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Thomas Henry Huxley - Lectures and Essays

T >> Thomas Henry Huxley >> Lectures and Essays




There are forty-four teeth. The incisors have no strong pit. The canines
seem to have been well developed in both sexes. The first of the seven
grinders, which, as I have said, is frequently absent, and, when it does
exist, is small in the horse, is a good-sized and permanent tooth, while
the grinder which follows it is but little larger than the hinder ones.
The crowns of the grinders are short, and though the fundamental pattern
of the horse-tooth is discernible, the front and back ridges are less
curved, the accessory pillars are wanting, and the valleys, much
shallower, are not filled up with cement.

Seven years ago, when I happened to be looking critically into the
bearing of palaeontological facts upon the doctrine of evolution, it
appeared to me that the _Anchitherium_, the _Hipparion_, and the modern
horses, constitute a series in which the modifications of structure
coincide with the order of chronological occurrence, in the manner in
which they must coincide, if the modern horses really are the result of
the gradual metamorphosis, in the course of the Tertiary epoch, of a
less specialised ancestral form. And I found by correspondence with the
late eminent French anatomist and palaeontologist, M. Lartet, that he had
arrived at the same conclusion from the same data.

That the _Anchitherium_ type had become metamorphosed into the
_Hipparion_ type, and the latter into the _Equine_ type, in the course
of that period of time which is represented by the latter half of the
Tertiary deposits, seemed to me to be the only explanation of the facts
for which there was even a shadow of probability.[3]

And, hence, I have ever since held that these facts afford evidence of
the occurrence of evolution, which, in the sense already defined, may be
termed demonstrative.

All who have occupied themselves with the structure of _Anchitherium_,
from Cuvier onwards, have acknowledged its many points of likeness to a
well-known genus of extinct Eocene mammals, _Palaeotherium_. Indeed, as
we have seen, Cuvier regarded his remains of _Anchitherium_ as those of
a species of _Palaeotherium_. Hence, in attempting to trace the pedigree
of the horse beyond the Miocene epoch and the Anchitheroid form, I
naturally sought among the various species of Palaeotheroid animals for
its nearest ally, and I was led to conclude that the _Palaeotherium
minus_ (_Plagiolophus_) represented the next step more nearly than any
form then known.

I think that this opinion was fully justifiable; but the progress of
investigation has thrown an unexpected light on the question, and has
brought us much nearer than could have been anticipated to a knowledge
of the true series of the progenitors of the horse.

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

Professor Marsh's kindness has enabled me to put before you a diagram,
every figure in which is an actual representation of some specimen which
is to be seen at Yale at this present time (Fig. 9).

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

[Illustration: FIG. 9.]

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

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

Thus, thanks to these important researches, it has become evident that,
so far as our present knowledge extends, the history of the horse-type
is exactly and precisely that which could have been predicted from a
knowledge of the principles of evolution. And the knowledge we now
possess justifies us completely in the anticipation, that when the still
lower Eocene deposits, and those which belong to the cretaceous epoch,
have yielded up their remains of ancestral equine animals, we shall
find, first, a form with four complete toes and a rudiment of the
innermost or first digit in front, with probably a rudiment of the fifth
digit in the hind foot;[4] while, in still older forms, the series of
the digits will be more and more complete, until we come to the
five-toed animals, in which, if the doctrine of evolution is well
founded, the whole series must have taken its orgin.

That is what I mean by demonstrative evidence of evolution. An inductive
hypothesis is said to be demonstrated when the facts are shown to be in
entire accordance with it. If that is not scientific proof, there are no
merely inductive conclusions which can be said to be proved. And the
doctrine of evolution, at the present time, rests upon exactly as secure
a foundation as the Copernican theory of the motions of the heavenly
bodies did at the time of its promulgation. Its logical basis is
precisely of the same character--the coincidence of the observed facts
with theoretical requirements.

The only way of escape, if it be a way of escape, from the conclusions
which I have just indicated, is the supposition that all these different
equine forms have been created separately at separate epochs of time;
and, I repeat, that of such an hypothesis as this there neither is, nor
can be, any scientific evidence; and, assuredly so far as I know, there
is none which is supported, or pretends to be supported, by evidence or
authority of any other kind. I can but think that the time will come
when such suggestions as these, such obvious attempts to escape the
force of demonstration, will be put upon the same footing as the
supposition made by some writers, who are I believe not completely
extinct at present, that fossils are mere simulacra, are no indications
of the former existence of the animals to which they seem to belong; but
that they are either sports of Nature, or special creations,
intended--as I heard suggested the other day--to test our faith.

