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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species



Produced by Sue Asscher asschers@bigpond.com
This Etext is based on the First Edition.




ON THE


ORIGIN OF SPECIES.







"But with regard to the material world, we can at least go so far as this--
we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."

W. Whewell: Bridgewater Treatise.



"To conclude, therefore, let no man out of a weak conceit of sobriety, or
an ill-applied moderation, think or maintain, that a man can search too far
or be too well studied in the book of God's word, or in the book of God's
works; divinity or philosophy; but rather let men endeavour an endless
progress or proficience in both."

Bacon: Advancement of Learning.



Down, Bromley, Kent,

October 1st, 1859.




ON


THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION,


OR THE


PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.


BY CHARLES DARWIN, M.A.,
FELLOW OF THE ROYAL, GEOLOGICAL, LINNAEAN, ETC., SOCIETIES;


AUTHOR OF 'JOURNAL OF RESEARCHES DURING H.M.S. BEAGLE'S VOYAGE
ROUND THE WORLD.'



LONDON:

JOHN MURRAY, ALBEMARLE STREET.

1859.



CONTENTS.


INTRODUCTION.


CHAPTER 1. VARIATION UNDER DOMESTICATION.

Causes of Variability.
Effects of Habit.
Correlation of Growth.
Inheritance.
Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species.
Origin of Domestic Varieties from one or more Species.
Domestic Pigeons, their Differences and Origin.
Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection.
Unknown Origin of our Domestic Productions.
Circumstances favourable to Man's power of Selection.


CHAPTER 2. VARIATION UNDER NATURE.

Variability.
Individual Differences.
Doubtful species.
Wide ranging, much diffused, and common species vary most.
Species of the larger genera in any country vary more than the species
of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.


CHAPTER 3. STRUGGLE FOR EXISTENCE.

Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of the
same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.


CHAPTER 4. NATURAL SELECTION.

Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.


CHAPTER 5. LAWS OF VARIATION.

Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long-lost characters.
Summary.


CHAPTER 6. DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.


CHAPTER 7. INSTINCT.

Instincts comparable with habits, but different in their origin.
Instincts graduated.
Aphides and ants.
Instincts variable.
Domestic instincts, their origin.
Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants.
Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects.
Summary.


CHAPTER 8. HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing.
Fertility of varieties when crossed and of their mongrel offspring not
universal.
Hybrids and mongrels compared independently of their fertility.
Summary.


CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition and
of denudation.
On the poorness of our palaeontological collections.
On the intermittence of geological formations.
On the absence of intermediate varieties in any one formation.
On the sudden appearance of groups of species.
On their sudden appearance in the lowest known fossiliferous strata.


CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species.
On their different rates of change.
Species once lost do not reappear.
Groups of species follow the same general rules in their appearance
and disappearance as do single species.
On Extinction.
On simultaneous changes in the forms of life throughout the world.
On the affinities of extinct species to each other and to living
species.
On the state of development of ancient forms.
On the succession of the same types within the same areas.
Summary of preceding and present chapters.


CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.


CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.


CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY
ORGANS.

CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.


CHAPTER 14. RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.



ON THE ORIGIN OF SPECIES.



INTRODUCTION.

When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants
of that continent. These facts seemed to me to throw some light on the
origin of species--that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to
me, in 1837, that something might perhaps be made out on this question
by patiently accumulating and reflecting on all sorts of facts which
could possibly have any bearing on it. After five years' work I
allowed myself to speculate on the subject, and drew up some short
notes; these I enlarged in 1844 into a sketch of the conclusions,
which then seemed to me probable: from that period to the present day
I have steadily pursued the same object. I hope that I may be excused
for entering on these personal details, as I give them to show that I
have not been hasty in coming to a decision.

