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The Darwinian Hypothesis*

by Thomas H. Huxley



DARWIN ON THE ORIGIN OF SPECIES.


THERE is a growing immensity in the speculations of science to which no
human thing or thought at this day is comparable. Apart from the
results which science brings us home and securely harvests, there is an
expansive force and latitude in its tentative efforts, which lifts us
out of ourselves and transfigures our mortality. We may have a
preference for moral themes, like the Homeric sage, who had seen and
known much:—

     “Cities of men
     And manners, climates, councils, governments”;

yet we must end by confession that

     “The windy ways of men
     Are but dust which rises up
     And is lightly laid again,”

in comparison with the work of nature, to which science testifies, but
which has no boundaries in time or space to which science can
approximate.

There is something altogether out of the reach of science, and yet the
compass of science is practically illimitable. Hence it is that from
time to time we are startled and perplexed by theories which have no
parallel in the contracted moral world; for the generalizations of
science sweep on in ever-widening circles, and more aspiring flights,
through a limitless creation. While astronomy, with its telescope,
ranges beyond the known stars, and physiology, with its microscope, is
subdividing infinite minutiae, we may expect that our historic
centuries may be treated as inadequate counters in the history of the
planet on which we are placed. We must expect new conceptions of the
nature and relations of its denizens, as science acquires the materials
for fresh generalizations; nor have we occasion for alarms if a highly
advanced knowledge, like that of the eminent Naturalist before us,
confronts us with an hypothesis as vast as it is novel. This hypothesis
may or may not be sustainable hereafter; it may give way to something
else, and higher science may reverse what science has here built up
with so much skill and patience, but its sufficiency must be tried by
the tests of science alone, if we are to maintain our position as the
heirs of Bacon and the acquitters of Galileo. We must weigh this
hypothesis strictly in the controversy which is coming, by the only
tests which are appropriate, and by no others whatsoever.

The hypothesis to which we point, and of which the present work of Mr.
Darwin is but the preliminary outline, may be stated in his own
language as follows:—“Species originated by means of natural selection,
or through the preservation of the favoured races in the struggle for
life.” To render this thesis intelligible, it is necessary to interpret
its terms. In the first place, what is a species? The question is a
simple one, but the right answer to it is hard to find, even if we
appeal to those who should know most about it. It is all those animals
or plants which have descended from a single pair of parents; it is the
smallest distinctly definable group of living organisms; it is an
eternal and immutable entity; it is a mere abstraction of the human
intellect having no existence in nature. Such are a few of the
significations attached to this simple word which may be culled from
authoritative sources; and if, leaving terms and theoretical subtleties
aside, we turn to facts and endeavour to gather a meaning for
ourselves, by studying the things to which, in practice, the name of
species is applied, it profits us little. For practice varies as much
as theory. Let the botanist or the zoologist examine and describe the
productions of a country, and one will pretty certainly disagree with
the other as to the number, limits, and definitions of the species into
which he groups the very same things. In these islands, we are in the
habit of regarding mankind as of one species, but a fortnight’s steam
will land us in a country where divines and savants, for once in
agreement, vie with one another in loudness of assertion, if not in
cogency of proof, that men are of different species; and, more
particularly, that the species negro is so distinct from our own that
the Ten Commandments have actually no reference to him. Even in the
calm region of entomology, where, if anywhere in this sinful world,
passion and prejudice should fail to stir the mind, one learned
coleopterist will fill ten attractive volumes with descriptions of
species of beetles, nine-tenths of which are immediately declared by
his brother beetle-mongers to be no species at all.

The truth is that the number of distinguishable living creatures almost
surpasses imagination. At least a hundred thousand such kinds of
insects alone have been described and may be identified in collections,
and the number of separable kinds of living things is underestimated at
half a million. Seeing that most of these obvious kinds have their
accidental varieties, and that they often shade into others by
imperceptible degrees, it may well be imagined that the task of
distinguishing between what is permanent and what fleeting, what is a
species and what a mere variety, is sufficiently formidable.

