[1859]
The hypothesis 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 two botanists or two zoologists 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 100,000 such kinds of insects alone have
been described and may be identified in collections, and the number of
separable kinds of living things is under-estimated 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 discordancies of structure, the bond of union of
their present 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 versâ_, 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 _à 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. [Footnote: See page 60
_Note_.] 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 the lapse of which 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 fossilised exuviæ 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
primæval 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 which 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
palæontological 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 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
carcase 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 recognise 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 connection?
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 twenty
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 generalisations 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 twenty 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 _suâ 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
organisation 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 twenty 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 the principle of natural selection, as
greatly as Lamarck over-estimated 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.