Charles Darwin.

The origin of species online

. (page 47 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 47 of 50)
Font size
QR-code for this ebook

face. Sexual selection has given the most brilliant colors, elegant patterns,
and other ornaments to the males, and sometimes to both sexes, of many
birds, butterflies, and other animals. With birds it has often rendered the
voice of the male musical to the female, as well as to our ears. Flowers and
fruit have been rendered conspicuous by brilliant colors in contrast with the
green foliage, in order that the flowers may be easily seen, visited and fer-
tilized by insects, and the seeds disseminated by birds. How it comes that
certain colors, sounds, and forms should give pleasure to man and the lower
animals, that is, how the sense of beauty in its simplest form was first ac-
quired, we do not know any more than how certain odors and flavors were
first rendered agreeable.

As natural selection acts by competition, it adapts and improves the in-
habitants of each country only in relation to their co-inhabitants; so that we
need feel no surprise at the species of any one country, although on the ordi-
nary view supposed to have been created and specially adapted for that
country, being beaten and supplanted by the naturalized productions from
another land. Nor ought we to marvel if all the contrivances in nature be not,
as far as we can judge, absolutely perfect, as in the case even of the human
eye; or if some of them be abhorrent to our ideas of fitness. We need not
marvel at the sting of the bee, when used against an enemy, causing the bee's
own death; at drones being produced in such great numbers for one single


act, and being then slaughtered by their sterile sisters; at the astonishing
waste of pollen by our fir-trees; at the instinctive hatred of the queen bee
for her own fertile daughters; at ichneumonidae feeding within the living
bodies of caterpillars; or at other such cases. The wonder, indeed, is, on the
theory of natural selection, that more cases of the want of absolute perfec-
tion have not been detected.

The complex and little known laws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed the
production of distinct species. In both cases physical conditions seem to have
produced some direct and definite effect, but how much we cannot say. Thus,
when varieties enter any new station, they occasionally assume some of the
characters proper to the species of that station. With both varieties and
species, use and disuse seem to have produced a considerable effect; for it is
impossible to resist this conclusion when we look, for instance, at the logger-
headed duck, which has wings incapable of flight, in nearly the same condi-
tion as in the domestic duck; or when we look at the burrowing tucu-tucu,
which is occasionally blind, and then at certain moles, which are habitually
blind and have their eyes covered with skin; or when we look at the blind
animals inhabiting the dark caves of America and Europe. With varieties
and species, correlated variation seems to have played an important part, so
that when one part has been modified other parts have been necessarily
modified. With both varieties and species, reversions to long-lost characters
occasionally occur. How inexplicable on the theory of creation is the occa-
sional appearance of stripes on the shoulders and legs of the several species of
the horse-genus and of their hybrids ! How simply is this fact explained if we
believe that these species are all descended from a striped progenitor, in the
same manner as the several domestic breeds of the pigeon are descended from
the blue and barred rock-pigeon !

On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the same
genus differ from each other, be more variable than generic characters in
which they all agree ? Why, for instance, should the color of a flower be more
likely to vary in any one species of a genus, if the other species possess dif-
ferently colored flowers, than if all possessed the same colored flowers? If
species are only well-marked varieties, of which the characters have become
in a high degree permanent, we can understand this fact; for they have
already varied since they branched off from a common progenitor in cer-
tain characters, by which they have come to be specifically distinct from each
other; therefore these same characters would be more likely again to vary
than the generic characters which have been inherited without change for
an immense period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in one species alone of a genus, and
therefore, as we may naturally infer, of great importance to that species,
should be eminently liable to variation; but, on our view, this part has un-
dergone, since the several species branched ofT from a common progenitor,
an unusual amount of variability and modification, and therefore we might