But is it not possible to apply a test whereby a true species may be
known from a mere variety? Is there no criterion of species? Great
authorities affirm that there is—that the unions of members of the same
species are always fertile, while those of distinct species are either
sterile, or their offspring, called hybrids, are so. It is affirmed not
only that this is an experimental fact, but that it is a provision for
the preservation of the purity of species. Such a criterion as this
would be invaluable; but, unfortunately, not only is it not obvious how
to apply it in the great majority of cases in which its aid is needed,
but its general validity is stoutly denied. The Hon. and Rev. Mr.
Herbert, a most trustworthy authority, not only asserts as the result
of his own observations and experiments that many hybrids are quite as
fertile as the parent species, but he goes so far as to assert that the
particular plant _Crinum capense_ is much more fertile when crossed by
a distinct species than when fertilised by its proper pollen! On the
other hand, the famous Gaertner, though he took the greatest pains to
cross the primrose and the cowslip, succeeded only once or twice in
several years; and yet it is a well-established fact that the primrose
and the cowslip are only varieties of the same kind of plant. Again,
such cases as the following are well established. The female of species
A, if crossed with the male of species B, is fertile; but, if the
female of B is crossed with the male of A, she remains barren. Facts of
this kind destroy the value of the supposed criterion.

If, weary of the endless difficulties involved in the determination of
species, the investigator, contenting himself with the rough practical
distinction of separable kinds, endeavours to study them as they occur
in nature—to ascertain their relations to the conditions which surround
them, their mutual harmonies and discordances of structure, the bond of
union of their parts and their past history, he finds himself,
according to the received notions, in a mighty maze, and with, at most,
the dimmest adumbration of a plan. If he starts with any one clear
conviction, it is that every part of a living creature is cunningly
adapted to some special use in its life. Has not his Paley told him
that that seemingly useless organ, the spleen, is beautifully adjusted
as so much packing between the other organs? And yet, at the outset of
his studies, he finds that no adaptive reason whatsoever can be given
for one-half of the peculiarities of vegetable structure; he also
discovers rudimentary teeth, which are never used, in the gums of the
young calf and in those of the foetal whale; insects which never bite
have rudimental jaws, and others which never fly have rudimental wings;
naturally blind creatures have rudimental eyes; and the halt have
rudimentary limbs. So, again, no animal or plant puts on its perfect
form at once, but all have to start from the same point, however
various the course which each has to pursue. Not only men and horses,
and cats and dogs, lobsters and beetles, periwinkles and mussels, but
even the very sponges and animalcules commence their existence under
forms which are essentially undistinguishable; and this is true of all
the infinite variety of plants. Nay, more, all living beings march side
by side along the high road of development, and separate the later the
more like they are; like people leaving church, who all go down the
aisle, but having reached the door some turn into the parsonage, others
go down the village, and others part only in the next parish. A man in
his development runs for a little while parallel with, though never
passing through, the form of the meanest worm, then travels for a space
beside the fish, then journeys along with the bird and the reptile for
his fellow travellers; and only at last, after a brief companionship
with the highest of the four-footed and four-handed world, rises into
the dignity of pure manhood. No competent thinker of the present day
dreams of explaining these indubitable facts by the notion of the
existence of unknown and undiscoverable adaptations to purpose. And we
would remind those who, ignorant of the facts, must be moved by
authority, that no one has asserted the incompetence of the doctrine of
final causes, in its application to physiology and anatomy, more
strongly than our own eminent anatomist, Professor Owen, who, speaking
of such cases, says (_On the Nature of Limbs_, pp. 39, 40): “I think it
will be obvious that the principle of final adaptations fails to
satisfy all the conditions of the problem.”