In fact, the whole evidence is in favour of evolution, and there is none
against it. And I say this, although perfectly well aware of the seeming
difficulties which have been built up upon what appears to the
uninformed to be a solid foundation. I meet constantly with the argument
that the doctrine of evolution cannot be well founded, because it
requires the lapse of a very vast period of time; while the duration of
life upon the earth thus implied is inconsistent with the conclusions
arrived at by the astronomer and the physicist. I may venture to say
that I am familiar with those conclusions, inasmuch as some years ago,
when President of the Geological Society of London, I took the liberty
of criticising them, and of showing in what respects, as it appeared to
me, they lacked complete and thorough demonstration. But, putting that
point aside, suppose that, as the astronomers, or some of them, and some
physical philosophers, tell us, it is impossible that life could have
endured upon the earth for as long a period as is required by the
doctrine of evolution--supposing that to be proved--I desire to be
informed, what is the foundation for the statement that evolution does
require so great a time? The biologist knows nothing whatever of the
amount of time which may be required for the process of evolution. It is
a matter of fact that the equine forms which I have described to you
occur, in the order stated, in the Tertiary formations. But I have not
the slightest means of guessing whether it took a million of years, or
ten millions, or a hundred millions, or a thousand millions of years, to
give rise to that series of changes. A biologist has no means of
arriving at any conclusion as to the amount of time which may be needed
for a certain quantity of organic change. He takes his time from the
geologist. The geologist, considering the rate at which deposits are
formed and the rate at which denudation goes on upon the surface of the
earth, arrives at more or less justifiable conclusions as to the time
which is required for the deposit of a certain thickness of rocks; and
if he tells me that the Tertiary formations required 500,000,000 years
for their deposit, I suppose he has good ground for what he says, and I
take that as a measure of the duration of the evolution of the horse
from the _Orohippus_ up to its present condition. And, if he is right,
undoubtedly evolution is a very slow process and requires a great deal
of time. But suppose, now, that an astronomer or a physicist--for
instance, my friend Sir William Thomson--tells me that my geological
authority is quite wrong; and that he has weighty evidence to show that
life could not possibly have existed upon the surface of the earth
500,000,000 years ago, because the earth would have then been too hot to
allow of life, my reply is: "That is not my affair; settle that with the
geologist, and when you have come to an agreement among yourselves I
will adopt your conclusion." We take our time from the geologists and
physicists; and it is monstrous that having taken our time from the
physical philosopher's clock, the physical philosopher should turn round
upon us, and say we are too fast or too slow. What we desire to know is,
is it a fact that evolution took place? As to the amount of time which
evolution may have occupied, we are in the hands of the physicist and
the astronomer, whose business it is to deal with those questions.

I have now, ladies and gentlemen, arrived at the conclusion of the task
which I set before myself when I undertook to deliver these lectures. My
purpose has been, not to enable those among you who have paid no
attention to these subjects before, to leave this room in a condition to
decide upon the validity or the invalidity of the hypothesis of
evolution; but I have desired to put before you the principles upon
which all hypotheses respecting the history of Nature must be judged;
and furthermore, to make apparent the nature of the evidence and the
amount of cogency which is to be expected and may be obtained from it.
To this end, I have not hesitated to regard you as genuine students and
persons desirous of knowing the truth. I have not shrunk from taking you
through long discussions, that I fear may have sometimed tried your
patience; and I have inflicted upon you details which were
indispensable, but which may well have been wearisome. But I shall
rejoice--I shall consider that I have done you the greatest service
which it was in my power to do--if I have thus convinced you that the
great question which we have been discussing is not one to be dealt with
by rhetorical flourishes, or by loose and superficial talk; but that it
requires the keen attention of the trained intellect and the patience of
the accurate observer.




