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Though we are quite clearabout the constancy of the order of Nature,at the present time, and in the present state of things, it by no means necessarilyfollows that we are justified in ex

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Title: American Addresses, with aLecture on the Study of Biology

Author: Thomas Henry Huxley

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ADDRESSES, WITH A LECTURE ON

THE STUDY OF BIOLOGY***

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Weatherford, and the Project Gutenberg

Online Distributed Proofreading Team (http://www.pgdp.net)

AMERICAN ADDRESSES,

regulæ, secundum quas

omnia fiunt et ex unis

formis in alias mutantur,

sunt ubique et semper

eadem."

Ethices, Pars

tertia,Præfatio

London:

MACMILLAN AND CO

1877

LONDON:

R CLAY, SONS, AND TAYLOR, PRINTERS,

BREAD STREET HILL, QUEEN VICTORIA STREET.

I THREE LECTURES ON EVOLUTION. (New York,September 18, 20, 22, 1876)

LECTURE I. The Three Hypothesesrespecting The History ofNature

LECTURE II. The Hypothesis ofEvolution The Neutral and theFavourable Evidence

LECTURE III. The DemonstrativeEvidence of Evolution

OCCASION OF THE OPENING

OF THE JOHN HOPKINS

UNIVERSITY (Baltimore,September 12, 1876)

BIOLOGY, IN CONNECTION WITH THE LOAN COLLECTION OF SCIENTIFIC APPARATUS. (South KensingtonMuseum, December 16, 1876)

NEW YORK.

LECTURES ON EVOLUTION.

a matter of the deepest interest to all of usthat we should form just conceptions ofthe constitution of that system and of itspast history With relation to this universe,man is, in extent, little more than amathematical point; in duration but a

fleeting shadow; he is a mere reed shaken

in the winds of force But, as Pascal longago remarked, although a mere reed, he is

a thinking reed; and in virtue of thatwonderful capacity of thought, he has thepower of framing for himself a symbolicconception of the universe, which,although doubtless highly imperfect andinadequate as a picture of the great whole,

is yet sufficient to serve him as a chart forthe guidance of his practical affairs It hastaken long ages of toilsome and oftenfruitless labour to enable man to looksteadily at the shifting scenes of thephantasmagoria of Nature, to notice what

is fixed among her fluctuations, and what

irregularities; and it is only comparativelylately, within the last few centuries, that

the conception of a universal order and of

a definite course of things, which we termthe course of Nature, has emerged

But, once originated, the conception of theconstancy of the order of Nature hasbecome the dominant idea of modernthought To any person who is familiarwith the facts upon which that conception

is based, and is competent to estimatetheir significance, it has ceased to beconceivable that chance should have anyplace in the universe, or that events shoulddepend upon any but the natural sequence

of cause and effect We have come to lookupon the present as the child of the pastand as the parent of the future; and, as wehave excluded chance from a place in theuniverse, so we ignore, even as a

possibility, the notion of any interferencewith the order of Nature Whatever may

be men's speculative doctrines, it is quitecertain, that every intelligent personguides his life and risks his fortune uponthe belief that the order of Nature isconstant, and that the chain of naturalcausation is never broken

In fact, no belief which we entertain has

so complete a logical basis as that towhich I have just referred It tacitlyunderlies every process of reasoning; it isthe foundation of every act of the will It isbased 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 aprobable belief, and that our widest andsafest generalizations are simplystatements of the highest degree ofprobability Though we are quite clearabout the constancy of the order of Nature,

at the present time, and in the present state

of things, it by no means necessarilyfollows that we are justified in expandingthis generalisation into the infinite past,and in denying, absolutely, that there mayhave been a time when Nature did notfollow a fixed order, when the relations ofcause and effect were not definite, andwhen extra-natural agencies interferedwith the general course of Nature.Cautious men will allow that a universe

so different from that which we know mayhave existed; just as a very candid thinker

may admit that a world in which two andtwo do not make four, and in which twostraight lines do inclose a space, mayexist But the same caution which forcesthe admission of such possibilitiesdemands a great deal of evidence before itrecognises them to be anything moresubstantial And when it is asserted that,

so many thousand years ago, eventsoccurred in a manner utterly foreign to andinconsistent with the existing laws ofNature, men, who without beingparticularly cautious, are simply honestthinkers, unwilling to deceive themselves

or delude others, ask for trustworthyevidence 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 thesame way as the solution of any otherhistorical problem.

So far as I know, there are only threehypotheses which ever have beenentertained, or which well can beentertained, respecting the past history ofNature I will, in the first place, state thehypotheses, and then I will consider whatevidence bearing upon them is in ourpossession, and by what light of criticismthat evidence is to be interpreted

Upon the first hypothesis, the assumption

is, that phenomena of Nature similar tothose exhibited by the present world have

always existed; in other words, that theuniverse has existed from all eternity inwhat may be broadly termed its presentcondition.

