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THE ORIGIN OF SPECIES*

by Thomas H. Huxley


[footnote] *'The Westminster Review', April 1860.


MR. DARWIN'S long-standing and well-earned scientific eminence probably
renders him indifferent to that social notoriety which passes by the
name of success; but if the calm spirit of the philosopher have not yet
wholly superseded the ambition and the vanity of the carnal man within
him, he must be well satisfied with the results of his venture in
publishing the 'Origin of Species'. Overflowing the narrow bounds of
purely scientific circles, the "species question" divides with Italy
and the Volunteers the attention of general society. Everybody has read
Mr. Darwin's book, or, at least, has given an opinion upon its merits
or demerits; pietists, whether lay or ecclesiastic, decry it with the
mild railing which sounds so charitable; bigots denounce it with
ignorant invective; old ladies of both sexes consider it a decidedly
dangerous book, and even savants, who have no better mud to throw,
quote antiquated writers to show that its author is no better than an
ape himself; while every philosophical thinker hails it as a veritable
Whitworth gun in the armoury of liberalism; and all competent
naturalists and physiologists, whatever their opinions as to the
ultimate fate of the doctrines put forth, acknowledge that the work in
which they are embodied is a solid contribution to knowledge and
inaugurates a new epoch in natural history.

Nor has the discussion of the subject been restrained within the limits
of conversation. When the public is eager and interested, reviewers
must minister to its wants; and the genuine 'litterateur' is too much
in the habit of acquiring his knowledge from the book he judges - as the
Abyssinian is said to provide himself with steaks from the ox which
carries him - to be withheld from criticism of a profound scientific
work by the mere want of the requisite preliminary scientific
acquirement; while, on the other hand, the men of science who wish well
to the new views, no less than those who dispute their validity, have
naturally sought opportunities of expressing their opinions. Hence it
is not surprising that almost all the critical journals have noticed
Mr. Darwin's work at greater or less length; and so many disquisitions,
of every degree of excellence, from the poor product of ignorance, too
often stimulated by prejudice, to the fair and thoughtful essay of the
candid student of Nature, have appeared, that it seems an almost
hopeless task to attempt to say anything new upon the question.

But it may be doubted if the knowledge and acumen of prejudged
scientific opponents, or the subtlety of orthodox special pleaders,
have yet exerted their full force in mystifying the real issues of the
great controversy which has been set afoot, and whose end is hardly
likely to be seen by this generation; so that, at this eleventh hour,
and even failing anything new, it may be useful to state afresh that
which is true, and to put the fundamental positions advocated by Mr.
Darwin in such a form that they may be grasped by those whose special
studies lie in other directions. And the adoption of this course may
be the more advisable, because, notwithstanding its great deserts, and
indeed partly on account of them, the 'Origin of Species' is by no
means an easy book to read - if by reading is implied the full
comprehension of an author's meaning.

We do not speak jestingly in saying that it is Mr. Darwin's misfortune
to know more about the question he has taken up than any man living.
Personally and practically exercised in zoology, in minute anatomy, in
geology; a student of geographical distribution, not on maps and in
museums only, but by long voyages and laborious collection; having
largely advanced each of these branches of science, and having spent
many years in gathering and sifting materials for his present work, the
store of accurately registered facts upon which the author of the
'Origin of Species' is able to draw at will is prodigious.

But this very superabundance of matter must have been embarrassing to a
writer who, for the present, can only put forward an abstract of his
views; and thence it arises, perhaps, that notwithstanding the
clearness of the style, those who attempt fairly to digest the book
find much of it a sort of intellectual pemmican - a mass of facts
crushed and pounded into shape, rather than held together by the
ordinary medium of an obvious logical bond; due attention will, without
doubt, discover this bond, but it is often hard to find.