expect the part generally to be still variable. But a part may be developed
in the most unusual manner, like the wing of a bat, and yet not be more
variable than any other structure, if the part be common to many subordi-
nate forms, that is, if it has been inherited for a very long period; for in this
case it will have been rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater diffi-
culty than do corporeal structures on the theory of the natural selection of
successive, slight, but profitable modifications. We can thus understand why
nature moves by graduated steps in endowing different animals of the same
class with their severaL instincts. I have attempted to show how much light
the principle of gradation throws on the admirable architectural powers of
the hive-bee. Habit no doubt often comes into play in modifying instincts;
but it certainly is not indispensable, as we see in the case of neuter insects,
which leave no progeny to inherit the effects of long-continued habit. On the
view of all the species of the same genus having descended from a common i
parent, and having inherited much in common, we can understand how it
is that allied species, when placed under widely different conditions of life,
yet follow nearly the same instincts; why the thrushes of tropical and tem-
perate South America, for instance, line their nests with mud like our British
species. On the view of instincts having been slowly acquired through natural
selection, we need not marvel at some instincts being not perfect and liable
to mistakes, and at many instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents — in being absorbed into
each other by successive crosses, and in other such points — as do the
crossed offspring of acknowledged varieties. This similarity would be a
strange fact, if species had been independently created, and varieties had
been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme degree,
then the facts, which the record does give, strongly support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change, after equal intervals of
time, is widely different in different groups. The extinction of species and of
whole groups of species, which has played so conspicuous a part in the his-
tory of the organic world, almost inevitably follows from the principle of
natural selection; for old forms are supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain of
ordiriary generation is once broken. The gradual diffusion of dominant forms,
with the slow modification of their descendants, causes the forms of life,
after long intervals of time, to appear as if they had changed simultane-
ously throughout the world. The fact of the fossil remains of each formation
being in some degree intermediate in character between the fossils in the
formations above and below, is simply explained by their intermediate posi-
tion in the chain of descent. The grand fact that all extinct beings can be
classed with all recent beings, naturally follows from the living and the ex-


tinct being the offspring of common parents. As species have generally di-
verged in character during their long course of descent and modification, we
can understand why it is that the more ancient forms, or early progenitors
of each group, so often occupy a position in some degree intermediate be-
tween existing groups. Recent forms are generally looked upon as being, on
the whole, higher in the scale of organization than ancient forms; and they
must be higher, in so far as the later and more improved forms have con-
quered the older and less improved forms in the struggle for life; they have
also generally had their organs more specialized for different functions. This
fact is perfectly compatible with numerous beings still retaining simple and
but little improved structures, fitted for simple conditions of life; it is like-
wise compatible with some forms having retrograded in organization, by
having become at each stage of descent better fitted for new and degraded
habits of life. Lastly, the wonderful law of the long endurance of allied forms
on the same continent — of marsupials in Australia, of edentata in America,
and other such cases — is intelligible, for within the same country the existing
and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has been dur-
ing the long course of ages much migration from one part of the world to
another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading facts in
distribution. We can see why there should be so striking a parallelism in the
distribution of organic beings throughout space, and in their geological suc-
cession throughout time ; for in both cases the beings have been connected by
the bond of ordinary generation, and the means of modification have been
the same. We see the full meaning of the wonderful fact, which has struck
every traveller, namely, that on the same continent, under the most diverse
conditions, under heat and cold, on mountain and lowland, on deserts and
marshes, most of the inhabitants within each great class are plainly related;
for they are the descendants of the same progenitors and early colonists. On
this same principle of former migration, combined in most cases with modi-
fication, we can understand, by the aid of the Glacial period, the identity of
some few plants, and the close alliance of many others, on the most distant
mountains, and in the northern and southern temperate zones; and likewise
the close alliance of some of the inhabitants of the sea in the northern and
southern temperate latitudes, though separated by the whole intertropical
ocean. Although two countries may present physical conditions as closely
similar as the same species ever require, we need feel no surprise at their
inhabitants being widely different, if they have been for a long period com-
pletely sundered from each other; for as the relation of organism to organ-
ism is the most important of all relations, and as the two countries will have
received colonists at various periods and in different proportions, from some
other country or from each other, the course of modification in the two areas
will inevitably have been different.

On this view of migration, with subsequent modification^ we see why


oceanic islands are inhabited by only few species, but of these, why many are
peculiar or endemic forms. We clearly see why species belonging to those
groups of animals which cannot cross wide spaces of the ocean, as frogs and
terrestrial mammals, do not inhabit oceanic islands; and why, on the other
hand, new and peculiar species of bats, animals which can traverse the ocean,
are often found on islands far distant from any continent. Such cases as the
presence of peculiar species of bats on oceanic islands and the absence of all
other terrestrial mammals, are facts utterly inexplicable on the theory of in-
dependent acts of creation.

