On the Origin of Species by Means of Natural Selection
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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. On the other hand, these
would be strange facts if species have been independently created, and
varieties have been produced by secondary laws.
If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, 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
history of the organic world, almost inevitably follows on the principle of
natural selection; for old forms will be supplanted by new and improved
forms. Neither single species nor groups of species reappear when the
chain of ordinary generation has once been 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 simultaneously 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 position in the chain of descent. The grand fact
that all extinct organic beings belong to the same system with recent
beings, falling either into the same or into intermediate groups, follows
from the living and the extinct being the offspring of common parents. As
the groups which have descended from an ancient progenitor have generally
diverged in character, the progenitor with its early descendants will often
be intermediate in character in comparison with its later descendants; and
thus we can see why the more ancient a fossil is, the oftener it stands in
some degree intermediate between existing and allied groups. Recent forms
are generally looked at as being, in some vague sense, higher than ancient
and extinct forms; and they are in so far higher as the later and more
improved forms have conquered the older and less improved organic beings in
the struggle for life. Lastly, the 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 a confined
country, the recent and the extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world to
another, owing to former climatal 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 succession 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 must have 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 will generally be
descendants of the same progenitors and early colonists. On this same
principle of former migration, combined in most cases with modification, 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, under the most different climates; and likewise the close
alliance of some of the inhabitants of the sea in the northern and southern
temperate zones, though separated by the whole intertropical ocean.
Although two areas may present the same physical conditions of life, we
need feel no surprise at their inhabitants being widely different, if they
have been for a long period completely separated from each other; for as
the relation of organism to organism is the most important of all
relations, and as the two areas will have received colonists from some
third source or from each other, at various periods and in different
proportions, the course of modification in the two areas will inevitably be
different.
On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that many
should be peculiar. We can clearly see why those animals which cannot
cross wide spaces of ocean, as frogs and terrestrial mammals, should not
inhabit oceanic islands; and why, on the other hand, new and peculiar
species of bats, which can traverse the ocean, should so often be found on
islands far distant from any continent. Such facts as the presence of
peculiar species of bats, and the absence of all other mammals, on oceanic
islands, are utterly inexplicable on the theory of independent acts of
creation.
The existence of closely allied or representative species in any two areas,
implies, on the theory of descent with modification, that the same parents
formerly inhabited both areas; and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species
common to both still exist. Wherever many closely allied yet distinct
species occur, many doubtful forms and varieties of the same species
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 nearly all the plants
and animals of the Galapagos archipelago, of Juan Fernandez, and of the
other American islands being related in the most striking manner to the
plants and animals of the neighbouring American mainland; and those of the
Cape de Verde archipelago and other African islands to the African
mainland. It must be admitted that these facts receive no explanation on
the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group, and
with extinct groups often falling in between 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 affinities of the species and genera 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 being, are of hardly any importance
in classification; why characters derived from rudimentary parts, though of
no service to the being, are often of high classificatory value; and why
embryological characters are the most valuable of all. The real affinities
of all organic beings are due to inheritance or community of descent. The
natural system is a genealogical arrangement, in which we have to discover
the lines of descent by the most permanent characters, however slight their
vital importance may be.
The framework of bones being the same 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 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 intelligible on the view of the gradual modification of parts or
organs, which were alike in the early progenitor of each class. 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 can
clearly see why the embryos of mammals, birds, reptiles, and fishes should
be so closely alike, and should be 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 in 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 tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning
of rudimentary organs. But disuse 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 of acting on an
organ during early life; hence the organ will not be much reduced or
rendered rudimentary at this early age. The calf, for instance, has
inherited teeth, which never cut through the gums of the upper jaw, from an
early progenitor having well-developed teeth; and we may believe, that the
teeth in the mature animal were reduced, during successive generations, by
disuse or by the tongue and palate having been fitted by natural selection
to browse without their aid; whereas in the calf, the teeth have been left
untouched by selection or disuse, 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 organic being and each separate organ having been
specially created, how utterly inexplicable it is that parts, like the
teeth in the embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain stamp
of inutility! Nature may be said to have taken pains to reveal, by
rudimentary organs and by homologous structures, her scheme of
modification, which it seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of 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 between 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, without 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 any great change of which we do not see the intermediate
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 slow action of the coast-waves. The mind
cannot possibly grasp the full meaning of the term of a hundred million
years; it cannot add up and perceive the full effects of many slight
variations, accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced 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,' &c., and to think
that we give an explanation 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
my theory. A few naturalists, endowed with much flexibility of mind, and
who have already begun to doubt on 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 expressing his conviction;
for only thus can the load of prejudice by which this subject is
overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude 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 consequently have every external characteristic feature of true
species,--they admit that these have been produced by variation, but they
refuse to extend the same view to other and very 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 variation as a vera causa in one case, they
arbitrarily reject it in another, without assigning any distinction in the
two cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors seem
no more startled at a miraculous act of creation than at an ordinary birth.