But, if the doctrine of final causes will not help us to comprehend the
anomalies of living structure, the principle of adaptation must surely
lead us to understand why certain living beings are found in certain
regions of the world and not in others. The palm, as we know, will not
grow in our climate, nor the oak in Greenland. The white bear cannot
live where the tiger thrives, nor _vice versa_, and the more the
natural habits of animal and vegetable species are examined, the more
do they seem, on the whole, limited to particular provinces. But when
we look into the facts established by the study of the geographical
distribution of animals and plants it seems utterly hopeless to attempt
to understand the strange and apparently capricious relations which
they exhibit. One would be inclined to suppose _a priori_ that every
country must be naturally peopled by those animals that are fittest to
live and thrive in it. And yet how, on this hypothesis, are we to
account for the absence of cattle in the Pampas of South America, when
those parts of the New World were discovered? It is not that they were
unfit for cattle, for millions of cattle now run wild there; and the
like holds good of Australia and New Zealand. It is a curious
circumstance, in fact, that the animals and plants of the Northern
Hemisphere are not only as well adapted to live in the Southern
Hemisphere as its own autochthones, but are in many cases absolutely
better adapted, and so overrun and extirpate the aborigines. Clearly,
therefore, the species which naturally inhabit a country are not
necessarily the best adapted to its climate and other conditions. The
inhabitants of islands are often distinct from any other known species
of animal or plants (witness our recent examples from the work of Sir
Emerson Tennent, on Ceylon), and yet they have almost always a sort of
general family resemblance to the animals and plants of the nearest
mainland. On the other hand, there is hardly a species of fish, shell,
or crab common to the opposite sides of the narrow isthmus of Panama.
Wherever we look, then, living nature offers us riddles of difficult
solution, if we suppose that what we see is all that can be known of
it.

But our knowledge of life is not confined to the existing world.
Whatever their minor differences, geologists are agreed as to the vast
thickness of the accumulated strata which compose the visible part of
our earth, and the inconceivable immensity of the time of whose lapse
they are the imperfect, but the only accessible witnesses. Now,
throughout the greater part of this long series of stratified rocks are
scattered, sometimes very abundantly, multitudes of organic remains,
the fossilized exuviae of animals and plants which lived and died while
the mud of which the rocks are formed was yet soft ooze, and could
receive and bury them. It would be a great error to suppose that these
organic remains were fragmentary relics. Our museums exhibit fossil
shells of immeasurable antiquity, as perfect as the day they were
formed, whole skeletons without a limb disturbed—nay, the changed
flesh, the developing embryos, and even the very footsteps of primeval
organisms. Thus the naturalist finds in the bowels of the earth species
as well defined as, and in some groups of animals more numerous than,
those that breathe the upper air. But, singularly enough, the majority
of these entombed species are wholly distinct from those that now live.
Nor is this unlikeness without its rule and order. As a broad fact, the
further we go back in time the less the buried species are like
existing forms; and the further apart the sets of extinct creatures are
the less they are like one another. In other words, there has been a
regular succession of living beings, each younger set being in a very
broad and general sense somewhat more like those which now live.

It was once supposed that this succession had been the result of vast
successive catastrophes, destructions, and re-creations _en masse_; but
catastrophes are now almost eliminated from geological, or at least
palaeontological speculation; and it is admitted on all hands that the
seeming breaks in the chain of being are not absolute, but only
relative to our imperfect knowledge; that species have replaced
species, not in assemblages, but one by one; and that, if it were
possible to have all the phenomena of the past presented to us, the
convenient epochs and formations of the geologist, though having a
certain distinctness, would fade into one another with limits as
undefinable as those of the distinct and yet separable colours of the
solar spectrum.

Such is a brief summary of the main truths which have been established
concerning species. Are these truths ultimate and irresolvable facts,
or are their complexities and perplexities the mere expressions of a
higher law?

A large number of persons practically assume the former position to be
correct. They believe that the writer of the Pentateuch was empowered
and commissioned to teach us scientific as well as other truth, that
the account we find there of the creation of living things is simply
and literally correct, and that anything which seems to contradict it
is, by the nature of the case, false. All the phenomena which have been
detailed are, on this view, the immediate product of a creative fiat
and consequently are out of the domain of science altogether.

Whether this view prove ultimately to be true or false, it is, at any
rate, not at present supported by what is commonly regarded as logical
proof, even if it be capable of discussion by reason; and hence we
consider ourselves at liberty to pass it by, and to turn to those views
which profess to rest on a scientific basis only, and therefore admit
of being argued to their consequences. And we do this with the less
hesitation as it so happens that those persons who are practically
conversant with the facts of the case (plainly a considerable
advantage) have always thought fit to range themselves under the latter
category.