ON THE PHYSICAL BASIS OF LIFE

[1868]


In order to make the title of this discourse generally intelligible, I
have translated the term "Protoplasm," which is the scientific name of
the substance of which I am about to speak, by the words "the physical
basis of life." I suppose that, to many, the idea that there is such a
thing as a physical basis, or matter, of life may be novel--so widely
spread is the conception of life as a something which works through
matter, but is independent of it; and even those who are aware that
matter and life are inseparably connected, may not be prepared for the
conclusion plainly suggested by the phrase, "_the_ physical basis or
matter of life," that there is some one kind of matter which is common
to all living beings, and that their endless diversities are bound
together by a physical, as well as an ideal, unity. In fact, when first
apprehended, such a doctrine as this appears almost shocking to common
sense.

What, truly, can seem to be more obviously different from one another,
in faculty, in form, and in substance, than the various kinds of living
beings? What community of faculty can there be between the
brightly-coloured lichen, which so nearly resembles a mere mineral
incrustation of the bare rock on which it grows, and the painter, to
whom it is instinct with beauty, or the botanist, whom it feeds with
knowledge?

Again, think of the microscopic fungus--a mere infinitesimal ovoid
particle, which finds space and duration enough to multiply into
countless millions in the body of a living fly; and then of the wealth
of foliage, the luxuriance of flower and fruit, which lies between this
bald sketch of a plant and the giant pine of California, towering to the
dimensions of a cathedral spire, or the Indian fig, which covers acres
with its profound shadow, and endures while nations and empires come and
go around its vast circumference. Or, turning to the other half of the
world of life, picture to yourselves the great Finner whale, hugest of
beasts that live, or have lived, disporting his eighty or ninety feet of
bone, muscle, and blubber, with easy roll, among waves in which the
stoutest ship that ever left dockyard would flounder hopelessly; and
contrast him with the invisible animalcules--mere gelatinous specks,
multitudes of which could, in fact, dance upon the point of a needle
with the same ease as the angels of the Schoolmen could, in imagination.
With these images before your minds, you may well ask, what community of
form, or structure, is there between the animalcule and the whale; or
between the fungus and the fig-tree? And, _a fortiori_, between all
four?

Finally, if we regard substance, or material composition, what hidden
bond can connect the flower which a girl wears in her hair and the blood
which courses through her youthful veins; or, what is there in common
between the dense and resisting mass of the oak, or the strong fabric of
the tortoise, and those broad disks of glassy jelly which may be seen
pulsating through the waters of a calm sea, but which drain away to
mere films in the hand which raises them out of their element?

Such objections as these must, I think, arise in the mind of every one
who ponders, for the first time, upon the conception of a single
physical basis of life underlying all the diversities of vital
existence; but I propose to demonstrate to you that, notwithstanding
these apparent difficulties, a threefold unity--namely, a unity of
power or faculty, a unity of form, and a unity of substantial
composition--does pervade the whole living world.

No very abstruse argumentation is needed, in the first place, to prove
that the powers, or faculties, of all kinds of living matter, diverse as
they may be in degree, are substantially similar in kind.

Goethe has condensed a survey of all powers of mankind into the
well-known epigram:--

"Warum treibt sich das Volk so und schreit?
Es will sich ernaehren
Kinder zeugen, und die naehren so gut es vermag.

* * * * *

Weiter bringt es kein Mensch, stell' er
sich wie er auch will."

In physiological language this means, that all the multifarious and
complicated activities of man are comprehensible under three categories.
Either they are immediately directed towards the maintenance and
development of the body, or they effect transitory changes in the
relative positions of parts of the body, or they tend towards the
continuance of the species. Even those manifestations of intellect, of
feeling, and of will, which we rightly name the higher faculties, are
not excluded from this classification, inasmuch as to every one but the
subject of them, they are known only as transitory changes in the
relative positions of parts of the body. Speech, gesture, and every
other form of human action are, in the long run, resolvable into
muscular contraction, and muscular contraction is but a transitory
change in the relative positions of the parts of a muscle. But the
scheme which is large enough to embrace the activities of the highest
form of life, covers all those of the lower creatures. The lowest plant,
or animalcule, feeds, grows, and reproduces its kind. In addition, all
animals manifest those transitory changes of form which we class under
irritability and contractility; and, it is more than probable, that when
the vegetable world is thoroughly explored, we shall find all plants in
possession of the same powers, at one time or other of their existence.

