INHERITANCE.
WONDERFUL NATURE OF INHERITANCE--PEDIGREES OF OUR DOMESTICATED ANIMALS--
INHERITANCE NOT DUE TO CHANCE--TRIFLING CHARACTERS INHERITED--DISEASES
INHERITED--PECULIARITIES IN THE EYE INHERITED--DISEASES IN THE HORSE--
LONGEVITY AND VIGOUR--ASYMMETRICAL DEVIATIONS OF STRUCTURE--POLYDACTYLISM
AND REGROWTH OF SUPERNUMERARY DIGITS AFTER AMPUTATION--CASES OF SEVERAL
CHILDREN SIMILARLY AFFECTED FROM NON-AFFECTED PARENTS--WEAK AND FLUCTUATING
INHERITANCE: IN WEEPING TREES, IN DWARFNESS, COLOUR OF FRUIT AND FLOWERS--
COLOUR OF HORSES--NON-INHERITANCE IN CERTAIN CASES--INHERITANCE OF
STRUCTURE AND HABITS OVERBORNE BY HOSTILE CONDITIONS OF LIFE, BY
INCESSANTLY RECURRING VARIABILITY, AND BY REVERSION--CONCLUSION.
LIST OF ILLUSTRATIONS.
FIGURE.
1. DUN DEVONSHIRE PONY, WITH SHOULDER, SPINAL, AND LEG STRIPES.
2. HEAD OF JAPAN OR MASKED PIG.
3. HEAD OF WILD BOAR, AND OF "GOLDEN DAYS," A PIG OF THE YORKSHIRE LARGE
BREED.
4. OLD IRISH PIG WITH JAW-APPENDAGES.
5. HALF-LOP RABBIT.
6. SKULL OF WILD RABBIT.
7. SKULL OF LARGE LOP-EARED RABBIT.
8. PART OF ZYGOMATIC ARCH, SHOWING THE PROJECTING END OF THE MALAR BONE OF
THE AUDITORY MEATUS, OF RABBITS.
9. POSTERIOR END OF SKULL, SHOWING THE INTER-PARIETAL BONE, OF RABBITS.
10. OCCIPITAL FORAMEN OF RABBITS.
11. SKULL OF HALF-LOP RABBIT.
12. ATLAS VERTEBRAE OF RABBITS.
13. THIRD CERVICAL VERTEBRAE OF RABBITS.
14. DORSAL VERTEBRAE, FROM SIXTH TO TENTH INCLUSIVE, OF RABBITS.
15. TERMINAL BONE OF STERNUM OF RABBITS.
16. ACROMION OF SCAPULA OF RABBITS.
17. THE ROCK-PIGEON, OR COLUMBA LIVIA.
18. ENGLISH POUTER.
19. ENGLISH CARRIER.
20. ENGLISH BARB.
21. ENGLISH FANTAIL.
22. AFRICAN OWL.
23. SHORT-FACED ENGLISH TUMBLER.
24. SKULLS OF PIGEONS, VIEWED LATERALLY.
25. LOWER JAWS OF PIGEONS, SEEN FROM ABOVE.
26. SKULL OF RUNT, SEEN FROM ABOVE.
27. LATERAL VIEW OF JAWS OF PIGEONS.
28. SCAPULAE OF PIGEONS.
29. FURCULA OF PIGEONS.
30. SPANISH FOWL.
31. HAMBURGH FOWL.
32. POLISH FOWL.
33. OCCIPITAL FORAMEN OF THE SKULLS OF FOWLS.
34. SKULLS OF FOWLS, VIEWED FROM ABOVE, A LITTLE OBLIQUELY.
35. LONGITUDINAL SECTIONS OF SKULLS OF FOWLS, VIEWED LATERALLY.
36. SKULL OF HORNED FOWL, VIEWED FROM ABOVE, A LITTLE OBLIQUELY.
37. SIXTH CERVICAL VERTEBRAE OF FOWLS, VIEWED LATERALLY.
38. EXTREMITY OF THE FURCULA OF FOWLS, VIEWED LATERALLY.
39. SKULLS OF DUCKS, VIEWED LATERALLY, REDUCED TO TWO-THIRDS OF THE
NATURAL SIZE.