Again, from sheer want of room, much has to be taken for granted which
might readily enough be proved; and hence, while the adept, who can
supply the missing links in the evidence from his own knowledge,
discovers fresh proof of the singular thoroughness with which all
difficulties have been considered and all unjustifiable suppositions
avoided, at every reperusal of Mr. Darwin's pregnant paragraphs, the
novice in biology is apt to complain of the frequency of what he
fancies is gratuitous assumption.

Thus while it may be doubted if, for some years, any one is likely to be
competent to pronounce judgment on all the issues raised by Mr. Darwin,
there is assuredly abundant room for him, who, assuming the humbler,
though perhaps as useful, office of an interpreter between the 'Origin
of Species' and the public, contents himself with endeavouring to point
out the nature of the problems which it discusses; to distinguish
between the ascertained facts and the theoretical views which it
contains; and finally, to show the extent to which the explanation it
offers satisfies the requirements of scientific logic. At any rate, it
is this office which we purpose to undertake in the following pages.

It may be safely assumed that our readers have a general conception of
the nature of the objects to which the word "species" is applied; but
it has, perhaps, occurred to a few, even to those who are naturalists
'ex professo', to reflect, that, as commonly employed, the term has a
double sense and denotes two very different orders of relations. When
we call a group of animals, or of plants, a species, we may imply
thereby, either that all these animals or plants have some common
peculiarity of form or structure; or, we may mean that they possess
some common functional character. That part of biological science
which deals with form and structure is called Morphology - that which
concerns itself with function, Physiology - so that we may conveniently
speak of these two senses, or aspects, of "species" - the one as
morphological, the other as physiological. Regarded from the former
point of view, a species is nothing more than a kind of animal or
plant, which is distinctly definable from all others, by certain
constant, and not merely sexual, morphological peculiarities. Thus
horses form a species, because the group of animals to which that name
is applied is distinguished from all others in the world by the
following constantly associated characters. They have - 1, A vertebral
column; 2, Mammae; 3, A placental embryo; 4, Four legs; 5, A single
well-developed toe in each foot provided with a hoof; 6, A bushy tail;
and 7, Callosities on the inner sides of both the fore and the hind
legs. The asses, again, form a distinct species, because, with the
same characters, as far as the fifth in the above list, all asses have
tufted tails, and have callosities only on the inner side of the
fore-legs. If animals were discovered having the general characters of
the horse, but sometimes with callosities only on the fore-legs, and
more or less tufted tails; or animals having the general characters of
the ass, but with more or less bushy tails, and sometimes with
callosities on both pairs of legs, besides being intermediate in other
respects - the two species would have to be merged into one. They could
no longer be regarded as morphologically distinct species, for they
would not be distinctly definable one from the other.

However bare and simple this definition of species may appear to be, we
confidently appeal to all practical naturalists, whether zoologists,
botanists, or palaeontologists, to say if, in the vast majority of
cases, they know, or mean to affirm anything more of the group of
animals or plants they so denominate than what has just been stated.
Even the most decided advocates of the received doctrines respecting
species admit this.

"I apprehend," says Professor Owen*, "that few naturalists nowadays, in
describing and proposing a name for what they call 'a new species,' use
that term to signify what was meant by it twenty or thirty years ago;
that is, an originally distinct creation, maintaining its primitive
distinction by obstructive generative peculiarities. The proposer of
the new species now intends to state no more than he actually knows;
as, for example, that the differences on which he founds the specific
character are constant in individuals of both sexes, so far as
observation has reached; and that they are not due to domestication or
to artificially superinduced external circumstances, or to any outward
influence within his cognizance; that the species is wild, or is such
as it appears by Nature."

[footnote] *On the Osteology of the Chimpanzees and Orangs:
Transactions of the Zoological Society, 1858.

