'THE ORIGIN OF SPECIES.'
The accident came in this wise.
Alfred Russel Wallace, a young Welsh biologist, went out at twenty-four,
in 1848, to the Amazons River, in company with Bates (the author of 'The
Naturalist on the Amazons'), to collect birds and butterflies, and to
study tropical life in the richest region of equatorial America. Like
all other higher zoologists of their time, the two young explorers were
deeply interested in the profound questions of origin and metamorphosis,
and of geographical distribution, and in the letters that passed between
them before they started they avowed to one another that the object of
their quest was a solution of the pressing biological enigma of creation
or evolution. Starting with fresh hopes and a few pounds in pocket, on
an old, worn-out, and unseaworthy slave-trader, they often discussed
these deep problems of life and nature together upon the Sargasso sea,
or among the palms and lianas of the Brazilian woodlands. The air was
thick with whiffs and foretastes of evolutionism, and the two budding
naturalists of the Amazons expedition had inhaled them eagerly with
every breath. They saw among the mimicking organisms of that equatorial
zone strange puzzles to engage their deepest attention; they recognised
in the veins and spots that diversified the filmy membranes of insects'
wings the hieroglyphs of nature, writing as on a tablet for them to
decipher the story of the slow modification of species. In 1852--the
year when Herbert Spencer in England published his essay on the
'Development Hypothesis,' and when Naudin in France put forth his bold
and able paper on the 'Origin of Species'--Wallace once more returned to
Europe, and gave to the world his interesting 'Travels on the Amazons
and the Rio Negro.' Two years later the indefatigable traveller set out
a second time on a voyage of tropical exploration, among the islands of
the Malay archipelago, and for eight years he wandered about in Malay
huts and remote islets, gathering in solitude and isolation the enormous
store of minute facts which he afterwards lavished with so prodigal a
hand upon 'Tropical Nature,' and the 'Geographical Distribution of
Animals.'
While Wallace was still at Amboyna, he sent home in 1858 a striking
memoir, addressed to Darwin, with a request that he would forward it to
Sir Charles Lyell, for presentation to the Linnean Society. Darwin
opened and read his brother naturalist's paper, and found to his
surprise that it contained his own theory of natural selection, not
worked out in detail, as he himself was working it out, but still
complete in spirit and essence, with no important portion of the central
idea lacking to its full rotundity of conception. A jealous man would
have thrown obstacles in the way of publication; but both Darwin and
Wallace were born superior to the meannesses of jealousy. The elder
naturalist commended his young rival's paper at once to Sir Charles
Lyell, who sent it on immediately to the Linnean Society.
But Sir Charles Lyell and Sir Joseph Hooker, both of whom knew of
Darwin's work, thought it advisable that he should publish, in the
'Journal' of the Society, a few extracts from his own manuscripts, side
by side with Wallace's paper. Darwin, therefore, selected some essential
passages for the purpose from his own long-gathered and voluminous
notes, and the two contributions were read together before the Society
on July the 1st, 1858. That double communication marks the date of birth
of modern evolutionism. It is to the eternal credit of both thinkers
that each accepted his own true position with regard to the great
discovery in perfect sincerity. The elder naturalist never strove for a
moment to press his own claim to priority against the younger: the
younger, with singular generosity and courtesy, waived his own claim to
divide the honours of discovery in favour of the elder. Not one word
save words of fraternal admiration and cordial appreciation ever passed
the lips of either with regard to the other.
The distinctive notion of natural selection, indeed, like all true and
fruitful ideas, had more than once flashed for a moment across the
penetrating mind of more than one independent investigator. As early as
1813, Dr. Wells, the famous author of the theory of dew, applied that
particular conception to the single case of the production of special
races among mankind.
'Of the accidental varieties of man, which would occur among the first
few and scattered inhabitants of the middle regions of Africa,' he
wrote, 'some one would be better fitted than the others to bear the
diseases of the country. This race would consequently multiply, while
the others would decrease; not only from their inability to sustain the
attacks of disease, but from their incapacity of contending with their
more vigorous neighbours.... The same disposition to form varieties
still existing, a darker and a darker race would in the course of time
occur; and as the darkest would be the best fitted for the climate, this
would at last become the most prevalent, if not the only race in the
country.' Here we have not merely the radical concept of natural
selection, but also the subordinate idea of its exertion upon what
Darwin calls 'spontaneous variations.' What is wanting in the paper is
the application of the faintly descried law to the facts and
circumstances of general biology: Wells saw only a particular instance,
where Darwin and Wallace more vividly perceived a universal principle.
Again, in 1831, Mr. Patrick Matthew in that singular appendix to his
book on naval timber actually enunciates the same idea, applied this
time to the whole of nature, in words sometimes almost identical with
Darwin's own. 'As nature in all her modifications of life,' says this
unconscious discoverer, 'has a power of increase far beyond what is
needed to supply the place of what falls by Time's decay, those
individuals who possess not the requisite strength, swiftness,
hardihood, or cunning, fall prematurely without reproducing--either a
prey to their natural devourers, or sinking under disease, generally
induced by want of nourishment, their place being occupied by the more
perfect of their own kind, who are pressing on the means of
existence.... The self-regulating adaptive disposition of organised life
may, in part, be traced to the extreme fecundity of nature, who, as
before stated, has in all the varieties of her offspring a prolific
power much beyond (in many cases a thousandfold) what is necessary to
fill up the vacancies caused by senile decay. As the field of existence
is limited and preoccupied, it is only the hardier, more robust,
better-suited-to-circumstance individuals, who are able to struggle
forward to maturity, these inhabiting only the situations to which they
have superior adaptation and greater power of occupancy than any other
kind; the weaker and less circumstance-suited being prematurely
destroyed. This principle is in constant action; it regulates the
colour, the figure, the capacities, and instincts; those individuals in
each species whose colour and covering are best suited to concealment or
protection from enemies, or defence from inclemencies and vicissitudes
of climate, whose figure is best accommodated to health, strength,
defence, and support; whose capacities and instincts can best regulate
the physical energies to self-advantage according to circumstances--in
such immense waste of primary and youthful life those only come forward
to maturity from the strict ordeal by which nature tests their
adaptation to her standard of perfection and fitness to continue their
kind by reproduction.' Of the ideas expressed in these paragraphs, and
others which preceded them, Darwin himself rightly observes, 'He gives
precisely the same view on the origin of species as that propounded by
Mr. Wallace and myself. He clearly saw the full force of the principle
of natural selection.'