40. CERVICAL VERTEBRAE OF DUCKS, OF NATURAL SIZE.
41. PODS OF THE COMMON PEA.
42. PEACH AND ALMOND STONES, OF NATURAL SIZE, VIEWED EDGEWAYS.
43. PLUM STONES, OF NATURAL SIZE, VIEWED LATERALLY.
TABLE 1: PRINCIPAL ADDITIONS AND CORRECTIONS IN THIS (SECOND) EDITION.
First Edition, Volume I., Page 34.
Second Edition Volume I., Page 35.
Dr. Burt Wilder's observations on the brains of different breeds of the
Dog.
First Edition, Volume I., Page 38.
Second Edition Volume I., Page 40.
Degeneracy of Dogs imported into Guinea.
First Edition, Volume I., Page 51.
Second Edition Volume I., Page 54.
Difference in the number of the lumbar vertebrae in the races or species of
the Horse.
First Edition, Volume I., Page 102.
Second Edition Volume I., Page 106.
Hairy appendages to the throats of Goats.
First Edition, Volume I., Page 162.
Second Edition Volume I., Page 170.
Sexual differences in colour in the domestic Pigeon.
First Edition, Volume I., Page 217.
Second Edition Volume I., Page 228.
Movements like those of the Tumbler-pigeon, caused by injury to the brain.
First Edition, Volume I., Page 290.
Second Edition Volume I., Page 306.
Additional facts with respect to the Black-shouldered Peacock.
First Edition, Volume I., Page 296.
Second Edition Volume I., Page 312.
Ancient selection of Gold-fish in China.
First Edition, Volume I., Page 314.
Second Edition Volume I., Page 332.
Major Hallett's 'Pedigree Wheat.'
First Edition, Volume I., Page 326.
Second Edition Volume I., Page 345.
The common radish descended from Raphanus raphanistrum.
First Edition, Volume I., Page 374.
Second Edition Volume I., Page 398.
Several additional cases of bud-variation given.
First Edition, Volume I., Page 396.
Second Edition Volume I., Page 420.
An abstract of all the cases recently published of graft-hybrids in the
potato, together with a general summary on graft-hybridisation.
First Edition, Volume I., Page 399.
Second Edition Volume I., Page 429.
An erroneous statement with respect to the pollen of the date-palm
affecting the fruit of the Chamaerops omitted.
First Edition, Volume I., Page 400.
Second Edition Volume I., Page 430.
New cases of the direct action of pollen on the mother-plant.
First Edition, Volume I., Page 404.
Second Edition Volume I., Page 435.
Additional and remarkable instances of the action of the male parent on the
future progeny of the female.
First Edition, Volume II., Page 14.
Second Edition Volume I., Page 459.
An erroneous statement corrected, with respect to the regrowth of
supernumerary digits after amputation.
First Edition, Volume II., Page 23.
Second Edition Volume I., Page 467.
Additional facts with respect to the inherited effects of circumcision.
First Edition, Volume II., Page 23.
Second Edition Volume I., Page 467.
Dr. Brown-Sequard on the inherited effects of operations on the Guinea-pig.
First Edition, Volume II., Page 24.
Second Edition Volume I., Page 469.
Other cases of inherited mutilations.
First Edition, Volume II., Page 43.
Second Edition Volume II., Page 17.
An additional case of reversion due to a cross.
First Edition, Volume II., Page 72.
Second Edition Volume II., Page 48.
Inheritance as limited by sex.
First Edition, Volume II., Page 105.
Second Edition Volume II., Page 83.
Two varieties of maize which cannot be crossed.
First Edition, Volume II., Page 120.
Second Edition Volume II., Page 99.
Some additional facts on the advantages of cross-breeding in animals.
First Edition, Volume II., Page 123.
Second Edition Volume II., Page 103.
Discussion on the effects of close interbreeding in the case of man.
First Edition, Volume II., Page 135 to 141.
Second Edition Volume II., Page 117 to 122.
Additional cases of plants sterile with pollen from the same plant.
First Edition, Volume II., Page 149.
Second Edition Volume II., Page 131.
Mr. Sclater on the infertility of animals under confinement.
First Edition, Volume II., Page 152.
