not only altogether bird-like, but have the special
characters
of the feet of perching birds, while the body had a clothing of
true
feathers. Nevertheless, in some other respects, _Archæopteryx_ is
unlike
a bird and like a reptile. There is a long tail composed of
many
vertebræ. The structure of the wing differs in some very
remarkable
respects from that which it presents in a true bird. In the
latter, the
end of the wing answers to the thumb and two fingers of my hand;
but the
metacarpal bones, or those which answer to the bones of the
fingers
which lie in the palm of the hand, are fused together into one mass;
and
the whole apparatus, except the last joints of the thumb, is bound up
in
a sheath of integument, while the edge of the hand carries the
principal
quill feathers. In the _Archæopteryx_, the upper-arm bone is like
that
of a bird; and the two bones of the fore-arm are more or less like
those
of a bird, but the fingers are not bound together--they are free.
What
their number may have been is uncertain; but several, if not all,
of
them were terminated by strong curved claws, not like such as
are
sometimes found in birds, but such as reptiles possess; so that, in
the
_Archæopteryx_, we have an animal which, to a certain extent, occupies
a
midway place between a bird and a reptile. It is a bird so far as
its
foot and sundry other parts of its skeleton are concerned; it
is
essentially and thoroughly a bird by its feathers; but it is much
more
properly a reptile in the fact that the region which represents the
hand
has separate bones, with claws resembling those which terminate
the
fore-limb of a reptile. Moreover, it had a long reptile-like tail with
a
fringe of feathers on each side; while, in all true birds
hitherto
known, the tail is relatively short, and the vertebræ which
constitute
its skeleton are generally peculiarly
modified.
[Illustration: FIG. 5.--ICHTHYORNIS DISPAR
(Marsh).
(Side and upper views of half the lower jaw; and side and end
views of a
vertebra.)]
Like the _Anoplotherium_ and the
_Palæotherium_, therefore,
_Archaopteryx_ tends to fill up the interval
between groups which, in
the existing world, are widely separated, and to
destroy the value of
the definitions of zoological groups based upon our
knowledge of
existing forms. And such cases as these constitute evidence in
favour of
evolution, in so far as they prove that, in former periods of
the
world's history, there were animals which overstepped the bounds
of
existing groups, and tended to merge them into larger assemblages.
They
show that animal organisation is more flexible than our knowledge
of
recent forms might have led us to believe; and that many structural
permutations and combinations, of which the present world gives us no
indication, may nevertheless have existed. |
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