The majority of these competent persons have up to the present time
maintained two positions,—the first, that every species is, within
certain defined or definable limits, fixed and incapable of
modification; the second, that every species was originally produced by
a distinct creative act. The second position is obviously incapable of
proof or disproof, the direct operations of the Creator not being
subjects of science; and it must therefore be regarded as a corollary
from the first, the truth or falsehood of which is a matter of
evidence. Most persons imagine that the arguments in favour of it are
overwhelming; but to some few minds, and these, it must be confessed,
intellects of no small power and grasp of knowledge, they have not
brought conviction. Among these minds, that of the famous naturalist
Lamarck, who possessed a greater acquaintance with the lower forms of
life than any man of his day, Cuvier not excepted, and was a good
botanist to boot, occupies a prominent place.

Two facts appear to have strongly affected the course of thought of
this remarkable man—the one, that finer or stronger links of affinity
connect all living beings with one another, and that thus the highest
creature grades by multitudinous steps into the lowest; the other, that
an organ may be developed in particular directions by exerting itself
in particular ways, and that modifications once induced may be
transmitted and become hereditary. Putting these facts together,
Lamarck endeavoured to account for the first by the operation of the
second. Place an animal in new circumstances, says he, and its needs
will be altered; the new needs will create new desires, and the attempt
to gratify such desires will result in an appropriate modification of
the organs exerted. Make a man a blacksmith, and his brachial muscles
will develop in accordance with the demands made upon them, and in like
manner, says Lamarck, “the efforts of some short-necked bird to catch
fish without wetting himself have, with time and perseverance, given
rise to all our herons and long-necked waders.”

The Lamarckian hypothesis has long since been justly condemned, and it
is the established practice for every tyro to raise his heel against
the carcass of the dead lion. But it is rarely either wise or
instructive to treat even the errors of a really great man with mere
ridicule, and in the present case the logical form of the doctrine
stands on a very different footing from its substance.

If species have really arisen by the operation of natural conditions,
we ought to be able to find those conditions now at work; we ought to
be able to discover in nature some power adequate to modify any given
kind of animal or plant in such a manner as to give rise to another
kind, which would be admitted by naturalists as a distinct species.
Lamarck imagined that he had discovered this _vera causa_ in the
admitted facts that some organs may be modified by exercise; and that
modifications, once produced, are capable of hereditary transmission.
It does not seem to have occurred to him to inquire whether there is
any reason to believe that there are any limits to the amount of
modification producible, or to ask how long an animal is likely to
endeavour to gratify an impossible desire. The bird, in our example,
would surely have renounced fish dinners long before it had produced
the least effect on leg or neck.

Since Lamarck’s time, almost all competent naturalists have left
speculations on the origin of species to such dreamers as the author of
the “Vestiges,” by whose well-intentioned efforts the Lamarckian theory
received its final condemnation in the minds of all sound thinkers.
Notwithstanding this silence, however, the transmutation theory, as it
has been called, has been a “skeleton in the closet” to many an honest
zoologist and botanist who had a soul above the mere naming of dried
plants and skins. Surely, has such an one thought, nature is a mighty
and consistent whole, and the providential order established in the
world of life must, if we could only see it rightly, be consistent with
that dominant over the multiform shapes of brute matter. But what is
the history of astronomy, of all the branches of physics, of chemistry,
of medicine, but a narration of the steps by which the human mind has
been compelled, often sorely against its will, to recognize the
operation of secondary causes in events where ignorance beheld an
immediate intervention of a higher power? And when we know that living
things are formed of the same elements as the inorganic world, that
they act and react upon it, bound by a thousand ties of natural piety,
is it probable, nay is it possible, that they, and they alone, should
have no order in their seeming disorder, no unity in their seeming
multiplicity, should suffer no explanation by the discovery of some
central and sublime law of mutual connexion?