The second hypothesis is, that the presentstate of things has had only a limitedduration; and that, at some period in thepast, a condition of the world, essentiallysimilar to that which we now know, cameinto existence, without any precedentcondition from which it could havenaturally proceeded The assumption thatsuccessive states of Nature have arisen,each without any relation of naturalcausation to an antecedent state, is a meremodification of this second hypothesis.The third hypothesis also assumes that thepresent state of things has had but a

limited duration; but it supposes that thisstate has been evolved by a naturalprocess from an antecedent state, and thatfrom another, and so on; and, on thishypothesis, the attempt to assign any limit

to the series of past changes is, usually,given up

It is so needful to form clear and distinctnotions of what is really meant by each ofthese hypotheses that I will ask you toimagine what, according to each, wouldhave been visible to a spectator of theevents which constitute the history of theearth On the first hypothesis, however farback in time that spectator might beplaced, 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 theancestors of those which now live, andsimilar to them; the plants, in like manner,would be such as we know; and themountains, plains, and waters wouldforeshadow the salient features of ourpresent land and water This view washeld more or less distinctly, sometimescombined with the notion of recurrentcycles of change, in ancient times; and itsinfluence has been felt down to the presentday It is worthy of remark that it is ahypothesis which is not inconsistent withthe doctrine of Uniformitarianism, withwhich geologists are familiar Thatdoctrine was held by Hutton, and in hisearlier days by Lyell Hutton was struck

by the demonstration of astronomers thatthe perturbations of the planetary bodies,

however great they may be, yet sooner orlater right themselves; and that the solarsystem possesses a self-adjusting power

by which these aberrations are all broughtback to a mean condition Hutton imaginedthat the like might be true of terrestrialchanges; although no one recognised moreclearly than he the fact that the dry land isbeing constantly washed down by rain andrivers and deposited in the sea; and thatthus, in a longer or shorter time, theinequalities of the earth's surface must belevelled, and its high lands brought down

to the ocean But, taking into account theinternal forces of the earth, which,upheaving the sea-bottom give rise to newland, he thought that these operations ofdegradation and elevation mightcompensate each other; and that thus, for

any assignable time, the general features

of our planet might remain what they are

circumstances, there need be no limit tothe propagation of animals and plants, it isclear that the consistent working-out of theuniformitarian idea might lead to theconception of the eternity of the world.Not that I mean to say that either Hutton orLyell held this conception—assuredly not;they would have been the first to repudiate

it Nevertheless, the logical development

of their arguments tends directly towardsthis hypothesis

The second hypothesis supposes that thepresent order of things, at some no veryremote time, had a sudden origin, and thatthe world, such as it now is, had chaos for

its phenomenal antecedent That is thedoctrine which you will find stated mostfully and clearly in the immortal poem of

Commedia—Paradise Lost I believe it is

largely to the influence of that remarkablework, combined with the daily teachings

to which we have all listened in ourchildhood, that this hypothesis owes itsgeneral wide diffusion as one of thecurrent beliefs of English-speakingpeople If you turn to the seventh book of

Paradise Lost, you will find there stated

the hypothesis to which I refer, which isbriefly this: That this visible universe ofours came into existence at no greatdistance of time from the present; and thatthe parts of which it is composed madetheir appearance, in a certain definite

order, in the space of six natural days, insuch a manner that, on the first of thesedays, light appeared; that, on the second,the firmament, or sky, separated thewaters above, from the waters beneath thefirmament; that, on the third day, thewaters drew away from the dry land, andupon it a varied vegetable life, similar tothat which now exists, made itsappearance; that the fourth day wassignalised by the apparition of the sun, thestars, the moon, and the planets; that, onthe fifth day, aquatic animals originatedwithin the waters; that, on the sixth day,the earth gave rise to our four-footedterrestrial creatures, and to all varieties ofterrestrial animals except birds, whichhad appeared on the preceding day; and,finally, that man appeared upon the earth,

and the emergence of the universe fromchaos was finished Milton tells us,without the least ambiguity, what aspectator of these marvellous occurrenceswould have witnessed I doubt not that hispoem is familiar to all of you, but I shouldlike to recall one passage to your minds,

in order that I may be justified in what Ihave said regarding the perfectly concrete,definite picture of the origin of the animalworld which Milton draws He says:—

"The sixth, and of creation last,

aroseWith evening harps 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, straightOpening her fertile womb, teemed

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 freeHis hinder parts—then springs, as

broke from bonds,

And rampant shakes his brinded

mane; the ounce,

The libbard, and the tiger, as the

Bore up his branching head;

scarce from his mouldBehemoth, 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 thisstatement, nor as to what a man of Milton'sgenius expected would have been actuallyvisible to an eye-witness of this mode oforigination of living things

The third hypothesis, or the hypothesis ofevolution, supposes that, at anycomparatively late period of past time,our imaginary spectator would meet with

a state of things very similar to that whichnow obtains; but that the likeness of thepast to the present would graduallybecome less and less, in proportion to theremoteness of his period of observationfrom the present day; that the existingdistribution of mountains and plains, ofrivers and seas, would show itself to bethe product of a slow process of naturalchange operating upon more and morewidely different antecedent conditions ofthe mineral framework of the earth; until,

at length, in place of that framework, hewould behold only a vast nebulous mass,representing the constituents of the sun and

of the planetary bodies Preceding theforms of life which now exist, ourobserver would see animals and plants notidentical with them, but like them;

increasing their differences with theirantiquity and, at the same time, becomingsimpler and simpler; until, finally, theworld of life would present nothing butthat undifferentiated protoplasmic matterwhich, so far as our present knowledgegoes, is the common foundation of all vitalactivity.