Second Edition Volume II., Page 134.
The Aperea a distinct species from the Guinea-pig.
First Edition, Volume II., Page 230.
Second Edition Volume II., Page 215.
Professor Jager on hawks killing light-coloured pigeons.
First Edition, Volume II., Page 273.
Second Edition Volume II., Page 262.
Professor Weismann on the effects of isolation in the development of
species.
First Edition, Volume II., Page 281.
Second Edition Volume II., Page 271.
The direct action of the conditions of life in causing variation.
First Edition, Volume II., Page 317.
Second Edition Volume II., Page 309.
Mr. Romanes on rudimentary parts.
First Edition, Volume II., Page 324 to 328.
Second Edition Volume II., Page 316 to 327.
Some additional cases of correlated variability.
First Edition, Volume II., Page 339.
Second Edition Volume II., Page 333.
On Geoffroy St.-Hilaire's law of "soi pour soi."
First Edition, Volume II., Page 357 to 404.
Second Edition Volume II., Page 349 to 399.
The chapter on Pangenesis has been largely altered and re-modelled; but the
essential principles remain the same.
THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION.
INTRODUCTION.
The object of this work is not to describe all the many races of animals
which have been domesticated by man, and of the plants which have been
cultivated by him; even if I possessed the requisite knowledge, so gigantic
an undertaking would be here superfluous. It is my intention to give under
the head of each species only such facts as I have been able to collect or
observe, showing the amount and nature of the changes which animals and
plants have undergone whilst under man's dominion, or which bear on the
general principles of variation. In one case alone, namely in that of the
domestic pigeon, I will describe fully all the chief races, their history,
the amount and nature of their differences, and the probable steps by which
they have been formed. I have selected this case, because, as we shall
hereafter see, the materials are better than in any other; and one case
fully described will in fact illustrate all others. But I shall also
describe domesticated rabbits, fowls, and ducks, with considerable fulness.
The subjects discussed in this volume are so connected that it is not a
little difficult to decide how they can be best arranged. I have determined
in the first part to give, under the heads of the various animals and
plants, a large body of facts, some of which may at first appear but little
related to our subject, and to devote the latter part to general
discussions. Whenever I have found it necessary to give numerous details,
in support of any proposition or conclusion, small type has been used.
(Here shown with [].) The reader will, I think, find this plan a
convenience, for, if he does not doubt the conclusion or care about the
details, he can easily pass them over; yet I may be permitted to say that
some of the discussions thus printed deserve attention, at least from the
professed naturalist.
It may be useful to those who have read nothing about Natural Selection, if
I here give a brief sketch of the whole subject and of its bearing on the
origin of species. (Introduction/1. To any one who has attentively read my
'Origin of Species' this Introduction will be superfluous. As I stated in
that work that I should soon publish the facts on which the conclusions
given in it were founded, I here beg permission to remark that the great
delay in publishing this first work has been caused by continued ill-
health.) This is the more desirable, as it is impossible in the present
work to avoid many allusions to questions which will be fully discussed in
future volumes.
From a remote period, in all parts of the world, man has subjected many
animals and plants to domestication or culture. Man has no power of
altering the absolute conditions of life; he cannot change the climate of
any country; he adds no new element to the soil; but he can remove an
animal or plant from one climate or soil to another, and give it food on
which it did not subsist in its natural state. It is an error to speak of
man "tampering with nature" and causing variability. If a man drops a piece
of iron into sulphuric acid, it cannot be said strictly that he makes the
sulphate of iron, he only allows their elective affinities to come into
play. If organic beings had not possessed an inherent tendency to vary, man
could have done nothing. (Introduction/2. M. Pouchet has recently
('Plurality of Races' English Translation 1864 page 83 etc.) insisted that
variation under domestication throws no light on the natural modification
of species. I cannot perceive the force of his arguments, or, to speak more
accurately, of his assertions to this effect.) He unintentionally exposes
his animals and plants to various conditions of life, and variability
supervenes, which he cannot even prevent or check. Consider the simple case
of a plant which has been cultivated during a long time in its native
country, and which consequently has not been subjected to any change of
climate. It has been protected to a certain extent from the competing roots
of plants of other kinds; it has generally been grown in manured soil; but
probably not richer than that of many an alluvial flat; and lastly, it has
been exposed to changes in its conditions, being grown sometimes in one
district and sometimes in another, in different soils. Under such
circumstances, scarcely a plant can be named, though cultivated in the
rudest manner, which has not given birth to several varieties. It can
hardly be maintained that during the many changes which this earth has
undergone, and during the natural migrations of plants from one land or
island to another, tenanted by different species, that such plants will not
often have been subjected to changes in their conditions analogous to those
which almost inevitably cause cultivated plants to vary. No doubt man
selects varying individuals, sows their seeds, and again selects their
varying offspring. But the initial variation on which man works, and
without which he can do nothing, is caused by slight changes in the
conditions of life, which must often have occurred under nature. Man,
therefore, may be said to have been trying an experiment on a gigantic
scale; and it is an experiment which nature during the long lapse of time
has incessantly tried. Hence it follows that the principles of
domestication are important for us. The main result is that organic beings
thus treated have varied largely, and the variations have been inherited.
