Charles Darwin.

The origin of species online

. (page 1 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 1 of 50)
Font size
QR-code for this ebook

Digitized by the Internet Archive

in 2010 with funding from

Boston Library Consortium IVIember Libraries




New York







Introduction •; •: . . . i



Causes of variability — Effects of habit and the use or disuse of parts — Corre-
lated variation — 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
— Principles of selection, anciently followed, their effects — Methodical and
unconscious selection — Unknown origin of our domestic productions — Cir-
cumstances favorable to man's power of selection 4



Variability — Individual differences — Doubtful species — Wide ranging, much
diffused, and common species, vary most — Species of the larger genera in
each country vary more frequently 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 2>



Its bearing on natural selection — The term used in a wide sense — Geometrical
ratio of increase — Rapid increase of naturalized animals and plants —
Nature of the checks to increase — Competition universal — Effects of cli-
mate — 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 39



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 favorable and unfavorable to the results
of 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
naturalization — Action of Natural Selection, through Divergence of Char-
acter and Extinction, on the descendants from a common parent — Explains
the grouping of all organic beings — Advance in organization — Low forms



preserved — Convergence of character — Indefinite multiplication of species

— Summary 51



Effects of changed conditions — Use and disuse, combined with natural selection;
organs of flight and of vision — Acclimatization — Correlated variation —
Compensation and economy of growth — False correlations — Multiple, rudi-
mentary, and lowly organized 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 — Sum-
mary 89



Difficulties of the theory of descent with modification — 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 — Modes of transition — Cases of diffi-
culty — Natura non facit saltum — Organs of small importance — Organs not
in all cases absolutely perfect — The law of Unity of Type and of the Condi-
tions of Existence embraced by the theory of Natural Selection . . . 112



Longevity — Modifications not necessarily simultaneous — Modifications appar-
ently of no direct service — Progressive development — Characters of small
functional importance, the most constant — Supposed incompetence of nat-
ural selection to account for the incipient stages of useful structures —
Causes which interfere with the acquisition through natural selection
of useful structures — Gradations of structure with changed functions —
Widely different organs in members of the same class, developed from
one and the same source — Reasons for disbelieving in great and abrupt
modifications 140



Instincts comparable with habits, but different in their origin — Instincts gradu-
ated — Aphides and ants — Instincts variable — Domestic instincts, their
origin — Natural instincts of the cuckoo, molothrus, ostrich and parasitic
bees — Slave-making ants — Hive-bee, its cell-making instinct — Changes of
instinct and structure not necessarily simultaneous — Difficulties of the
theory of the Natural Selection of instincts — Neuter or sterile insects — Sum-
mary 171





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, not accumulated
by natural selection — Causes of the sterility of first crosses and of hybrids
— Parallelism between the effects of changed conditions of life and of
crossing — Dimorphism and Trimorphism — Fertility of varieties when
crossed and of their mongrel offspring not universal — Hybrids and mon-
grels compared independently of their fertility — Summary . . . , 196




On the absence of intermediate varieties at the present day — On the nature of
extinct intermediate varieties; on their number — On the lapse of time, as
inferred from the rate of denudation and of deposition — On the lapse of
time as estimated by years — On the poorness of our palaeontological collec-
tions — On the intermittence of geological formations — On the denudation
of granitic areas — 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 — Antiquity of
the habitable earth 220



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 — Sunmiary of pre-
ceding and present chapter 242



Present distribution cannot be accounted for by differences in physical condi-
tions — Importance of barriers — Affinity of the productions of the same con-
tinent — 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 — Alternate Glacial periods in the north and south . . . 263



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 coloniza-
tion from the nearest source with subsequent modification — Summary of
the last and present chapter 286




Classification, groups subordinate to groups — Natural system — Rules and diffi-
culties in classification, explained on the theory of descent with modification
— Classification of varieties — Descent always used in classification— Ana-
logical 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 ex-
plained — Summary 302



Recapitulation of the objections to the theory of Natural Selection — Recapitula-
tion of the general and special circumstances in its favor — Causes of the
general circumstances in its favor — 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 — Con-
cluding remarks 33^

Appendix 357

Glossary of Scientific Terms . 365

"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." — Whewell : Bridgewater Treatise.

"The only distinct meaning of the word 'natural' is stated, fixed or settled;
since what is natural as much requires and presupposes an intelligent agent
to render it so, i. e., to effect it continually or at stated times, as what is
supernatural or miraculous does to effect it for once." — Butler: Analogy
of Revealed Religion.

"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 endeavor an endless progress
or proficience in both." — Bacon : Advancement of Learning.



When on board H. M. S. Beagle, as naturalist, I was much struck with cer-
tain facts in the distribution of the organic beings inhabiting South America,
and in the geological relations of the present to the past inhabitants of that
continent. These facts, as will be seen in the latter chapters of this volume,
seemed to throw some light on the origin of species — that mystery of mys-
teries, 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 en-
larged in 1 844 into a sketch of the conclusions which then seemed to me prob-
able: 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 ( 1859) nearly finished; but as it will take me many 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. In 1858 he sent 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 — honored 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 can-
not 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
may 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 con-
clusions 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 op-
posite 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 is here impossible.