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 a natural process,"and "This is not a natural process;" butthat the whole might be compared to thatwonderful process of development whichmay be seen going on every day under oureyes, in virtue of which there arises, out of

homogeneous substance which we call anegg, the complicated organization of one

of the higher animals That, in a fewwords, is what is meant by the hypothesis

of evolution

I have already suggested that in dealingwith these three hypotheses, inendeavouring to form a judgment as towhich of them is the more worthy ofbelief, or whether none is worthy of belief

—in which case our condition of mindshould be that suspension of judgmentwhich is so difficult to all but trainedintellects—we should be indifferent to all

à priori considerations The question is a

question of historical fact The universehas come into existence somehow orother, and the problem is, whether it cameinto existence in one fashion, or whether itcame into existence in another; and, as anessential preliminary to further discussion,permit me to say two or three words as tothe nature and the kinds of historicalevidence.

The evidence as to the occurrence of anyevent in past time may be ranged undertwo heads which, for convenience' sake, Iwill speak of as testimonial evidence and

as circumstantial evidence By testimonialevidence I mean human testimony; and bycircumstantial evidence I mean evidencewhich is not human testimony Let meillustrate by a familiar example what I

understand by these two kinds ofevidence, and what is to be saidrespecting their value.

Suppose that a man tells you that he saw aperson strike another and kill him; that istestimonial evidence of the fact of murder.But it is possible to have circumstantialevidence of the fact of murder; that is tosay, you may find a man dying with awound upon his head having exactly theform and character of the wound which ismade by an axe, and, with due care intaking surrounding circumstances intoaccount, you may conclude with the utmostcertainty that the man has been murdered;that his death is the consequence of a blowinflicted by another man with thatimplement We are very much in the habit

of considering circumstantial evidence as

of less value than testimonial evidence,and it may be that, where thecircumstances are not perfectly clear andintelligible, it is a dangerous and unsafekind of evidence; but it must not beforgotten that, in many cases,circumstantial is quite as conclusive astestimonial evidence, and that, notunfrequently, it is a great deal weightierthan testimonial evidence For example,take the case to which I referred just now.The circumstantial evidence may be betterand more convincing than the testimonialevidence; for it may be impossible, underthe conditions that I have defined, tosuppose that the man met his death fromany cause but the violent blow of an axe

circumstantial evidence in favour of amurder having been committed, in thatcase, is as complete and as convincing asevidence can be It is evidence which isopen to no doubt and to no falsification.But the testimony of a witness is open tomultitudinous doubts He may have beenmistaken He may have been actuated bymalice It has constantly happened thateven an accurate man has declared that athing has happened in this, that, or theother way, when a careful analysis of thecircumstantial evidence has shown that itdid not happen in that way, but in someother way.

We may now consider the evidence infavour of or against the three hypotheses.Let me first direct your attention to what is

to be said about the hypothesis of theeternity of the state of things in which wenow live What will first strike you is, that

it is a hypothesis which, whether true orfalse, is not capable of verification by anyevidence For, in order to obtain eithercircumstantial or testimonial evidencesufficient to prove the eternity of duration

of the present state of nature, you musthave an eternity of witnesses or an infinity

of circumstances, and neither of these isattainable It is utterly impossible thatsuch evidence should be carried beyond acertain point of time; and all that could besaid, at most, would be, that so far as theevidence could be traced, there wasnothing to contradict the hypothesis Butwhen you look, not to the testimonialevidence—which, considering the relative

insignificance of the antiquity of humanrecords, might not be good for much inthis case—but to the circumstantialevidence, then you find that this hypothesis

is absolutely incompatible with suchevidence as we have; which is of so plainand so simple a character that it isimpossible in any way to escape from theconclusions which it forces upon us

You are, doubtless, all aware that theouter substance of the earth, which alone

is accessible to direct observation, is not

of a homogeneous character, but that it ismade up of a number of layers or strata,the titles of the principal groups of whichare placed upon the accompanyingdiagram Each of these groups represents

a number of beds of sand, of stone, of

clay, of slate, and of various othermaterials.

Fig 1.—Ideal Section of the Crust of the

Earth

On careful examination, it is found that thematerials of which each of these layers ofmore or less hard rock are composed are,for the most part, of the same nature asthose which are at present being formedunder known conditions on the surface ofthe earth For example, the chalk, whichconstitutes a great part of the Cretaceousformation in some parts of the world, ispractically identical in its physical andchemical characters with a substancewhich is now being formed at the bottom

of the Atlantic Ocean, and covers anenormous area; other beds of rock arecomparable with the sands which arebeing formed upon sea-shores, packedtogether, and so on Thus, omitting rocks

of igneous origin, it is demonstrable that