This has apparently been one chief cause of the belief long held by some
few naturalists that species in a state of nature undergo change.
I shall in this volume treat, as fully as my materials permit, the whole
subject of variation under domestication. We may thus hope to obtain some
light, little though it be, on the causes of variability,--on the laws
which govern it, such as the direct action of climate and food, the effects
of use and disuse, and of correlation of growth,--and on the amount of
change to which domesticated organisms are liable. We shall learn something
of the laws of inheritance, of the effects of crossing different breeds,
and on that sterility which often supervenes when organic beings are
removed from their natural conditions of life, and likewise when they are
too closely interbred. During this investigation we shall see that the
principle of Selection is highly important. Although man does not cause
variability and cannot even prevent it, he can select, preserve, and
accumulate the variations given to him by the hand of nature almost in any
way which he chooses; and thus he can certainly produce a great result.
Selection may be followed either methodically and intentionally, or
unconsciously and unintentionally. Man may select and preserve each
successive variation, with the distinct intention of improving and altering
a breed, in accordance with a preconceived idea; and by thus adding up
variations, often so slight as to be imperceptible by an uneducated eye, he
has effected wonderful changes and improvements. It can, also, be clearly
shown that man, without any intention or thought of improving the breed, by
preserving in each successive generation the individuals which he prizes
most, and by destroying the worthless individuals, slowly, though surely,
induces great changes. As the will of man thus comes into play, we can
understand how it is that domesticated breeds show adaptation to his wants
and pleasures. We can further understand how it is that domestic races of
animals and cultivated races of plants often exhibit an abnormal character,
as compared with natural species; for they have been modified not for their
own benefit, but for that of man.
In another work I shall discuss, if time and health permit, the variability
of organic beings in a state of nature; namely, the individual differences
presented by animals and plants, and those slightly greater and generally
inherited differences which are ranked by naturalists as varieties or
geographical races. We shall see how difficult, or rather how impossible it
often is, to distinguish between races and sub-species, as the less well-
marked forms have sometimes been denominated; and again between sub-species
and true species. I shall further attempt to show that it is the common and
widely ranging, or, as they may be called, the dominant species, which most
frequently vary; and that it is the large and flourishing genera which
include the greatest number of varying species. Varieties, as we shall see,
may justly be called incipient species.
But it may be urged, granting that organic beings in a state of nature
present some varieties,--that their organisation is in some slight degree
plastic; granting that many animals and plants have varied greatly under
domestication, and that man by his power of selection has gone on
accumulating such variations until he has made strongly marked and firmly
inherited races; granting all this, how, it may be asked, have species
arisen in a state of nature? The differences between natural varieties are
slight; whereas the differences are considerable between the species of the
same genus, and great between the species of distinct genera. How do these
lesser differences become augmented into the greater difference? How do
varieties, or as I have called them incipient species, become converted
into true and well-defined species? How has each new species been adapted
to the surrounding physical conditions, and to the other forms of life on
which it in any way depends? We see on every side of us innumerable
adaptations and contrivances, which have justly excited the highest
admiration of every observer. There is, for instance, a fly (Cecidomyia
(Introduction/3. Leon Dufour in 'Annales des Science. Nat.' (3rd series,
Zoolog.) tome 5 page 6.)) which deposits its eggs within the stamens of a
Scrophularia, and secretes a poison which produces a gall, on which the
larva feeds; but there is another insect (Misocampus) which deposits its
eggs within the body of the larva within the gall, and is thus nourished by
its living prey; so that here a hymenopterous insect depends on a dipterous
insect, and this depends on its power of producing a monstrous growth in a
particular organ of a particular plant. So it is, in a more or less plainly
marked manner, in thousands and tens of thousands of cases, with the lowest
as well as with the highest productions of nature.