In 1852, once more, so eminent and confirmed an evolutionist as Mr.
Herbert Spencer himself had hit upon a glimpse of the same great truth,
strange to say without perceiving the width and scope of its
implications. 'All mankind,' he wrote in that year in an essay on
population in the 'Westminster Review,' 'in turn subject themselves more
or less to the discipline described; they either may or may not advance
under it; but, in the nature of things, only those who _do_ advance
under it eventually survive. For, necessarily, families and races whom
this increasing difficulty of getting a living which excess of fertility
entails does not stimulate to improvements in production.... are on the
high road to extinction; and must ultimately be supplanted by those whom
the pressure does so stimulate.... And here, indeed, without further
illustration, it will be seen that premature death, under all its forms,
and from all its causes, cannot fail to work in the same direction. For
as those prematurely carried off must, in the average of cases, be those
in whom the power of self-preservation is the least, it unavoidably
follows that those left behind to continue the race must be those in
whom the power of self-preservation is the greatest, must be the select
of their generation.' In this striking pre-Darwinian passage we have a
partial perception of what Mr. Spencer afterwards described as the
survival of the fittest; but, as our great philosopher himself remarks,
it 'shows how near one may be to a great generalisation without seeing
it.' For not only does Mr. Spencer, like Wells before him, limit the
application of the principle to the case of humanity; but, unlike Wells,
he overlooks the all-important factor of spontaneous variation, and the
power of natural selection, acting upon such, to produce specific and
generic divergences of structure. In short, in his own words, the
paragraph 'contains merely a passing recognition of the selective
process, and indicates no suspicion of the enormous range of its
effects, or of the conditions under which a large part of its effects
are produced.' On the other hand, it must be noted that both Spencer and
Matthew, like Darwin himself, based their ideas largely upon the
Malthusian principle, and thus held the two true keys of the situation
fairly within their unconscious hands.
Frankly to recognise these various foreshadowings of the distinctive
Darwinian theory of natural selection is not in any way to undermine the
foundations of Charles Darwin's own real and exceptional greatness. On
the contrary, the mere fact that his views were so far anticipated by
Wells, Matthew, Spencer, and others, and were simultaneously arrived at
across half the globe by the independent intellect of Alfred Russel
Wallace, is in itself the very best proof and finest criterion of
Charles Darwin's genuine apostleship. No truly grand and fruitful idea
was ever yet the sole property of a single originator. Great
discoveries, says an acute critic, must always be concerned with some
problem of the time which many of the world's foremost minds are just
then cudgelling their active brains about. It was so with the discovery
of the differential calculus, and of the planet Neptune; with the
interpretation of the Egyptian hieroglyphics, and of the cuneiform
inscriptions; with the undulatory theory of light, with the mechanical
equivalent of heat, with the doctrine of the correlation and
conservation of energies, with the invention of the steam engine, the
locomotive, the telegraph and the telephone; with the nebular
hypothesis, and with spectrum analysis. It was so, too, with the
evolutionary movement. The fertile upturning of virgin sod in the
biological field which produced Darwin's forerunners, as regards the
idea of descent with modification, in the persons of Buffon, Lamarck,
and Erasmus Darwin, necessarily produced a little later, under the fresh
impetus of the Malthusian conception, his forerunners or coadjutors, as
regards the idea of natural selection, in the persons of Wells, Matthew,
and Wallace. It was Darwin's task to recognise the universal, where
Wells and Spencer had seen only the particular; to build up a vast and
irresistible inductive system, where Matthew and Wallace had but thrown
out a pregnant hint of wonderful _a priori_ interest and suggestiveness.
It is one thing to draw out the idea of a campaign, another thing to
carry it to a successful conclusion; one thing rudely to sketch a
ground-plan, another thing finally to pile aloft to the sky the front of
an august and imposing fabric.
As soon as the papers at the Linnean had been read and printed, Darwin
set to work in real earnest to bring out the first instalment of his
great work. That instalment was the 'Origin of Species.' The first
edition was ready for the public on November the 24th, 1859.
In his own mind Darwin regarded that immortal work merely in the light
of an abstract of his projected volumes. So immense were his collections
and so voluminous his notes that the 'Origin of Species' itself seemed
to him like a mere small portion of the contemplated publication. And
indeed he did ultimately work out several other portions of his original
plan in his detailed treatises on the Variation of Animals and Plants
under Domestication, on the Effects of Cross and Self-Fertilisation, and
on the Descent of Man and Sexual Selection. But the immense and
unexpected vogue of his first volume, the almost immediate revolution
which it caused in biological and general opinion, and the all but
universal adhesion to his views of all the greatest and most rising
naturalists, to a great extent saved him the trouble of carrying out in
full the task he had originally contemplated as necessary. Younger and
less occupied labourers took part of the work off their leader's hands;
the great chief was left to prosecute his special researches in some
special lines, and was relieved from the necessity of further proving in
minuter detail what he had already proved with sufficient cogency to
convince all but the wilfully blind or the hopelessly stupid.