Questions of this kind have assuredly often arisen, but it might have
been long before they received such expression as would have commanded
the respect and attention of the scientific world, had it not been for
the publication of the work which prompted this article. Its author,
Mr. Darwin, inheritor of a once celebrated name, won his spurs in
science when most of those now distinguished were young men, and has
for the last 20 years held a place in the front ranks of British
philosophers. After a circumnavigatory voyage, undertaken solely for
the love of his science, Mr. Darwin published a series of researches
which at once arrested the attention of naturalists and geologists; his
generalizations have since received ample confirmation, and now command
universal assent, nor is it questionable that they have had the most
important influence on the progress of science. More recently Mr.
Darwin, with a versatility which is among the rarest of gifts, turned
his attention to a most difficult question of zoology and minute
anatomy; and no living naturalist and anatomist has published a better
monograph than that which resulted from his labours. Such a man, at all
events, has not entered the sanctuary with unwashed hands, and when he
lays before us the results of 20 years’ investigation and reflection we
must listen even though we be disposed to strike. But, in reading his
work it must be confessed that the attention which might at first be
dutifully, soon becomes willingly, given, so clear is the author’s
thought, so outspoken his conviction, so honest and fair the candid
expression of his doubts. Those who would judge the book must read it;
we shall endeavour only to make its line of argument and its
philosophical position intelligible to the general reader in our own
way.

The Baker-street Bazaar has just been exhibiting its familiar annual
spectacle. Straight-backed, small-headed, big-barrelled oxen, as
dissimilar from any wild species as can well be imagined, contended for
attention and praise with sheep of half-a-dozen different breeds and
styes of bloated preposterous pigs, no more like a wild boar or sow
than a city alderman is like an ourang-outang. The cattle show has
been, and perhaps may again be, succeeded by a poultry show, of whose
crowing and clucking prodigies it can only be certainly predicated that
they will be very unlike the aboriginal _Phasianus gallus_. If the
seeker after animal anomalies is not satisfied, a turn or two in Seven
Dials will convince him that the breeds of pigeons are quite as
extraordinary and unlike one another and their parent stock, while the
Horticultural Society will provide him with any number of corresponding
vegetable aberrations from nature’s types. He will learn with no little
surprise, too, in the course of his travels, that the proprietors and
producers of these animal and vegetable anomalies regard them as
distinct species, with a firm belief, the strength of which is exactly
proportioned to their ignorance of scientific biology, and which is the
more remarkable as they are all proud of their skill in _originating_
such “species.”

On careful inquiry it is found that all these, and the many other
artificial breeds or races of animals and plants, have been produced by
one method. The breeder—and a skilful one must be a person of much
sagacity and natural or acquired perceptive faculty—notes some slight
difference, arising he knows not how, in some individuals of his stock.
If he wish to perpetuate the difference, to form a breed with the
peculiarity in question strongly marked, he selects such male and
female individuals as exhibit the desired character, and breeds from
them. Their offspring are then carefully examined, and those which
exhibit the peculiarity the most distinctly are selected for breeding,
and this operation is repeated until the desired amount of divergence
from the primitive stock is reached. It is then found that by
continuing the process of selection—always breeding, that is, from
well-marked forms, and allowing no impure crosses to interfere,—a race
may be formed, the tendency of which to reproduce itself is exceedingly
strong; nor is the limit to the amount of divergence which may be thus
produced known, but one thing is certain, that, if certain breeds of
dogs, or of pigeons, or of horses, were known only in a fossil state,
no naturalist would hesitate in regarding them as distinct species.

But, in all these cases we have _human interference_. Without the
breeder there would be no selection, and without the selection no race.
Before admitting the possibility of natural species having originated
in any similar way, it must be proved that there is in nature some
power which takes the place of man, and performs a selection _sua
sponte_. It is the claim of Mr. Darwin that he professes to have
discovered the existence and the _modus operandi_ of this natural
selection, as he terms it; and, if he be right, the process is
perfectly simple and comprehensible, and irresistibly deducible from
very familiar but well nigh forgotten facts.