This problem of the conversion of varieties into species,--that is, the
augmentation of the slight differences characteristic of varieties into the
greater differences characteristic of species and genera, including the
admirable adaptations of each being to its complex organic and inorganic
conditions of life,--has been briefly treated in my 'Origin of Species.' It
was there shown that all organic beings, without exception, tend to
increase at so high a ratio, that no district, no station, not even the
whole surface of the land or the whole ocean, would hold the progeny of a
single pair after a certain number of generations. The inevitable result is
an ever-recurrent Struggle for Existence. It has truly been said that all
nature is at war; the strongest ultimately prevail, the weakest fail; and
we well know that myriads of forms have disappeared from the face of the
earth. If then organic beings in a state of nature vary even in a slight
degree, owing to changes in the surrounding conditions, of which we have
abundant geological evidence, or from any other cause; if, in the long
course of ages, inheritable variations ever arise in any way advantageous
to any being under its excessively complex and changing relations of life;
and it would be a strange fact if beneficial variations did never arise,
seeing how many have arisen which man has taken advantage of for his own
profit or pleasure; if then these contingencies ever occur, and I do not
see how the probability of their occurrence can be doubted, then the severe
and often-recurrent struggle for existence will determine that those
variations, however slight, which are favourable shall be preserved or
selected, and those which are unfavourable shall be destroyed.
This preservation, during the battle for life, of varieties which possess
any advantage in structure, constitution, or instinct, I have called
Natural Selection; and Mr. Herbert Spencer has well expressed the same idea
by the Survival of the Fittest. The term "natural selection" is in some
respects a bad one, as it seems to imply conscious choice; but this will be
disregarded after a little familiarity. No one objects to chemists speaking
of "elective affinity;" and certainly an acid has no more choice in
combining with a base, than the conditions of life have in determining
whether or not a new form be selected or preserved. The term is so far a
good one as it brings into connection the production of domestic races by
man's power of selection, and the natural preservation of varieties and
species in a state of nature. For brevity sake I sometimes speak of natural
selection as an intelligent power;--in the same way as astronomers speak of
the attraction of gravity as ruling the movements of the planets, or as
agriculturists speak of man making domestic races by his power of
selection. In the one case, as in the other, selection does nothing without
variability, and this depends in some manner on the action of the
surrounding circumstances on the organism. I have, also, often personified
the word Nature; for I have found it difficult to avoid this ambiguity; but
I mean by nature only the aggregate action and product of many natural
laws,--and by laws only the ascertained sequence of events.
It has been shown from many facts that the largest amount of life can be
supported on each area, by great diversification or divergence in the
structure and constitution of its inhabitants. We have, also, seen that the
continued production of new forms through natural selection, which implies
that each new variety has some advantage over others, inevitably leads to
the extermination of the older and less improved forms. These latter are
almost necessarily intermediate in structure, as well as in descent,
between the last-produced forms and their original parent-species. Now, if
we suppose a species to produce two or more varieties, and these in the
course of time to produce other varieties, the principal of good being
derived from diversification of structure will generally lead to the
preservation of the most divergent varieties; thus the lesser differences
characteristic of varieties come to be augmented into the greater
differences characteristic of species, and, by the extermination of the
older intermediate forms, new species end by being distinctly defined
objects. Thus, also, we shall see how it is that organic beings can be
classed by what is called a natural method in distinct groups--species
under genera, and genera under families.