The extraordinary and unprecedented success of the 'Origin of Species'
is the truest test of the advance it made upon all previous evolutionary
theorising. Those who had never been convinced before were now convinced
by sheer force of reasoning; those who believed and those who wavered
had their faith confirmed into something like the reposeful calm of
absolute certitude.
Let us consider, therefore, what exactly were the additions which
Charles Darwin offered in his epoch-making work to the pre-existing
conceptions of evolutionists.
In 1852, seven years before the publication of Darwin's masterpiece, Mr.
Herbert Spencer wrote as follows in an essay in the 'Leader' on creation
and evolution. The expressions of so profound and philosophical a
biologist may be regarded as the high-water mark of evolutionary
thinking up to the date of the appearance of Wallace and Darwin's
theory:--
'Even could the supporters of the development hypothesis merely show
that the production of species by the process of modification is
conceivable, they would be in a better position than their opponents.
But they can do much more than this; they can show that the process of
modification has effected and is effecting great changes in all
organisms, subject to modifying influences ... they can show that any
existing species--animal or vegetable--when placed under conditions
different from its previous ones, immediately begins to undergo certain
changes of structure fitting it for the new conditions. They can show
that in successive generations these changes continue until ultimately
the new conditions become the natural ones. They can show that in
cultivated plants and domesticated animals, and in the several races of
men, these changes have uniformly taken place. They can show that the
degrees of difference, so produced, are often, as in dogs, greater than
those on which distinctions of species are in other cases founded. They
can show that it is a matter of dispute whether some of these modified
forms _are_ varieties or modified species. They can show too that the
changes daily taking place in ourselves; the facility that attends long
practice, and the loss of aptitude that begins when practice ceases; the
development of every faculty, bodily, moral or intellectual, according
to the use made of it, are all explicable on this same principle. And
thus they can show that throughout all organic nature there _is_ at
work a modifying influence of the kind they assign as the cause of these
specific differences, an influence which, though slow in its action,
does in time, if the circumstances demand it, produce marked changes; an
influence which, to all appearance, would produce in the millions of
years, and under the great varieties of condition which geological
records imply, any amount of change.'
This admirable passage, written seven years before the publication of
the 'Origin of Species,' contains explicitly almost every idea that
ordinary people, not specially biological in their interests, now
associate with the name of Darwin. That is to say, it contains, in a
very philosophical and abstract form, the theory of 'descent with
modification' _without_ the distinctive Darwinian adjunct of 'natural
selection' or 'survival of the fittest.' Yet it was just that particular
lever, dexterously applied, and carefully weighted with the whole weight
of his endlessly accumulated inductive instances, that finally enabled
our modern Archimedes in so short a time to move the world. The public,
that was deaf to the high philosophy of Herbert Spencer, listened at
once to the practical wisdom of Charles Darwin. They did not care at all
for the _a priori_ proof, but they believed forthwith as soon as a
cautious and careful investigator laid bare before their eyes in minute
detail the _modus operandi_ of nature herself.
The main argument of Darwin's chief work runs somewhat after the
following fashion[2]:--
Variation, to a greater or less degree, is a common and well-known fact
in nature. More especially, animals and plants under domestication tend
to vary from one another far more than do the individuals of any one
species in the wild state. Rabbits in a warren are all alike in shape,
size, colour, and features: rabbits in a hutch vary indefinitely in the
hue of their fur, the length of their ears, the character of their coat,
and half a dozen other minor particulars, well known to the observant
souls of boys and fanciers. This great variability, though partly
perhaps referable to excess of food, is probably due on the whole to
their having been raised under conditions of life not so uniform as, and
somewhat different from, those to which the parent species is commonly
exposed in a state of nature. In other words, variability is one result
of altered and more varied surrounding circumstances.
Again, this variability is usually indefinite. You cannot say what
direction it will take, or to what particular results it is likely in
any special instance to lead. Marked differences sometimes occur even
between the young of the same litter, or between the seedlings sown from
the same capsule. As a rule, the variations exhibit themselves in
connection with sexual reproduction; but sometimes, as in the case of
'sporting plants,' a new bud suddenly produces leaves or flowers of a
different character from the rest of those on the self-same stem, thus
showing that the tendency to vary is inherent, as it were, in the
organism itself. Upon this fundamental fact of the existence in nature
of numerous and indefinite variations, the whole theory of natural
selection is ultimately built up. In illustrating by example the immense
variability of domesticated creatures, Darwin lays great stress upon the
case of pigeons, with which he was familiar from his long experience as
a breeder and fancier in his own home at Down. Naturalists are almost
universally of opinion that all the breeds of domestic pigeons, from the
carrier to the tumbler, from the runt to the fantail, are alike
descended from the wild rock pigeon of the European coasts. The immense
amount of variation which this original species has undergone in
domestication may be seen by comparing the numberless breeds of pigeon
now exhibited at all our poultry shows with one another.