If we consider, in fact, that by far the largest proportion of recorded
existing species are known only by the study of their skins, or bones,
or other lifeless exuvia; that we are acquainted with none, or next to
none, of their physiological peculiarities, beyond those which can be
deduced from their structure, or are open to cursory observation; and
that we cannot hope to learn more of any of those extinct forms of life
which now constitute no inconsiderable proportion of the known Flora
and Fauna of the world: it is obvious that the definitions of these
species can be only of a purely structural, or morphological, character.
It is probable that naturalists would have avoided much confusion of
ideas if they had more frequently borne the necessary limitations of
our knowledge in mind. But while it may safely be admitted that we are
acquainted with only the morphological characters of the vast majority
of species - the functional or physiological, peculiarities of a few have
been carefully investigated, and the result of that study forms a large
and most interesting portion of the physiology of reproduction.

The student of Nature wonders the more and is astonished the less, the
more conversant he becomes with her operations; but of all the
perennial miracles she offers to his inspection, perhaps the most
worthy of admiration is the development of a plant or of an animal from
its embryo. Examine the recently laid egg of some common animal, such
as a salamander or newt. It is a minute spheroid in which the best
microscope will reveal nothing but a structureless sac, enclosing a
glairy fluid, holding granules in suspension. But strange
possibilities lie dormant in that semi-fluid globule. Let a moderate
supply of warmth reach its watery cradle, and the plastic matter
undergoes changes so rapid, yet so steady and purposelike in their
succession, that one can only compare them to those operated by a
skilled modeller upon a formless lump of clay. As with an invisible
trowel, the mass is divided and subdivided into smaller and smaller
portions, until it is reduced to an aggregation of granules not too
large to build withal the finest fabrics of the nascent organism. And,
then, it is as if a delicate finger traced out the line to be occupied
by the spinal column, and moulded the contour of the body; pinching up
the head at one end, the tail at the other, and fashioning flank and
limb into due salamandrine proportions, in so artistic a way, that,
after watching the process hour by hour, one is almost involuntarily
possessed by the notion, that some more subtle aid to vision than an
achromatic, would show the hidden artist, with his plan before him,
striving with skilful manipulation to perfect his work.

As life advances, and the young amphibian ranges the waters, the terror
of his insect contemporaries, not only are the nutritious particles
supplied by its prey, by the addition of which to its frame, growth
takes place, laid down, each in its proper spot, and in such due
proportion to the rest, as to reproduce the form, the colour, and the
size, characteristic of the parental stock; but even the wonderful
powers of reproducing lost parts possessed by these animals are
controlled by the same governing tendency. Cut off the legs, the tail,
the jaws, separately or all together, and, as Spallanzani showed long
ago, these parts not only grow again, but the redintegrated limb is
formed on the same type as those which were lost. The new jaw, or leg,
is a newt's, and never by any accident more like that of a frog. What
is true of the newt is true of every animal and of every plant; the
acorn tends to build itself up again into a woodland giant such as that
from whose twig it fell; the spore of the humblest lichen reproduces
the green or brown incrustation which gave it birth; and at the other
end of the scale of life, the child that resembled neither the paternal
nor the maternal side of the house would be regarded as a kind of
monster.

So that the one end to which, in all living beings, the formative
impulse is tending - the one scheme which the Archaeus of the old
speculators strives to carry out, seems to be to mould the offspring
into the likeness of the parent. It is the first great law of
reproduction, that the offspring tends to resemble its parent or
parents, more closely than anything else.

Science will some day show us how this law is a necessary consequence of
the more general laws which govern matter; but, for the present, more
can hardly be said than that it appears to be in harmony with them. We
know that the phenomena of vitality are not something apart from other
physical phenomena, but one with them; and matter and force are the two
names of the one artist who fashions the living as well as the
lifeless. Hence living bodies should obey the same great laws as other
matter - nor, throughout Nature, is there a law of wider application
than this, that a body impelled by two forces takes the direction of
their resultant. But living bodies may be regarded as nothing but
extremely complex bundles of forces held in a mass of matter, as the
complex forces of a magnet are held in the steel by its coercive force;
and, since the differences of sex are comparatively slight, or, in other
words, the sum of the forces in each has a very similar tendency, their
resultant, the offspring, may reasonably be expected to deviate but
little from a course parallel to either, or to both.