As all the inhabitants of each country may be said, owing to their high
rate of reproduction, to be striving to increase in numbers; as each form
comes into competition with many other forms in the struggle for life,--for
destroy any one and its place will be seized by others; as every part of
the organisation occasionally varies in some slight degree, and as natural
selection acts exclusively by the preservation of variations which are
advantageous under the excessively complex conditions to which each being
is exposed, no limit exists to the number, singularity, and perfection of
the contrivances and co-adaptations which may thus be produced. An animal
or a plant may thus slowly become related in its structure and habits in
the most intricate manner to many other animals and plants, and to the
physical conditions of its home. Variations in the organisation will in
some cases be aided by habit, or by the use and disuse of parts, and they
will be governed by the direct action of the surrounding physical
conditions and by correlation of growth.
On the principles here briefly sketched out, there is no innate or
necessary tendency in each being to its own advancement in the scale of
organisation. We are almost compelled to look at the specialisation or
differentiation of parts or organs for different functions as the best or
even sole standard of advancement; for by such division of labour each
function of body and mind is better performed. And as natural selection
acts exclusively through the preservation of profitable modifications of
structure, and as the conditions of life in each area generally become more
and more complex from the increasing number of different forms which
inhabit it and from most of these forms acquiring a more and more perfect
structure, we may confidently believe, that, on the whole, organisation
advances. Nevertheless a very simple form fitted for very simple conditions
of life might remain for indefinite ages unaltered or unimproved; for what
would it profit an infusorial animalcule, for instance, or an intestinal
worm, to become highly organised? Members of a high group might even
become, and this apparently has often occurred, fitted for simpler
conditions of life; and in this case natural selection would tend to
simplify or degrade the organisation, for complicated mechanism for simple
actions would be useless or even disadvantageous.
The arguments opposed to the theory of Natural Selection, have been
discussed in my 'Origin of Species,' as far as the size of that work
permitted, under the following heads: the difficulty in understanding how
very simple organs have been converted by small and graduated steps into
highly perfect and complex organs; the marvellous facts of Instinct; the
whole question of Hybridity; and, lastly, the absence in our known
geological formations of innumerable links connecting all allied species.
Although some of these difficulties are of great weight, we shall see that
many of them are explicable on the theory of natural selection, and are
otherwise inexplicable.
In scientific investigations it is permitted to invent any hypothesis, and
if it explains various large and independent classes of facts it rises to
the rank of a well-grounded theory. The undulations of the ether and even
its existence are hypothetical, yet every one now admits the undulatory
theory of light. The principle of natural selection may be looked at as a
mere hypothesis, but rendered in some degree probable by what we positively
know of the variability of organic beings in a state of nature,--by what we
positively know of the struggle for existence, and the consequent almost
inevitable preservation of favourable variations,--and from the analogical
formation of domestic races. Now this hypothesis may be tested,--and this
seems to me the only fair and legitimate manner of considering the whole
question,--by trying whether it explains several large and independent
classes of facts; such as the geological succession of organic beings,
their distribution in past and present times, and their mutual affinities
and homologies. If the principle of natural selection does explain these
and other large bodies of facts, it ought to be received. On the ordinary
view of each species having been independently created, we gain no
scientific explanation of any one of these facts. We can only say that it
has so pleased the Creator to command that the past and present inhabitants
of the world should appear in a certain order and in certain areas; that He
has impressed on them the most extraordinary resemblances, and has classed
them in groups subordinate to groups. But by such statements we gain no new
knowledge; we do not connect together facts and laws; we explain nothing.
It was the consideration of such large groups of facts as these which first
led me to take up the present subject. When I visited during the voyage of
H.M.S. "Beagle," the Galapagos Archipelago, situated in the Pacific Ocean
about 500 miles from South America, I found myself surrounded by peculiar
species of birds, reptiles, and plants, existing nowhere else in the world.