But variation gives us only half the elements of the ultimate problem,
even in the case of domestic kinds. For the other half, we must have
recourse to human selection, which, by picking out for seed or breeding
purposes certain specially favoured varieties, has produced at last all
the purposive or intentional diversity between the different existing
stocks or breeds. In these artificially produced domestic races we see
everywhere special adaptations to man's particular use or fancy. The
dray-horse has been fashioned for purposes of strength and
sure-footedness in draught, the race-horse for purposes of fleetness in
running. In the fox-hound, man has encouraged the special properties
that tend to produce a good day's hunting; in the sheepdog, those that
make for the better maintenance and safety of a herd. The cauliflower is
a cabbage, with specialised and somewhat abortive flower-heads; the
fuller's teasel is a sport of the wild form, with curved hooks specially
adapted by a freak of nature for the teasing of wool. So in every case
man, by deliberately picking out for breeding or seeding purposes the
accidental variations which happened best to suit his own needs, has
succeeded at last in producing races admirably fitted in the minutest
particulars for the special functions to which they are applied. There
appears indeed to be hardly any limit to the almost infinite plasticity
and modifiability of domestic animals. 'It would seem,' said a great
sheep-breeder, speaking of sheep, 'as if farmers had chalked out upon a
wall a form perfect in itself, and then proceeded to give it existence.'
Now, what is thus true within narrow limits, and in a short space of
time about the deliberate action of man, Darwin showed to be also true
within wider limits and spread over longer geological epochs about the
unconscious action of nature. And herein consisted his great advance
upon the earlier evolutionism of Lamarck, Goethe, and Erasmus Darwin.
For while these instinctive pioneers of the evolutionary spirit saw
clearly that animals and plants betrayed signs of common descent from
one or a few original ancestors, they did not see what was the mechanism
by which such organisms had been differentiated into so many distinct
genera and species. They caught, indeed, at the analogy of variation
under domestication and in the wild state, but they missed the subtler
and deeper analogy between human and natural selection. Now, variation
alone would give us a world consisting not of definite kinds fairly well
demarcated one from the other, but of innumerable unclassified and
unorganisable individuals, all shading off indefinitely one into the
other, and incapable of being reduced by human ingenuity to any orderly
hierarchical system. Furthermore, it would give us creatures without
special adaptation of any kind to the peculiar circumstances of their
own environment. To account for adaptation, for the almost perfect
fitness of every plant and every animal to its position in life, for the
existence (in other words) of definitely correlated parts and organs, we
must call in the aid of survival of the fittest. Without that potent
selective agent, our conception of the becoming of life is a mere chaos;
order and organisation are utterly inexplicable save by the brilliant
illuminating ray of the Darwinian principle. That is why Darwin
destroyed at one blow the specious arguments of the early teleologists;
he showed that where Chambers and even Erasmus Darwin had seen the
working of a final cause, we ought rather to recognise the working of an
efficient cause, whose outcome necessarily but fallaciously simulates
the supposed features of an _a priori_ finality.
From art, then, Darwin harks back once more to nature. He proceeds to
show that variability occurs among all wild plants and animals, though
not so frequently under ordinary circumstances as in the case of
domesticated species. Individual differences everywhere occur between
plant and plant, between animal and animal. Sometimes these differences
are so very numerous that it is impossible to divide the individuals at
all into well-marked kinds; for example, among British wild-roses,
brambles, hawkweeds and epilobes, with a few other very variable
families, Babington makes as many as 251 distinct species, where
Bentham gives only 112--a margin of 139 doubtful forms of shadowy
indefiniteness. Varieties, in fact, are always arising, and dominant
species in particular always tend to vary most in every direction. The
reason why variation is not so marked in the wild state as under
domestication is of course because the conditions are there less
diverse; but where the conditions of wild things are most diverse, as in
the case of dominant kinds, which range over a wide space of country or
of ocean, abundant individual variations habitually occur. Local
varieties thus produced are regarded by Darwin as incipient species:
they are the raw material on which natural selection gradually exerts
itself in the struggle for existence.
Granting individual variability, then, how do species arise in nature?
And how are all the exquisite adaptations of part to whole, and of whole
to environment, gradually initiated, improved, and perfected?
Here Malthus and the struggle for life come in to help us.
For the world is perpetually over-populated. It is not, as many good
people fearfully imagine, on a half-comprehension of the Malthusian
principle, shortly going to be over-populated; it is now, it has always
been, and it will always be, pressed close up to the utmost possible
limit of population. Reproduction is everywhere and in all species for
ever outrunning means of subsistence; and starvation or competition is
for ever keeping down the number of the offspring to the level of the
average or normal supply of raw material. A single red campion produces
in a year three thousand seeds; but there are not this year three
thousand times as many red campions as there were last summer, nor will
there be three thousand times as many more in the succeeding season. The
roe of a cod contains sometimes nearly ten million eggs; but supposing
each of these produced a young fish which arrived at maturity, the whole
sea would immediately become a solid mass of closely packed codfish.
Linnæus reckoned that if an annual plant had two seeds, each of which
produced two seedlings in the succeeding season, and so on continually,
in twenty years their progeny would amount to a million plants. A
struggle for existence necessarily results from this universal tendency
of animals and plants to increase faster than the means of subsistence,
whether those means be food, as in the first case, or carbonic acid,
water, and sunshine as in the second. Animals are all perpetually
battling with one another for the food-supply of the moment; plants are
perpetually battling with one another for their share of the soil, the
rainfall, and the sunshine.