My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been
induced to do this, as Mr. Wallace, who is now studying the natural
history of the Malay archipelago, has arrived at almost exactly the
same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I
would forward it to Sir Charles Lyell, who sent it to the Linnean
Society, and it is published in the third volume of the Journal of
that Society. Sir C. Lyell and Dr. Hooker, who both knew of my
work--the latter having read my sketch of 1844--honoured me by
thinking it advisable to publish, with Mr. Wallace's excellent memoir,
some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my
accuracy. No doubt errors will have crept in, though I hope I have
always been cautious in trusting to good authorities alone. I can here
give only the general conclusions at which I have arrived, with a few
facts in illustration, but which, I hope, in most cases will suffice.
No one can feel more sensible than I do of the necessity of hereafter
publishing in detail all the facts, with references, on which my
conclusions have been grounded; and I hope in a future work to do
this. For I am well aware that scarcely a single point is discussed in
this volume on which facts cannot be adduced, often apparently leading
to conclusions directly opposite to those at which I have arrived. A
fair result can be obtained only by fully stating and balancing the
facts and arguments on both sides of each question; and this cannot
possibly be here done.

I much regret that want of space prevents my having the satisfaction
of acknowledging the generous assistance which I have received from
very many naturalists, some of them personally unknown to me. I
cannot, however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who for the last fifteen years has aided me
in every possible way by his large stores of knowledge and his
excellent judgment.

In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such
a conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.

The author of the 'Vestiges of Creation' would, I presume, say that,
after a certain unknown number of generations, some bird had given
birth to a woodpecker, and some plant to the misseltoe, and that these
had been produced perfect as we now see them; but this assumption
seems to me to be no explanation, for it leaves the case of the
coadaptations of organic beings to each other and to their physical
conditions of life, untouched and unexplained.

It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement
of my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I
may venture to express my conviction of the high value of such
studies, although they have been very commonly neglected by
naturalists.

From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and,
what is equally or more important, we shall see how great is the power
of man in accumulating by his Selection successive slight variations.
I will then pass on to the variability of species in a state of
nature; but I shall, unfortunately, be compelled to treat this subject
far too briefly, as it can be treated properly only by giving long
catalogues of facts. We shall, however, be enabled to discuss what
circumstances are most favourable to variation. In the next chapter
the Struggle for Existence amongst all organic beings throughout the
world, which inevitably follows from their high geometrical powers of
increase, will be treated of. This is the doctrine of Malthus, applied
to the whole animal and vegetable kingdoms. As many more individuals
of each species are born than can possibly survive; and as,
consequently, there is a frequently recurring struggle for existence,
it follows that any being, if it vary however slightly in any manner
profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus
be NATURALLY SELECTED. From the strong principle of inheritance, any
selected variety will tend to propagate its new and modified form.

This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less
improved forms of life and induces what I have called Divergence of
Character. In the next chapter I shall discuss the complex and little
known laws of variation and of correlation of growth. In the four
succeeding chapters, the most apparent and gravest difficulties on the
theory will be given: namely, first, the difficulties of transitions,
or in understanding how a simple being or a simple organ can be
changed and perfected into a highly developed being or elaborately
constructed organ; secondly the subject of Instinct, or the mental
powers of animals, thirdly, Hybridism, or the infertility of species
and the fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time;
in the eleventh and twelfth, their geographical distribution
throughout space; in the thirteenth, their classification or mutual
affinities, both when mature and in an embryonic condition. In the
last chapter I shall give a brief recapitulation of the whole work,
and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due
allowance for our profound ignorance in regard to the mutual relations
of all the beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why another allied
species has a narrow range and is rare? Yet these relations are of the
highest importance, for they determine the present welfare, and, as I
believe, the future success and modification of every inhabitant of
this world. Still less do we know of the mutual relations of the
innumerable inhabitants of the world during the many past geological
epochs in its history. Although much remains obscure, and will long
remain obscure, I can entertain no doubt, after the most deliberate
study and dispassionate judgment of which I am capable, that the view
which most naturalists entertain, and which I formerly
entertained--namely, that each species has been independently
created--is erroneous. I am fully convinced that species are not
immutable; but that those belonging to what are called the same genera
are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are
the descendants of that species. Furthermore, I am convinced that
Natural Selection has been the main but not exclusive means of
modification.