Who, for instance, has duly reflected upon all the consequences of the
marvellous struggle for existence which is daily and hourly going on
among living beings? Not only does every animal live at the expense of
some other animal or plant, but the very plants are at war. The ground
is full of seeds that cannot rise into seedlings; the seedlings rob one
another of air, light and water, the strongest robber winning the day,
and extinguishing his competitors. Year after year, the wild animals
with which man never interferes are, on the average, neither more nor
less numerous than they were; and yet we know that the annual produce
of every pair is from one to perhaps a million young,—so that it is
mathematically certain that, on the average, as many are killed by
natural causes as are born every year, and those only escape which
happen to be a little better fitted to resist destruction than those
which die. The individuals of a species are like the crew of a
foundered ship, and none but good swimmers have a chance of reaching
the land.

Such being unquestionably the necessary conditions under which living
creatures exist, Mr. Darwin discovers in them the instrument of natural
selection. Suppose that in the midst of this incessant competition some
individuals of a species (A) present accidental variations which happen
to fit them a little better than their fellows for the struggle in
which they are engaged, then the chances are in favour, not only of
these individuals being better nourished than the others, but of their
predominating over their fellows in other ways, and of having a better
chance of leaving offspring, which will of course tend to reproduce the
peculiarities of their parents. Their offspring will, by a parity of
reasoning, tend to predominate over their contemporaries, and there
being (suppose) no room for more than one species such as A, the weaker
variety will eventually be destroyed by the new destructive influence
which is thrown into the scale, and the stronger will take its place.
Surrounding conditions remaining unchanged, the new variety (which we
may call B)—supposed, for argument’s sake, to be the best adapted for
these conditions which can be got out of the original stock—will remain
unchanged, all accidental deviations from the type becoming at once
extinguished, as less fit for their post than B itself. The tendency of
B to persist will grow with its persistence through successive
generations, and it will acquire all the characters of a new species.

But, on the other hand, if the conditions of life change in any degree,
however slight, B may no longer be that form which is best adapted to
withstand their destructive, and profit by their sustaining, influence;
in which case if it should give rise to a more competent variety (C),
this will take its place and become a new species; and thus, by
_natural selection_, the species B and C will be successively derived
from A.

That this most ingenious hypothesis enables us to give a reason for
many apparent anomalies in the distribution of living beings in time
and space, and that it is not contradicted by the main phenomena of
life and organization appear to us to be unquestionable; and so far it
must be admitted to have an immense advantage over any of its
predecessors. But it is quite another matter to affirm absolutely
either the truth or falsehood of Mr. Darwin’s views at the present
stage of the inquiry. Goethe has an excellent aphorism defining that
state of mind which he calls _Thätige Skepsis_—active doubt. It is
doubt which so loves truth that it neither dares rest in doubting, nor
extinguish itself by unjustified belief; and we commend this state of
mind to students of species, with respect to Mr. Darwin’s or any other
hypothesis, as to their origin. The combined investigations of another
20 years may, perhaps, enable naturalists to say whether the modifying
causes and the selective power, which Mr. Darwin has satisfactorily
shown to exist in nature, are competent to produce all the effects he
ascribes to them, or whether, on the other hand, he has been led to
over-estimate the value of his principle of natural selection, as
greatly as Lamarck overestimated his _vera causa_ of modification by
exercise.

But there is, at all events, one advantage possessed by the more recent
writer over his predecessor. Mr. Darwin abhors mere speculation as
nature abhors a vacuum. He is as greedy of cases and precedents as any
constitutional lawyer, and all the principles he lays down are capable
of being brought to the test of observation and experiment. The path he
bids us follow professes to be, not a mere airy track, fabricated of
ideal cobwebs, but a solid and broad bridge of facts. If it be so, it
will carry us safely over many a chasm in our knowledge, and lead us to
a region free from the snares of those fascinating but barren Virgins,
the Final Causes, against whom a high authority has so justly warned
us. “My sons, dig in the vineyard,” were the last words of the old man
in the fable; and, though the sons found no treasure, they made their
fortunes by the grapes.

 * _Times_, December 26th, 1850.