The case of the plant is a very important one to understand in this
connection, because it is probable that most people greatly
misunderstand the biological meaning of the phrase 'struggle for
existence.' They imagine that the struggle is chiefly conducted between
different species, whereas in reality it is chiefly conducted between
members of the same species. It is not so much the battle between the
tiger and the antelope, between the wolf and the bison, between the
snake and the bird, that ultimately results in natural selection or
survival of the fittest, as the struggle between tiger and tiger,
between bison and bison, between snake and snake, between antelope and
antelope. A human analogy may help to make this difficult principle a
little clearer. The baker does not fear the competition of the butcher
in the struggle for life: it is the competition of the other bakers that
sometimes inexorably crushes him out of existence. The lawyer does not
press hard upon the doctor, nor the architect upon the journeyman
painter. A war in the Soudan or in South Africa is far less fatal to the
workman in our great towns than the ceaseless competition of his
fellow-workmen. It is not the soldier that kills the artisan, but the
number of other artisans who undersell him and crowd to fill up every
vacant position. In this way the great enemies of the individual
herbivore are not the carnivores, but the other herbivores. The lion
eats the antelope, to be sure; but the real struggle lies between lion
and lion for a fair share of meat, or between antelope and antelope for
a fair share of pasturage. _Homo homini lupus_, says the old proverb,
and so, we may add, in a wider sense, _lupus lupo lupus_, also. Of
course, the carnivore plays a great part in the selective process; but
he is the selector only; the real competition is between the selected.
Now, let us take the case of the plant. A thousand seedlings occupy the
space where few alone can ultimately grow; and between these seedlings
the struggle is fierce, the strongest and best adapted ultimately
surviving. To take Darwin's own example, the mistletoe, which is a
parasite, cannot truly be said to struggle with the apple tree on which
it fastens; for if too many parasites cover a tree, it perishes, and so
they kill themselves as well as their host, all alike dying together.
But several seedling mistletoes growing together on the same branch may
fairly be said to struggle with one another for light and air; and since
mistletoe seeds are disseminated by birds and dropped by them in the
angles of branches, the mistletoe may also be said to compete with other
berry-bearing bushes, like cornel and hawthorn, for the ministrations of
the fruit-eating birds. The struggle is fierce between allied kinds, and
fiercest of all between individual members of the same species.
Owing to this constant struggle, variations, however slight, and from
whatever cause arising, if in any degree profitable to the individual
which presents them, will tend to the preservation of the particular
organism, and, being on the average inherited by its offspring, will
similarly tend to increase and multiply in the world at large. This is
the principle of natural selection or survival of the fittest--the great
principle which Darwin and Wallace added to the evolutionism of Lamarck
and his successors.
Let us take a single concrete example. In the desert, with its
monotonous sandy colouring, a black insect or a white insect, still more
a red insect or a blue insect, would be immediately detected and
promptly devoured by its natural enemies, the birds and lizards. But any
greyish or yellowish insects would be less likely to attract attention
at first sight, and would be overlooked as long as there were any more
conspicuous individuals of their own kind about for the birds and
lizards to feed on at their leisure. Hence, in a very short time, the
desert would be depopulated of all but the greyest and yellowest
insects; and among these the birds would pick out those which differed
most markedly in hue or shade from the sand around them. But those
which happened to vary most in the direction of a sandy or spotty colour
would be most likely to survive, and to become the parents of future
generations. Thus, in the course of long ages, all the insects which
inhabit deserts have become sand-coloured; because the least sandy were
perpetually picked out for destruction by their ever-watchful foes,
while the most sandy escaped and multiplied and replenished the earth
with their own likes.
Conversely, the birds and the lizards again would probably begin by
being black, and white, and blue, and green, like most other birds and
lizards in the world generally. But the insect would have ample warning
of the near approach of such conspicuous self-advertising enemies, and
would avoid them accordingly whenever they appeared within range of his
limited vision, either by lying close, or by shamming death, or by
retreating precipitately to holes and crannies. Therefore, whatever
individual birds or lizards happened to vary most in the direction of
grey or sand-colour, and so to creep unobserved upon the unguarded
insects, would succeed best on the average in catching beetles or desert
grasshoppers. Hence, by the slow dying out of the more highly coloured
and distinctive insect-eaters, before the severe competition of the
greyest and sandiest, all the birds and lizards of the desert have
become at last as absolutely sand-coloured as the insects themselves.
Only the greyest insect could escape the bird; only the greyest bird,
_en revanche_, could surprise and devour the unwary insect.
Sir Charles Lyell and the elder De Candolle had already seen the great
importance of the struggle for existence in the organic world, but
neither of them had observed the magnificent corollary of natural
selection, which flows from it almost as a mathematical necessity when
once suggested; for, given indefinite variability, and a geometrical,
rate of increase, it must needs follow that some varieties will be
better suited to the circumstances than others, and therefore that they
will survive on the average in increased proportions. A passage from one
of Lyell's early letters will show how near he too went to this great
luminous generalisation, and yet how utterly he missed the true
implications of his own vague and chaotic idea. He writes thus to Sir
John Herschel in 1836, while Darwin was still but homeward bound on the
voyage of the 'Beagle':--
'In regard to the origination of new species, I am very glad to find
that you think it probable that it may be carried on through the
intervention of intermediate causes.... An insect may be made in one of
its transformations to resemble a dead stick, or a leaf, or a lichen, or
a stone, so as to be somewhat less easily found by its enemies; or if
this would make it too strong, an occasional variety of the species may
have this advantage conferred on it; or if this would be still too much,
one sex of a certain variety. Probably there is scarcely a dash of
colour on the wing or body of which the choice would be quite arbitrary,
or which might not affect its duration for thousands of years.'