The existence of closely allied representative species in any two areas, im-
plies, on the theory of descent with modification, that the same parent forms
formerly inhabited both areas : and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species are still
common to both. Wherever many closely allied yet distinct species occur,
doubtful forms and varieties belonging to the same groups likewise occur.
It is a rule of high generality that the inhabitants of each area are related
to the inhabitants of the nearest source whence immigrants might have been
derived. We see this in the striking relation of nearly all the plants and ani-
mals of the Galapagos Archipelago, of Juan Fernandez, and of the other
American islands, to the plants and animals of those of the mainland; and
of those of the Cape de Verde Archipelago, and of the other African islands
to the African mainland. It must be admitted that these facts receive no ex-
planation on the theory of creation.

The fact, as we have seen, that all past and present organic beings can be ar-
ranged within a few great classes, in groups subordinate to groups, and with the
extinct groups often falling in between the recent groups, is intelligible on the
theory of natural selection with its contingencies of extinction and divergence
of character. On these same principles we see how it is that the mutual affini-
ties of the forms within each class are so complex and circuitous. We see why
certain characters are far more serviceable than others for classification ; why
adaptive characters, though of paramount importance to the beings, are of
hardly any importance in classification; why characters derived from rudi-
mentary parts, though of no service to the beings, are often of high classifi-
catory value; and why embryological characters are often the most valuable
of all. The real affinities of all organic beings, in contra-distinction to their
adaptive resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement with the acquired grades of
difference, marked by the terms, varieties, species, genera, families, etc. ; and
we have to discover the lines of descent by the most permanent characters,
whatever they may be, and of however slight vital importance.

The similar framework of bones in the hand of a man, wing of a bat, fin
of the porpoise, and leg of the horse — the .same number of vertebrae forming
the neck of the giraffe and of the elephant — and innumerable other such
facts, at once explain themselves on the theory of descent with slow and
slight successive modifications. The similarity of pattern in the wing and in
the leg of a bat, though used for such different purpose — in the jaws and


legs of a crab — in the petals, stamens, and pistils of a flower, is likewise, to a
large extent, intelligible on the view of the gradual modification of parts or
organs, which were aboriginally alike in an early progenitor in each of these
classes. On the principle of successive variations not always supervening at
an early age, and being inherited at a corresponding not early period of life,
we clearly see why the embryos of mammals, birds, reptiles, and fishes should
be so closely similar and so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those of a fish which has to breathe the air
dissolved in water by the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life ; and
we can understand on this view the meaning of rudimentary organs. But dis-
use and selection will generally act on each creature, when it has come to
maturity and has to play its full part in the struggle for existence, and will
thus have little power on an organ during early life; hence the organ will
not be reduced or rendered rudimentary at this early age. The calf, for in-
stance, has inherited teeth, which never cut through the gums of the upper
jaw, from an early progenitor having well-developed teeth; and we may be-
lieve that the teeth in the mature animal were formerly reduced by disuse,
owing to the tongue and palate, or lips, having become excellently fitted
through natural selection to browse without their aid; whereas in the calf,
the teeth have been left unaffected, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the present
day. On the view of each organism with all its separate parts having been
specially created, how utterly inexplicable is it that organs bearing the plain
stamp of inutility, such as the teeth in the embryonic calf, or the shrivelled
wings under the soldered wing-covers of many beetles, should so frequently
occur. Nature may be said to have taken pains to reveal her scheme of modi-
fication, by means of rudimentary organs, of embryological and homologous
structures, but we are too blind to understand her meaning.

I have now recapitulated the facts and considerations which have thor-
oughly convinced me that species have been modified, during a long course of
descent. This has been effected chiefly through the natural selection of
numerous successive, slight, favorable variations ; aided in an important man-
ner by the inherited effects of the use and disuse of parts; and in an unim-
portant manner, that is, in relation to adaptive structures, whether past or
present, by the direct action of external conditions, and by variations which
seem to us in our ignorance to arise spontaneously. It appears that I formerly
underrated the frequency and value of these latter forms of variation, as
leading to permanent modifications of structure independently of natural
selection. But as my conclusions have lately been much misrepresented, and
it has been stated that I attribute the modification of species exclusively to
natural selection, I may be permitted to remark that in the first edition of
this work, and subsequently, I placed in a most conspicuous position —
namely, at the close of the Introduction — the following words: "I am con-


vinced that natural selection has been the main but not the exclusive means
of modification." This has been of no avail. Great is the power of steady mis-
representation; but the history of science shows that fortunately this power r
does not long endure.