I am not now alluding to such phaenomena, at once rare and conspicuous,
as those exhibited by the leaflets of the sensitive plants, or the
stamens of the barberry, but to much more widely spread, and at the same
time, more subtle and hidden, manifestations of vegetable contractility.
You are doubtless aware that the common nettle owes its stinging
property to the innumerable stiff and needle-like, though exquisitely
delicate, hairs which cover its surface. Each stinging-needle tapers
from a broad base to a slender summit, which, though rounded at the end,
is of such microscopic fineness that it readily penetrates, and breaks
off in, the skin. The whole hair consists of a very delicate outer case
of wood, closely applied to the inner surface of which is a layer of
semi-fluid matter, full of innumerable granules of extreme minuteness.
This semi-fluid lining is protoplasm, which thus constitutes a kind of
bag, full of a limpid liquid, and roughly corresponding in form with the
interior of the hair which it fills. When viewed with a sufficiently
high magnifying power, the protoplasmic layer of the nettle hair is seen
to be in a condition of unceasing activity. Local contractions of the
whole thickness of its substance pass slowly and gradually from point to
point, and give rise to the appearance of progressive waves, just as the
bending of successive stalks of corn by a breeze produces the apparent
billows of a cornfield.

But, in addition to these movements, and independently of them, the
granules are driven, in relatively rapid streams, through channels in
the protoplasm which seem to have a considerable amount of persistence.
Most commonly, the currents in adjacent parts of the protoplasm take
similar directions; and, thus, there is a general stream up one side of
the hair and down the other. But this does not prevent the existence of
partial currents which take different routes; and sometimes trains of
granules may be seen coursing swiftly in opposite directions within a
twenty-thousandth of an inch of one another; while, occasionally,
opposite streams come into direct collision, and, after a longer or
shorter struggle, one predominates. The cause of these currents seems to
lie in contractions of the protoplasm which bounds the channels in which
they flow, but which are so minute that the best microscopes show only
their effects, and not themselves.

The spectacle afforded by the wonderful energies prisoned within the
compass of the microscopic hair of a plant, which we commonly regard as
a merely passive organism, is not easily forgotten by one who has
watched its display, continued hour after hour, without pause or sign of
weakening. The possible complexity of many other organic forms,
seemingly as simple as the protoplasm of the nettle, dawns upon one; and
the comparison of such a protoplasm to a body with an internal
circulation, which has been put forward by an eminent physiologist,
loses much of its startling character. Currents similar to those of the
hairs of the nettle have been observed in a great multitude of very
different plants, and weighty authorities have suggested that they
probably occur, in more or less perfection, in all young vegetable
cells. If such be the case, the wonderful noonday silence of a tropical
forest is, after all, due only to the dulness of our hearing; and could
our ears catch the murmur of these tiny Maelstroms, as they whirl in the
innumerable myriads of living cells which constitute each tree, we
should be stunned, as with the roar of a great city.

Among the lower plants, it is the rule rather than the exception, that
contractility should be still more openly manifested at some periods of
their existence. The protoplasm of _Algae_ and _Fungi_ becomes, under
many circumstances, partially, or completely, freed from its woody case,
and exhibits movements of its whole mass, or is propelled by the
contractility of one, or more, hair-like prolongations of its body,
which are called vibratile cilia. And, so far as the conditions of the
manifestation of the phaenomena of contractility have yet been studied,
they are the same for the plant as for the animal. Heat and electric
shocks influence both, and in the same way, though it may be in
different degrees. It is by no means my intention to suggest that there
is no difference in faculty between the lowest plant and the highest, or
between plants and animals. But the difference between the powers of the
lowest plant, or animal, and those of the highest, is one of degree, not
of kind, and depends, as Milne-Edwards long ago so well pointed out,
upon the extent to which the principle of the division of labour is
carried out in the living economy. In the lowest organism all parts are
competent to perform all functions, and one and the same portion of
protoplasm may successfully take on the function of feeding, moving, or
reproducing apparatus. In the highest, on the contrary, a great number
of parts combine to perform each function, each part doing its allotted
share of the work with great accuracy and efficiency, but being useless
for any other purpose.

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