CHAPTER 1. VARIATION UNDER DOMESTICATION.

Causes of Variability.
Effects of Habit.
Correlation of Growth.
Inheritance.
Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species.
Origin of Domestic Varieties from one or more Species.
Domestic Pigeons, their Differences and Origin.
Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection.
Unknown Origin of our Domestic Productions.
Circumstances favourable to Man's power of Selection.

When we look to the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and
animals which have been cultivated, and which have varied during all
ages under the most different climates and treatment, I think we are
driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not
so uniform as, and somewhat different from, those to which the
parent-species have been exposed under nature. There is, also, I
think, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several
generations to the new conditions of life to cause any appreciable
amount of variation; and that when the organisation has once begun to
vary, it generally continues to vary for many generations. No case is
on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.

It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of
the embryo causes monstrosities; and monstrosities cannot be separated
by any clear line of distinction from mere variations. But I am
strongly inclined to suspect that the most frequent cause of
variability may be attributed to the male and female reproductive
elements having been affected prior to the act of conception. Several
reasons make me believe in this; but the chief one is the remarkable
effect which confinement or cultivation has on the functions of the
reproductive system; this system appearing to be far more susceptible
than any other part of the organisation, to the action of any change
in the conditions of life. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even in the many cases when the male and female
unite. How many animals there are which will not breed, though living
long under not very close confinement in their native country! This is
generally attributed to vitiated instincts; but how many cultivated
plants display the utmost vigour, and yet rarely or never seed! In
some few such cases it has been found out that very trifling changes,
such as a little more or less water at some particular period of
growth, will determine whether or not the plant sets a seed. I cannot
here enter on the copious details which I have collected on this
curious subject; but to show how singular the laws are which determine
the reproduction of animals under confinement, I may just mention that
carnivorous animals, even from the tropics, breed in this country
pretty freely under confinement, with the exception of the
plantigrades or bear family; whereas, carnivorous birds, with the
rarest exceptions, hardly ever lay fertile eggs. Many exotic plants
have pollen utterly worthless, in the same exact condition as in the
most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature, perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised
at this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.

Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility;
and variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret
kept in hutches), showing that their reproductive system has not been
thus affected; so will some animals and plants withstand domestication
or cultivation, and vary very slightly--perhaps hardly more than in a
state of nature.

A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new
and sometimes very different character from that of the rest of the
plant. Such buds can be propagated by grafting, etc., and sometimes by
seed. These "sports" are extremely rare under nature, but far from
rare under cultivation; and in this case we see that the treatment of
the parent has affected a bud or offset, and not the ovules or pollen.
But it is the opinion of most physiologists that there is no essential
difference between a bud and an ovule in their earliest stages of
formation; so that, in fact, "sports" support my view, that
variability may be largely attributed to the ovules or pollen, or to
both, having been affected by the treatment of the parent prior to the
act of conception. These cases anyhow show that variation is not
necessarily connected, as some authors have supposed, with the act of
generation.

Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller has remarked, have apparently been exposed
to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with
the laws of reproduction, and of growth, and of inheritance; for had
the action of the conditions been direct, if any of the young had
varied, all would probably have varied in the same manner. To judge
how much, in the case of any variation, we should attribute to the
direct action of heat, moisture, light, food, etc., is most difficult:
my impression is, that with animals such agencies have produced very
little direct effect, though apparently more in the case of plants.
Under this point of view, Mr. Buckman's recent experiments on plants
seem extremely valuable. When all or nearly all the individuals
exposed to certain conditions are affected in the same way, the change
at first appears to be directly due to such conditions; but in some
cases it can be shown that quite opposite conditions produce similar
changes of structure. Nevertheless some slight amount of change may, I
think, be attributed to the direct action of the conditions of
life--as, in some cases, increased size from amount of food, colour
from particular kinds of food and from light, and perhaps the
thickness of fur from climate.