Now, this comes in some ways perilously near to Darwin indeed; but in
the most important point of all it is wide apart from him as the pole is
from the equator. For Lyell thought of all this as a matter of external
teleological arrangement; he imagined a deliberate power from outside
settling it all by design beforehand, and granting to varieties or
species these special peculiarities in a manner that was at bottom
essentially supernatural, or in other words miraculous; whereas Darwin
thinks of it as the necessary result of the circumstances themselves, an
inevitable outcome of indefinite variability _plus_ the geometrical rate
of increase. Where Lyell sees a final cause, Darwin sees an efficient
cause; and this distinction is fundamental. It marks Darwin's position
as that of a great philosophical thinker, who can dash aside at once all
metaphysical cobwebs, and penetrate to the inmost recesses of things,
unswerved by the vain but specious allurements of obvious and misleading
teleological fallacies.
Darwin also laid great stress on the immense complexity of the relations
which animals and plants bear to one another, in the struggle for
existence. For example, on the heathy uplands near Farnham in Surrey,
large spaces were at one time enclosed, on which, within ten years,
self-grown fir-trees from the wind-borne seeds of distant clumps sprang
up so thickly as actually to choke one another with their tiny branches.
All over the heaths outside, when Darwin looked for them, he could not
find a single fir, except the old clumps on the hilltops, from which the
seedlings themselves had originally sprung. But, on looking closer among
the stems of the heath, he descried a number of very tiny firs, which
had been perpetually browsed down by the cattle on the commons; and one
of them, with twenty-six rings of growth, had during many years
endeavoured unsuccessfully to raise its head above the surrounding
heather. Hence, as soon as the land was enclosed, and the cattle
excluded, it became covered at once with a thick growth of vigorous
young fir-trees. Yet who would ever have supposed beforehand that the
mere presence or absence of cattle would absolutely have determined the
very existence of the Scotch fir throughout a wide range of well-adapted
sandy English upland?
To take another curious instance mentioned by Darwin. In Paraguay,
unlike the greater part of neighbouring South America, neither horses
nor cattle have ever run wild. This is due to the presence of a
parasitic fly, which lays its eggs in their bodies when first born, the
maggots killing off the tender young in their first stages. But if any
cause were to alter the number of the dangerous flies, then cattle and
wild horses would abound; and this would alter the vegetation, as Darwin
himself observed in other parts of America; and the change in the
vegetation would affect the insects; and that again the insectivorous
birds; and so on in ever widening circles of incalculable complexity.
Once more, to quote the most famous instance of all, the visits of
humble-bees are absolutely necessary in order to place the pollen in the
right position for setting the seeds of purple clover. Heads from which
Darwin excluded the bees produced no seeds at all. Hence, if humble-bees
became extinct in England, the red clover, too, would die off: and
indeed, in New Zealand, where there are no humble-bees, and where the
efforts to introduce them for this very purpose have been uniformly
unsuccessful, the clover never sets its seed at all, and fresh stocks
have to be imported at great expense every year from Europe. But the
number of humble-bees in any district largely depends upon the number of
field-mice, which destroy the combs and nests in immense quantities.
The number of mice, again, is greatly affected by the proportion of cats
in the neighbourhood; so that Colonel Newman, who paid much attention to
this subject, found humble-bees most numerous in the neighbourhood of
villages and small towns, an effect which he attributed to the abundance
of cats, and the consequent scarcity of the destructive field-mice. Yet
here once more, who could suppose beforehand that the degree to which
the purple clover set its seeds was in part determined by the number of
cats kept in houses in the surrounding district?
One of Darwin's own favourite examples of the action of natural
selection, which he afterwards expanded largely in his work on Orchids
and in several other volumes, is that which relates to the origin of
conspicuous flowers. Many plants have a sweet excretion, which is
eliminated sometimes even by the leaves, as in the case of the common
laurel. This juice, though small in quantity, is eagerly sought and
eaten by insects. Now let us suppose that, in some variety of an
inconspicuous flower, similar nectar was produced in the neighbourhood
of the petals and stamens. Insects, in seeking the nectar, would dust
their bodies over with the pollen, and would carry it away with them to
the next flower visited. This would result in an act of crossing; and
that act, as Darwin afterwards abundantly proved in a separate and very
laborious treatise, gives rise to exceptionally vigorous seedlings,
which would therefore have the best chance of flourishing and surviving
in the struggle for existence. The flowers which produced most honey
would oftenest be visited, and oftenest crossed; so that they would
finally form a new species. The more brightly coloured among them,
again, would be more readily discriminated than the less brightly
coloured; and this would give them such an advantage that in the long
run, as we actually see, almost all habitually insect-fertilised flowers
would come to have brilliant petals. The germ of this luminous idea,
once more, is to be found in Sprengel's remarkable work on the
fertilisation of flowers--a work far in advance of its time in many
ways, and to which Darwin always expressed his deep obligations; but, as
in so many other instances, while Sprengel looked upon all the little
modifications and adaptations of flower and insect to one another as the
result of distinct creative design, Darwin looked upon them as the
result of natural selection, working upon the basis of indeterminate
spontaneous variations.
How do these variations arise? Not by chance, of course (for in the
strict scientific sense nothing on earth can be considered as really
fortuitous), but as the outcome for the most part of very minute organic
causes, whose particular action it is impossible for us to predict with
our present knowledge. Some physical cause in each case there must
necessarily be; and indeed it is often possible to show that certain
changes of condition in the parent do result in variations in the
offspring, though what special direction the variation will take can
never be foretold with any accuracy. In short, our ignorance of the laws
of variation is profound, but our knowledge of the fact is clear and
certain. The fact alone is essential to the principle of natural
selection; the cause, though in itself an interesting subject of
inquiry, may be safely laid aside for the present as comparatively
unimportant. What we have actually given to us in the concrete universe
is, organisms varying perpetually in minute points, and a rapid rate of
increase causing every minute point of advantage to be exceptionally
favoured in the struggle for existence.