Represent the reason of the law to ourselves by what physical metaphor
or analogy we will, however, the great matter is to apprehend its
existence and the importance of the consequences deducible from it. For
things which are like to the same are like to one another; and if; in a
great series of generations, every offspring is like its parent, it
follows that all the offspring and all the parents must be like one
another; and that, given an original parental stock, with the
opportunity of undisturbed multiplication, the law in question
necessitates the production, in course of time, of an indefinitely
large group, the whole of whose members are at once very similar and
are blood relations, having descended from the same parent, or pair of
parents. The proof that all the members of any given group of animals,
or plants, had thus descended, would be ordinarily considered
sufficient to entitle them to the rank of physiological species, for
most physiologists consider species to be definable as "the offspring
of a single primitive stock."

But though it is quite true that all those groups we call species 'may',
according to the known laws of reproduction, have descended from a
single stock, and though it is very likely they really have done so,
yet this conclusion rests on deduction and can hardly hope to establish
itself upon a basis of observation. And the primitiveness of the
supposed single stock, which, after all, is the essential part of the
matter, is not only a hypothesis, but one which has not a shadow of
foundation, if by "primitive" be meant "independent of any other living
being." A scientific definition, of which an unwarrantable hypothesis
forms an essential part, carries its condemnation within itself; but,
even supposing such a definition were, in form, tenable, the
physiologist who should attempt to apply it in Nature would soon find
himself involved in great, if not inextricable, difficulties. As we
have said, it is indubitable that offspring 'tend' to resemble the
parental organism, but it is equally true that the similarity attained
never amounts to identity, either in form or in structure. There is
always a certain amount of deviation, not only from the precise
characters of a single parent, but when, as in most animals and many
plants, the sexes are lodged in distinct individuals, from an exact mean
between the two parents. And indeed, on general principles, this
slight deviation seems as intelligible as the general similarity, if we
reflect how complex the co-operating "bundles of forces" are, and how
improbable it is that, in any case, their true resultant shall coincide
with any mean between the more obvious characters of the two parents.
Whatever be its cause, however, the co-existence of this tendency to
minor variation with the tendency to general similarity, is of vast
importance in its bearing on the question of the origin of species.

As a general rule, the extent to which an offspring differs from its
parent is slight enough; but, occasionally, the amount of difference is
much more strongly marked, and then the divergent offspring receives
the name of a Variety. Multitudes, of what there is every reason to
believe are such varieties, are known, but the origin of very few has
been accurately recorded, and of these we will select two as more
especially illustrative of the main features of variation. The first
of them is that of the "Ancon," or "Otter" sheep, of which a careful
account is given by Colonel David Humphreys, F.R.S., in a letter to Sir
Joseph Banks, published in the Philosophical Transactions for 1813. It
appears that one Seth Wright, the proprietor of a farm on the banks of
the Charles River, in Massachusetts, possessed a flock of fifteen ewes
and a ram of the ordinary kind. In the year 1791, one of the ewes
presented her owner with a male lamb, differing, for no assignable
reason, from its parents by a proportionally long body and short bandy
legs, whence it was unable to emulate its relatives in those sportive
leaps over the neighbours' fences, in which they were in the habit of
indulging, much to the good farmer's vexation.

The second case is that detailed by a no less unexceptionable authority
than Reaumur, in his 'Art de faire eclore les Poulets'. A Maltese
couple, named Kelleia, whose hands and feet were constructed upon the
ordinary human model, had born to them a son, Gratio, who possessed six
perfectly movable fingers on each hand, and six toes, not quite so well
formed, on each foot. No cause could be assigned for the appearance of
this unusual variety of the human species.