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,

o 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 species had not been independ-
ently created, but had descended, like varieties, from other species. Neverthe-
less, 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 justly excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, etc., as the only possible cause of variation.
In one limited sense, as we shall hereafter see, this may be true ; but it is pre-
posterous 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 mistletoe, which
draws its nourishment from certain trees, which has seeds that must be trans-
ported 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.

It is, therefore, of the highest importance to gain a clear insight into the

y means of modificaticgi ^and coa dapta^tion. At the commencement of my obser-
vations it seemed to me probaBTe""tTiat a careful study of domesticated animals
and of cultivated plants would offer the best chance of making out this ob-
scure problem. Nor have I been disappointed; in this and in all other per-
plexing cases I have invariably found that our knowledge, imperfect though
it be, of variationjinder domestication, afforded the best and safest clew. I
may venture" to express my conviction of the high value of such studies, al"
though 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 im-
portant, 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 wh at circ um-
stances are most favorable to yadatig n. In the next chapter the struggle for
existence among~altl3T^nic beings throughout the world, which inevitably
follows from the high geometrical ratio of their increase, will be considered.
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 pos-


sibly 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 condi-
tions of life, will have a better chance of surviving, and thus be naturally
s elected . From the strong principle of inheritance, any selected variety will
tend to propagate its new^a^TirodiSed 'fomi7

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 in-
evitably causes much extension of the less improved forms of life, and leads
to what I have called divergence of character. In the next chapter I shall dis-
cuss the complex and little known laws of variation. In the five succeeding
chapters, the most apparent and gi'avest difficulties in accepting the theory
will be given; namely, first, the difficulties of transitions, or how a simple
being or a simple organ can be changed and perfected into a highly developed
being or into an elaborately constructed organ; secondly, the subject of in-
stinct, 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 twelfth and
thirteenth, their geographical distribution throughout space ; in the fourteenth,
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 rer
gard to the origin of species and varieties, if he make due allowance for our
profound ignorance in regard to the mutual relations of the many beings
which five around us. V/ho 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 dispas-
sionate judgment of which I am capable, that the view which most natural-
ists until recently entertained^^d which I formerly entertained — namely,
that each species has been indepen dently xreated — is erroneou s. I am fully
convmced-Ararripecies 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 naturalselectio n has been the most important, but not the exclusive,
means oflnodification.


Variation Under Domestication

Causes of Variability — Effects of Habit and the Use or Disuse of Parts — Correlated
Variation — Inheritance — Character of Domestic Varieties — Difficulty of dis-
tinguishing between Varieties and Species — Origin of Domestic Varieties from
one or more Species — Domestic Pigeons, their Differences and Origin — Princi-
ples of Selection, anciently followed, their Effects — Methodical and Unconscious
Selection — Unknown Origin of our Domestic Productions — Circumstances favor-
able to Man's Power of Selection.


When we compare 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 more from each other than do the individuals of
any one species or variety in a state of nature. And if 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,
we are driven to conclude that this great variability is 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 had been exposed
under nature. There is, also, some probability in the view propounded by
Andrew Knight, that this variability may be partly connected with excess of
food.^It seems clear that organic beings must be exposed during several gen-
erations to new conditions to cause any great amount of variation ; and that,
when the organization has once begun to vary, it generally continues varying
for many generations.^ No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat, still yield
new varieties; our oldest domesticated animals are still capable of rapid im-
provement or modification.

As far as I am able to judge, after long attending to the subject, the condi-
tions of life appear to act in two ways — directly on the whole organization or
on certain parts alone, and indirectly by affecting the reproductive system.
With respect to the direct action, we must bear in mind that in every case, as
Professor Weismann has lately insisted, and as I have incidentally shown in
my work on "Variation under Domestication," there are two factors : namely,
the nature of the organism and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations sometimes
arise under, as far as we can judge, dissimilar conditions; and, on the other
hand, dissimilar variations arise under conditions which appear to be nearly
uniform. The effects on the offspring are either definite or indefinite. They
may be considered as definite when all or nearly all the offspring of individ-
uals exposed to certain conditions during several generations are modified in
the same manner. It is extremely difficult to come to any conclusion in regard
to the extent of the changes which have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size from the


amount of food, color from the nature of the food, thickness of the skin and
hair from climate, etc. Each of the endless variations which. we see in the
plumage of our fowls must have had some efficient cause; and if the same
cause were to act uniformly during a long series of generations on many in-
dividuals, all probably would be modified in the same manner. Such facts as
the complex and extraordinary outgrowths which variably follow from the in-
sertion of a minute drop of poison by a gall-producing insect, show us what
singular modifications might result in the case of plants from a chemical
change in the nature of the sap.

Indefinite variability is a much more common result of changed conditions
than definite variability, and has probably played a more important part in
the formation of our domestic races. We see indefinite variability in the end-
less slight peculiarities which distinguish the individuals of the same species,

Online LibraryCharles DarwinThe origin of species → online text (page 1 of 50)