But Darwin is remarkable among all broachers of new theories for the
extraordinary candour and openness of his method. He acknowledged
beforehand all the difficulties in the way of his theory, and though he
himself confessed that some of them were serious (a statement which
subsequent research has often rendered unnecessary), he met many of them
with cogent arguments by anticipation, and demolished objections before
they could even be raised against him by hostile critics. Of these
objections, only two need here be mentioned. The first is the question,
why is not all nature even now a confused mass of transitional forms?
Why do genera and species exist as we see them at present in broad
distinction one from the other? To this Darwin answers rightly that,
where the process of species-making is still going on, we do actually
find fine gradations and transitional forms existing between genera,
varieties, and species.[3] But, furthermore, as natural selection acts
solely by the preservation of useful modifications, each better-adapted
new form will always tend in a fully stocked country to oust and
exterminate its own unimproved parent type, as well as all other
competing but less perfect varieties. Thus natural selection and
extinction of intermediates go for ever hand in hand. The more perfect
the new variety, the more absolutely will it kill off the intermediate
forms. The second great difficulty lies in the question of the origin of
instinct, which, as Darwin shows, by careful inductive instances, may
have arisen by the slow and gradual accumulation of numerous slight yet
profitable variations.
I have dwelt at some length upon those portions of the 'Origin of
Species' which deal in detail with the theory of natural selection, the
chief contribution which Darwin made to the evolutionary movement,
because it is impossible otherwise fully to understand the great gulf
which separates his evolutionism from the earlier evolutionism of
Lamarck and his followers. But it is impracticable here to give any idea
of the immense wealth of example and illustration which Darwin brought
to the elucidation of every part of his complex problem. In order to
gain a full conception of this side of his nature, we must turn to the
original treatise itself, and still more to the subsequent volumes in
which the ground-work of observations and experiments on which he based
his theory was more fully detailed for the specialist public.
The remainder of Darwin's epoch-making work deals, strictly speaking,
rather with the general theory of 'descent with modification' than with
the special doctrine of natural selection. It restates and reinforces,
by the light of the new additional concept, and with fuller facts and
later knowledge, the four great arguments already known in favour of
organic evolution as a whole, the argument from Geological Succession,
the argument from Geographical Distribution, the argument from
Embryological Development, and the argument from Classificatory
Affinities. Each of these we may briefly summarise.
The geological record is confessedly imperfect. At the time when Darwin
first published the 'Origin of Species,' it had disclosed to our view
comparatively few intermediate or transitional forms between the chief
great classes of plants or animals; since that time, in singular
confirmation of the Darwinian hypothesis, it has disclosed an immense
number of such connecting types, amongst which may be more particularly
noticed the 'missing links' between the birds and reptiles, the
ancestors of the horses, the camels, and the pigs, and the common
progenitor of the ruminants and the pachyderms, two great groups classed
by Cuvier as distinct orders--all of which instances were incorporated
by Darwin in later editions of his 'Origin of Species.' But, apart from
these special and newly discovered cases, the whole general course of
geological history 'agrees admirably with the theory of descent with
modification through variation and natural selection.' The simpler
animals of early times are followed by the more complex and more
specialised animals of later geological periods. As each main group of
animals appears upon the stage of life, it appears in a very central and
'generalised' form; as time goes on, we find its various members
differing more and more widely from one another, and assuming more and
more specialised adaptive forms. And in each country it is found, as a
rule, that the extinct animals of the later formations bear a close
general resemblance and relationship to the animals which now inhabit
the same regions. For example, the fossil mammals from the Australian
caves are nearly allied to the modern kangaroos, phalangers, and
wombats; and the gigantic extinct sloths and armadillos of South America
are reproduced in their smaller representatives at the present day. So,
too, the moa of New Zealand was a huge apteryx; and the birds
disentombed from the bone-caves of Brazil show close affinities to the
toucans and jacanars that still scream and flit in countless flocks
among Brazilian forests. The obvious implication is that the animals now
inhabiting any given area are the modified descendants of those that
formerly inhabited it. 'On the theory of descent with modification, the
great law of the succession of the same types within the same areas is
at once explained.'
This last consideration leads us up to the argument from Geographical
Distribution. In considering the various local faunas and floras on the
face of the globe, no point strikes one more forcibly than the fact that
neither their similarities nor their dissimilarities can be accounted
for by climate or physical conditions. The animals of South Africa do
not in the least resemble the animals of the corresponding belt of South
America; the Australian beasts and birds and trees are utterly unlike
those of France and Germany; the fishes and crustaceans of the Pacific
at Panama are widely different from those of the Caribbean at the same
point, separated from them only by the narrow belt of intervening
isthmus. On the other hand, within the same continuous areas of sea or
land, however great the differences of physical conditions, we find
everywhere closely related types in possession of the most distinct and
varied situations. On the burning plains of La Plata we get the agouti
and the bizcacha as the chief rodents; we ascend the Cordillera, and
close to the eternal snows we discover, not hares and rabbits like those
of Europe, but a specialised chilly mountain form of the same distinctly
South American type. We turn to the rivers, and we see no musk-rat or
beaver, but the coypu and capybara, slightly altered varieties of the
original bizcacha ancestor. Australia has no wolf, but it has instead
fierce and active carnivorous marsupials; it has no mice, but some of
its tiny kangaroo-like creatures fulfil analogous functions in its
animal economy. Everywhere the evidence points to the conclusion that
local species have been locally evolved from pre-existing similar
species. The oceanic isles, of which Darwin had had so large an
experience, and especially his old friends the Galapagos, come in
usefully for this stage of the question. They are invariably inhabited,
as Darwin pointed out, and as Wallace has since abundantly shown in the
minutest detail, by waifs and strays from neighbouring continents,
altered and specialised by natural selection in accordance with the
conditions of their new habitat. As a rule, they point back to the
districts whence blow the strongest and most prevalent winds; and the
modifications they have undergone are largely dependent upon the nature
of the other species with which they have to compete, or to whose habits
they must needs accommodate themselves. In such cases it is easy to see
how far Darwin's special conception of natural selection helps to
explain and account for facts not easily explicable by the older
evolutionism of mere descent with modification.
