As this whole volume is one long
argument, it may be convenient to the reader to have the leading facts and inferences
briefly recapitulated…
Under domestication we see much variability. This seems
to be mainly due to the reproductive system being eminently susceptible to
changes in the conditions of life so that this system, when not rendered
impotent, fails to reproduce offspring exactly like the parent-form.
Variability is governed by many complex laws, -- by correlation of growth, by
use and disuse, and by the direct action of the physical conditions of life.
There is much difficulty in ascertaining how much modification our domestic
productions have undergone; but we may safely infer that the amount has been
large, and that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On the
other hand we have evidence that variability, when it has once come into play,
does not wholly cease; for new varieties are still occasionally produced by our
most anciently domesticated productions.
Man does not actually produce
variability; he only unintentionally exposes organic beings to new conditions
of life, and then nature acts on the organisation, and causes variability. But
man can and does select the variations given to him by nature, and thus
accumulate them in any desired manner. He thus adapts animals and plants for
his own benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful to him at the time,
without any thought of altering the breed. It is certain that he can largely
influence the character of a breed by selecting, in each successive generation,
individual differences so slight as to be quite inappreciable by an uneducated
eye. This process of selection has been the great agency in the production of
the most distinct and useful domestic breeds. That many of the breeds produced
by man have to a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal
species.
There is no
obvious reason why the principles which have acted so efficiently under
domestication should not have acted under nature. In the preservation of
favoured individuals and races, during the constantly-recurrent Struggle for
Existence, we see the most powerful and ever-acting means of selection. The
struggle for existence inevitably follows from the high geometrical ratio of
increase which is common to all organic beings. This high rate of increase is
proved by calculation, by the effects of a succession of peculiar seasons, and
by the results of naturalisation, as explained in the third chapter. More
individuals are born than can possibly survive. A grain in the balance will
determine which individual shall live and which shall die, -- which variety or
species shall increase in number, and which shall decrease, or finally become
extinct. As the individuals of the same species come in all respects into the
closest competition with each other, the struggle will generally be most severe
between them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same genus. But
the struggle will often be very severe between beings most remote in the scale
of nature: The slightest advantage in one being, at any age or during any
season, over those with which it comes into competition, or better adaptation
in however slight a degree to the surrounding physical conditions, will turn
the balance.
With
animals having separated sexes there will in most cases be a struggle between
the males for possession of the females. The most vigorous individuals, or
those which have most successfully struggled with their conditions of life,
will generally leave most progeny. But success will often depend on having
special weapons or means of defence, or on the charms of the males; and the
slightest advantage will lead to victory…
If
then we have under nature variability and a powerful agent always ready to act
and select, why should we doubt that variations in any way useful to beings,
under their excessively complex relations of life, would be preserved,
accumulated, and inherited ? Why, if man can by patience select variations most
useful to himself, should nature fail in selecting variations useful, under
changing conditions of life, to her living products ? What limit can be put to
this power, acting during long ages and rigidly scrutinising the whole
constitution, structure, and habits of each creature, -- favouring the good and
rejecting the bad? I can see no limit to this power, in slowly and beautifully
adapting each form to the most complex relations of life. The theory of natural
selection, even if we looked no further than this, seems to me to be in itself
probable. I have already recapitulated, as fairly as I could, the opposed
difficulties and objections: now let us turn to the special facts and arguments
in favour of the theory…
As
each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modified descendants of each species will be
enabled to increase by so much the more as they become more diversified in
habits and structure, so as to be enabled to seize on many and widely different
places in the economy of nature, there will be a constant tendency in natural
selection to preserve the most divergent offspring of any one species. Hence
during a long-continued course of modification, the slight differences, characteristic
of varieties of the same species, tend to be augmented into the greater
differences characteristic of species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older, less improved and
intermediate varieties; and thus species are rendered to a large extent defined
and distinct objects. Dominant species belonging to the larger groups tend to
give birth to new and dominant forms; so that each large group tends to become
still larger, and at the same time more divergent in character. But as all
groups cannot thus succeed in increasing in size, for the world would not hold
them, the more dominant groups beat the less dominant. This tendency in the
large groups to go on increasing in size and diverging in character, together
with the almost inevitable contingency of much extinction, explains the
arrangement of all the forms of life, in groups subordinate to groups, all
within a few great classes, which we now see everywhere around us, and which
has prevailed throughout all time. This grand fact of the grouping of all
organic beings seems to me utterly inexplicable on the theory of creation.
As natural
selection acts solely by accumulating slight, successive, favourable
variations, it can produce no great or sudden modification; it can act only by
very short and slow steps. Hence the canon of ‘Natura non facit saltum,’[1]
which every fresh addition to our knowledge tends to make more strictly
correct, is on this theory simply intelligible. We can plainly see why nature
is prodigal in variety, though niggard in innovation. But why this should be a
law of nature if each species has been independently created, no man can
explain.
Many
other facts are, as it seems to me, explicable on this theory. How strange it
is that a bird, under the form of woodpecker, should have been created to prey
on insects on the ground; that upland geese, which never or rarely swim, should
have been created with webbed feet; that a thrush should have been created to
dive and feed on sub- aquatic insects; and that a petrel should have been
created with habits and structure fitting it for the life of an auk or grebe!
and so on in endless other cases. But on the view of each species constantly
trying to increase in number, with natural selection always ready to adapt the
slowly varying descendants of each to any unoccupied or ill-occupied place in
nature, these facts cease to be strange, or perhaps might even have been
anticipated.
As
natural selection acts by competition, it adapts the inhabitants of each
country only in relation to the degree of perfection of their associates; so
that we need feel no surprise at the inhabitants of any one country, although
on the ordinary view supposed to have been specially created and adapted for that
country, being beaten and supplanted by the naturalised 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; and if some of them be abhorrent to
our ideas of fitness. We need not marvel at the sting of the bee causing the
bee's own death; at drones being produced in such vast 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[2]
feeding within the live bodies of caterpillars; and at other such cases. The
wonder indeed is, on the theory of natural selection, that more cases of the
want of absolute perfection have not been observed…
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[3] 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 purposes, -- 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 willfully 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 gallfly 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 foretell 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.
[1] ‘Nature does not make leaps.’
[2] The ichneumonidae are a family of wasps, which includes some parasitic wasps that lay their eggs within the bodies of caterpillars. The larvae that hatch from these eggs nourish themselves on the body of the caterpillar, eventually killing their host. Darwin questioned whether a benevolent creator or designer would create or design such a cruel arrangement.
[3] Darwin is referring to several species of extinct giant ground sloths, whose fossils he and others discovered.