But do they really believe that at innumerable periods in the earth's
history certain elemental atoms have been commanded suddenly to flash into
living tissues? Do they believe that at each supposed act of creation one
individual or many were produced? Were all the infinitely numerous kinds
of animals and plants created as eggs or seed, or as full grown? and in the
case of mammals, were they created bearing the false marks of nourishment
from the mother's womb? Although naturalists very properly demand a full
explanation of every difficulty from those who believe in the mutability of
species, on their own side they ignore the whole subject of the first
appearance of species in what they consider reverent silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away in
force. But some arguments of the greatest weight extend very far. All the
members of whole classes can be connected together by chains of affinities,
and all can be classified on the same principle, in groups subordinate to
groups. Fossil remains sometimes tend to fill up very wide intervals
between existing orders. Organs in a rudimentary condition plainly show
that an early progenitor had the organ in a fully developed state; and this
in some instances necessarily implies an enormous amount of modification in
the descendants. Throughout whole classes various structures are formed on
the same pattern, and at an embryonic age the species closely resemble each
other. Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same class. I believe that
animals have descended from at most only four or five progenitors, and
plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy may
be a deceitful guide. Nevertheless all living things have much in common,
in their chemical composition, their germinal vesicles, their cellular
structure, and their laws of growth and reproduction. We see this even in
so trifling a circumstance as that the same poison often similarly affects
plants and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. Therefore I should infer
from analogy that probably all the organic beings which have ever lived on
this earth have descended from some one primordial form, into which life
was first breathed.
When the views entertained in this volume on the origin of species, or when
analogous views are generally admitted, we can dimly foresee that there
will be a considerable revolution in natural history. Systematists will be
able to pursue their labours as at present; but they will not be
incessantly haunted by the shadowy doubt whether this or that form be in
essence a species. This I feel sure, and I speak after experience, will be
no slight relief. The endless disputes whether or not some fifty species
of British brambles are true species will cease. Systematists will have
only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a far
more essential consideration than it is at present; for differences,
however slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as sufficient to raise both
forms to the rank of species. Hereafter we shall be compelled to
acknowledge that the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to be connected at
the present day by intermediate gradations, whereas species were formerly
thus connected. Hence, without quite rejecting the consideration of the
present existence of intermediate gradations between any two forms, we
shall be led to weigh more carefully and to value higher the actual amount
of difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names, as with the primrose and cowslip; and in this case
scientific and common language will come into accordance. In short, we
shall have to treat species in the same manner as those naturalists treat
genera, who admit that genera are merely artificial combinations made for
convenience. This may not be a cheering prospect; but we shall at least be
freed from the vain search for the undiscovered and undiscoverable essence
of the term species.
The other and more general departments of natural history will rise greatly
in interest. The terms used by naturalists of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, &c., will cease to be metaphorical, and will have a
plain signification. When we no longer look at an organic being as a
savage looks at a ship, as at something wholly beyond his comprehension;
when we regard every production of nature as one which has had a history;
when we contemplate every complex structure and instinct as the summing up
of many contrivances, each useful to the possessor, nearly in the same way
as when we look at any great mechanical invention as the summing up of the
labour, the experience, the reason, and even the blunders of numerous
workmen; when we thus view each organic being, how far more interesting, I
speak from experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the causes
and laws of variation, on correlation of growth, on the effects of use and
disuse, on the direct action of external conditions, and so forth. The
study of domestic productions will rise immensely in value. A new variety
raised by man will be a far more important and interesting subject for
study than one more species added to the infinitude of already recorded
species. Our classifications will come to be, as far as they can be so
made, genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigrees or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species,
which are called aberrant, and which may fancifully be called living
fossils, will aid us in forming a picture of the ancient forms of life.
Embryology will reveal to us the structure, in some degree obscured, of the
prototypes of each great class.
When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have within a not very
remote period descended from one parent, and have migrated from some one
birthplace; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled
to trace in an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences of the
inhabitants of the sea on the opposite sides of a continent, and the nature
of the various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme imperfection of
the record. The crust of the earth with its embedded remains must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual concurrence
of circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some
security the duration of these intervals by a comparison of the preceding
and succeeding organic forms. We must be cautious in attempting to
correlate as strictly contemporaneous two formations, which include few
identical species, by the general succession of their forms of life. As
species are produced and exterminated by slowly acting and still existing
causes, and not by miraculous acts of creation and by catastrophes; and as
the most important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,--the improvement of
one being entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual time.
A number of species, however, keeping in a body might remain for a long
period unchanged, whilst within this same period, several of these species,
by migrating into new countries and coming into competition with foreign
associates, might become modified; so that we must not overrate the
accuracy of organic change as a measure of time. During early periods of
the earth's history, when the forms of life were probably fewer and
simpler, the rate of change was probably slower; and at the first dawn of
life, when very few forms of the simplest structure existed, the rate of
change may have been slow in an extreme degree. The whole history of the
world, as at present known, although of a length quite incomprehensible by
us, will hereafter be recognised as a mere fragment of time, compared with
the ages which have elapsed since the first creature, the progenitor of
innumerable extinct and living descendants, was created.
In the distant future I see open fields for far more important researches.
Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be
thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Silurian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distant futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species of
each genus, and all the species of many genera, have left no descendants,
but have become utterly extinct. We can so far take a prophetic glance
into futurity as to foretel that it will be the common and widely-spread
species, belonging to the larger and dominant groups, which will ultimately
prevail and procreate new and dominant species. As all the living forms of
life are the lineal descendants of those which lived long before the
Silurian epoch, we may feel certain that the ordinary succession by
generation has never once been broken, and that no cataclysm has desolated
the whole world. Hence we may look with some confidence to a secure future
of equally inappreciable length. And as natural selection works solely by
and for the good of each being, all corporeal and mental endowments will
tend to progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth, and
to reflect that these elaborately constructed forms, so different from each
other, and dependent on each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with Reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
external 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
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. There is grandeur in
this view of life, with its several powers, having been originally breathed
into a few forms or into one; and that, whilst this planet has gone cycling
on according to the fixed law of gravity, from so simple a beginning
endless forms most beautiful and most wonderful have been, and are being,
evolved.
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