Embryology, the study of early development in the individual animal or
plant, also throws much side light upon the nature and ancestry of each
species or family. For example, gorse, which is a member of the
pea-flower tribe, has in its adult stage solid, spiny, thorn-like
leaves, none of which in the least resemble the foliage of the clover,
to which it is closely related; but the young seedling in its earliest
stages has trefoil leaves, which only slowly pass by infinitesimal
gradations into flat blades and finally into the familiar defensive
prickles. Here, natural selection under stress of herbivorous animals on
open heaths and commons has spared only those particular gorse-bushes
which varied in the direction of the stiffest and most inedible foliage;
but the young plant in its first days still preserves for us the trefoil
leaf which it shared originally with a vast group of clover-like
congeners. The adult barnacle, once more, presents a certain fallacious
external resemblance to a mollusk, and was actually so classed even by
the penetrating and systematic intellect of Cuvier; but a glance at the
larva shows an instructed eye at once that it is really a shell-making
and abnormal crustacean. On a wider scale, the embryos of mammals are at
first indistinguishable from those of birds or reptiles; the feet of
lizards, the hoofs of horses, the hands of man, the wings of the bat,
the pinions of birds, all arise from the same fundamental shapeless bud,
in the same spot of an almost identical embryo. Even the human foetus,
at a certain stage of its development, is provided with gill-slits,
which point dimly back to the remote ages when its ancestor was
something very like a fish. The embryo is a picture, more or less
obscured and blurred in its outline, of the common progenitor of a whole
great class of plants or animals.
Finally, classification points in the same way to the affiliation of all
existing genera and species upon certain early divergent ancestors. The
whole scheme of the biological system, as initiated by Linnæus and
improved by Cuvier, Jussieu, De Candolle, and their successors, is
essentially that of a genealogical tree. The prime central vertebrate
ancestor--to take the case of the creatures most familiar to the general
reader--appears to have been an animal not unlike the existing lancelet,
a mud-haunting, cartilaginous, undeveloped fish, whose main lineaments
are also embryologically preserved for us in the ascidian larva and the
common tadpole. From this early common centre have been developed,
apparently, in one direction the fishes, and in another the amphibian
tribes of frogs, newts, salamanders, and axolotls. From an early
amphibian, again, the common ancestor of birds, reptiles, and mammals
seems to have diverged: the intermediate links between bird and reptile
being faintly traced among the extinct deinosaurians and the
archæopteryx, some years subsequently to the first appearance of the
'Origin of Species;' while the ornithorhynchus, which to some extent
connects the mammals, and especially the marsupials, with the lower
egg-laying types of vertebrate, was already well-known and thoroughly
studied before the publication of Darwin's great work. Throughout, the
indications given by all the chief tribes of animals and plants point
back to slow descent and divergence from common ancestors; and all the
subsequent course of palæontological research has supplied us rapidly,
one after another, with the remains of just such undifferentiated
family starting-points.
Stress has mainly been laid, in this brief and necessarily imperfect
abstract, on the essentially Darwinian principle of natural selection.
But Darwin did not himself attribute everything to this potent factor in
the moulding of species. 'I am convinced,' he wrote pointedly in the
introduction to his first edition, 'that natural selection has been the
main but not the exclusive means of modification.' He attributed
considerable importance as well to the Lamarckian principle of use and
disuse, already so fully insisted upon before him by Mr. Herbert
Spencer. The chief factors in his compound theory, as given in his own
words at the end of his work, areas follows: 'Growth with Reproduction;
Inheritance, which is almost implied by reproduction; Variability, from
the indirect and direct action of the conditions of life, and from use
and disuse; a Ratio of Increase, so high as to lead to a Struggle for
Life, and as a consequence to Natural Selection, entailing Divergence of
Character, and the Extinction of the less improved forms. Thus, from the
war of nature, from famine and death, the most exalted object which we
are capable of conceiving, namely, the production of the higher animals,
directly follows.'
Such was the simple and inoffensive-looking bombshell which Darwin
launched from his quiet home at Down into the very midst of the
teleological camp in the peaceful year 1859. Subsequent generations will
remember the date as a crisis and turning-point in the history of
mankind.
[2] The remainder of the present chapter, which consists almost entirely
of an exposition of the doctrine of natural selection, may safely be
skipped by the reader already well acquainted with the Origin of
Species. The abstract is taken for the most part from the latest and
fullest enlarged edition, but attention is usually called in passing to
the points which did not appear in the first issue of 1859.
[3] The researches of Seebohm and others have since proved that this is
really the case to a far greater extent than Darwin was aware of in
1859, or, indeed, till many years afterward.