It can hardly be supposed that a false theory would explain, in so satisfac-
tory a manner as does the theory of natural selection, the several large classes
of facts above specified. It has recently been objected that this is an unsafe
method of arguing; but it is a method used in judging of the common events
of life, and has often been used by the greatest natural philosophers. The
undulatory theory of light has thus been arrived at; and the belief in the
revolution of the earth on its own axis was until lately supported by hardly
any direct evidence. It is no valid objection that science as yet throws no light
on the far higher problem of the essence or origin of life. Who can explain
what is the essence of the attraction of gravity? No one now objects to fol-
lowing out the results consequent on this unknown element of attraction;
notwithstanding that Leibnitz formerly accused Newton of introducing "oc-
cult qualities and miracles into philosophy."

I see no good reasons why the views given in this volume should shock the
religious feelings of any one. It is satisfactory, as showing how transient such
impressions are, to remember that the greatest discovery ever made by man,
namely, the law of the attraction of gravity, was also attacked by Leibnitz,
"as subversive of natural, and inferentially of revealed, religion." A cele-
brated author and divine has written to me that "he has gradually learned to
see that it is just as noble a conception of the Deity to believe that He created
a few original forms capable of self-development into other and needful
forms, as to believe that He required a fresh act of creation to supply the
voids caused by the action of His laws."

Why, it may be asked, until recently did nearly all the most eminent living
naturalists and geologists disbelieve in the mutability of species? It cannot be
asserted that organic beings in a state of nature are subject to no variation;
it cannot be proved that the amount of variation in the course of long ages
is a limited quantity; no clear distinction has been, or can be, drawn be-
tween species and well-marked varieties. It cannot be maintained that species
when intercrossed are invariably sterile and varieties invariably fertile; or
that sterility is a special endowment and sign of creation. The belief that
species were immutable productions was almost unavoidable as long as the
history of the world was thought to be of short duration; and now that we
have acquired some idea of the lapse of time, we are too apt to assume, with-
out proof, that the geological record is so perfect that it would have afforded
us plain evidence of the mutation of species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow in
admitting great changes of which we do not see the steps. The difficulty is
the same as that felt by so many geologists, when Lyell first insisted that long
lines of inland cliffs had been formed, and great valleys excavated, by the
agencies which we still see at work. The mind cannot possibly grasp the full


meaning of the term of even a million years; it cannot add up and perceive
the full effects of many slight variations, accumulated during an almost in-
finite number of generations.

Although I am fully convinced of the truth of the views given in this vol-
ume under the form of an abstract, I by no means expect to convince ex-
perienced naturalists whose minds are stocked with a multitude of facts all
viewed, during a long course of years, from a point of view directly opposite
to mine. It is so easy to hide our ignorance under such expressions as the
"plan of creation," "unity of design," etc., and to think that we give an ex-
planation when we only restate a fact. Any one whose disposition leads him
to attach more weight to unexplained difficulties than to the explanation of
a certain number of facts will certainly reject the theory. A few naturalists,
endowed with much flexibility of mind, and who have already begun to doubt
the immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will be
able to view both sides of the question with impartiality. Whoever is led to
believe that species are mutable will do good service by conscientiously ex-
pressing his conviction; for thus only can the load of prejudice by which this
subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a mul-
titude of reputed species in each genus are not real species; but that other
species are real, that is, have been independently created. This seems to me
a strange conclusion to arrive at. They admit that a multitude of forms,
which till lately they themselves thought were special creations, and which
are still thus looked at by the majority of naturalists, and which conse-
quently have all the external characteristic features of true species — they
admit that these have been produced by variation, but they refuse to extend
the same view to other and slightly different forms. Nevertheless, they do not
pretend that they can define, or even conjecture, which are the created forms
of life, and which are those produced by secondary laws. They admit varia-
tion as a vera causa in one case, they arbitrarily reject it in another, without

Online LibraryCharles DarwinThe origin of species → online text (page 47 of 50)