Two circumstances are well worthy of remark in both these cases. In
each, the variety appears to have arisen in full force, and, as it
were, 'per saltum'; a wide and definite difference appearing, at once,
between the Ancon ram and the ordinary sheep; between the six-fingered
and six-toed Gratio Kelleia and ordinary men. In neither case is it
possible to point out any obvious reason for the appearance of the
variety. Doubtless there were determining causes for these as for all
other phenomena; but they do not appear, and we can be tolerably certain
that what are ordinarily understood as changes in physical conditions,
as in climate, in food, or the like, did not take place and had nothing
to do with the matter. It was no case of what is commonly called
adaptation to circumstances; but, to use a conveniently erroneous
phrase, the variations arose spontaneously. The fruitless search after
final causes leads their pursuers a long way; but even those hardy
teleologists, who are ready to break through all the laws of physics in
chase of their favourite will-o'-the-wisp, may be puzzled to discover
what purpose could be attained by the stunted legs of Seth Wright's ram
or the hexadactyle members of Gratio Kelleia.

Varieties then arise we know not why; and it is more than probable that
the majority of varieties have arisen in this "spontaneous" manner,
though we are, of course, far from denying that they may be traced, in
some cases, to distinct external influences; which are assuredly
competent to alter the character of the tegumentary covering, to change
colour, to increase or diminish the size of muscles, to modify
constitution, and, among plants, to give rise to the metamorphosis of
stamens into petals, and so forth. But however they may have arisen,
what especially interests us at present is, to remark that, once in
existence, varieties obey the fundamental law of reproduction that like
tends to produce like; and their offspring exemplify it by tending to
exhibit the same deviation from the parental stock as themselves.
Indeed, there seems to be, in many instances, a pre-potent influence
about a newly-arisen variety which gives it what one may call an unfair
advantage over the normal descendants from the same stock. This is
strikingly exemplified by the case of Gratio Kelleia, who married a
woman with the ordinary pentadactyle extremities, and had by her four
children, Salvator, George, Andre, and Marie. Of these children
Salvator, the eldest boy, had six fingers and six toes, like his
father; the second and third, also boys, had five fingers and five toes,
like their mother, though the hands and feet of George were slightly
deformed. The last, a girl, had five fingers and five toes, but the
thumbs were slightly deformed. The variety thus reproduced itself
purely in the eldest, while the normal type reproduced itself purely in
the third, and almost purely in the second and last: so that it would
seem, at first, as if the normal type were more powerful than the
variety. But all these children grew up and intermarried with normal
wives and husband, and then, note what took place: Salvator had four
children, three of whom exhibited the hexadactyle members of their
grandfather and father, while the youngest had the pentadactyle limbs
of the mother and grandmother; so that here, notwithstanding a double
pentadactyle dilution of the blood, the hexadactyle variety had the
best of it. The same pre-potency of the variety was still more
markedly exemplified in the progeny of two of the other children, Marie
and George. Marie (whose thumbs only were deformed) gave birth to a boy
with six toes, and three other normally formed children; but George,
who was not quite so pure a pentadactyle, begot, first, two girls, each
of whom had six fingers and toes; then a girl with six fingers on each
hand and six toes on the right foot, but only five toes on the left;
and lastly, a boy with only five fingers and toes. In these instances,
therefore, the variety, as it were, leaped over one generation to
reproduce itself in full force in the next. Finally, the purely
pentadactyle Andre was the father of many children, not one of whom
departed from the normal parental type.

If a variation which approaches the nature of a monstrosity can strive
thus forcibly to reproduce itself, it is not wonderful that less
aberrant modifications should tend to be preserved even more strongly;
and the history of the Ancon sheep is, in this respect, particularly
instructive. With the "'cuteness" characteristic of their nation, the
neighbours of the Massachusetts farmer imagined it would be an
excellent thing if all his sheep were imbued with the stay-at-home
tendencies enforced by Nature upon the newly-arrived ram; and they
advised Wright to kill the old patriarch of his fold, and install the
Ancon ram in his place. The result justified their sagacious
anticipations, and coincided very nearly with what occurred to the
progeny of Gratio Kelleia. The young lambs were almost always either