Yet they nearly all bore an American stamp. In the song of the mocking-
thrush, in the harsh cry of the carrion-hawk, in the great candlestick-like
opuntias, I clearly perceived the neighbourhood of America, though the
islands were separated by so many miles of ocean from the mainland, and
differed much in their geological constitution and climate. Still more
surprising was the fact that most of the inhabitants of each separate
island in this small archipelago were specifically different, though most
closely related to each other. The archipelago, with its innumerable
craters and bare streams of lava, appeared to be of recent origin; and thus
I fancied myself brought near to the very act of creation. I often asked
myself how these many peculiar animals and plants had been produced: the
simplest answer seemed to be that the inhabitants of the several islands
had descended from each other, undergoing modification in the course of
their descent; and that all the inhabitants of the archipelago were
descended from those of the nearest land, namely America, whence colonists
would naturally have been derived. But it long remained to me an
inexplicable problem how the necessary degree of modification could have
been effected, and it would have thus remained for ever, had I not studied
domestic productions, and thus acquired a just idea of the power of
Selection. As soon as I had fully realised this idea, I saw, on reading
Malthus on Population, that Natural Selection was the inevitable result of
the rapid increase of all organic beings; for I was prepared to appreciate
the struggle for existence by having long studied the habits of animals.
Before visiting the Galapagos I had collected many animals whilst
travelling from north to south on both sides of America, and everywhere,
under conditions of life as different as it is possible to conceive,
American forms were met with--species replacing species of the same
peculiar genera. Thus it was when the Cordilleras were ascended, or the
thick tropical forests penetrated, or the fresh waters of America searched.
Subsequently I visited other countries, which in all their conditions of
life were incomparably more like parts of South America, than the different
parts of that continent are to each other; yet in these countries, as in
Australia or Southern Africa, the traveller cannot fail to be struck with
the entire difference of their productions. Again the reflection was forced
on me that community of descent from the early inhabitants of South America
would alone explain the wide prevalence of American types throughout that
immense area.
To exhume with one's own hands the bones of extinct and gigantic quadrupeds
brings the whole question of the succession of species vividly before one's
mind; and I found in South America great pieces of tesselated armour
exactly like, but on a magnificent scale, that covering the pigmy
armadillo; I had found great teeth like those of the living sloth, and
bones like those of the cavy. An analogous succession of allied forms had
been previously observed in Australia. Here then we see the prevalence, as
if by descent, in time as in space, of the same types in the same areas;
and in neither the case does the similarity of the conditions by any means
seem sufficient to account for the similarity of the forms of life. It is
notorious that the fossil remains of closely consecutive formations are
closely allied in structure, and we can at once understand the fact if they
are closely allied by descent. The succession of the many distinct species
of the same genus throughout the long series of geological formations seems
to have been unbroken or continuous. New species come in gradually one by
one. Ancient and extinct forms of life are often intermediate in character,
like the words of a dead language with respect to its several offshoots or
living tongues. All these facts seemed to me to point to descent with
modification as the means of production of new species.
The innumerable past and present inhabitants of the world are connected
together by the most singular and complex affinities, and can be classed in
groups under groups, in the same manner as varieties can be classed under
species and sub-varieties under varieties, but with much higher grades of
difference. These complex affinities and the rules for classification,
receive a rational explanation on the theory of descent, combined with the
principle of natural selection, which entails divergence of character and
the extinction of intermediate forms. How inexplicable is the similar
pattern of the hand of a man, the foot of a dog, the wing of a bat, the
flipper of a seal, on the doctrine of independent acts of creation! how
simply explained on the principle of the natural selection of successive
slight variations in the diverging descendants from a single progenitor! So
it is with certain parts or organs in the same individual animal or plant,
for instance, the jaws and legs of a crab, or the petals, stamens, and
pistils of a flower. During the many changes to which in the course of time
organic beings have been subjected, certain organs or parts have
occasionally become at first of little use and ultimately superfluous; and
the retention of such parts in a rudimentary and useless condition is
intelligible on the theory of descent. It can be shown that modifications
of structure are generally inherited by the offspring at the same age at
which each successive variation appeared in the parents; it can further be
shown that variations do not commonly supervene at a very early period of
embryonic growth, and on these two principles we can understand that most
wonderful fact in the whole circuit of natural history, namely, the close
similarity of the embryos within the same great class--for instance, those
of mammals, birds, reptiles, and fish.
It is the consideration and explanation of such facts as these which has
convinced me that the theory of descent with modification by means of
natural selection is in the main true. These facts have as yet received no
explanation on the theory of independent Creation; they cannot be grouped
together under one point of view, but each has to be considered as an
ultimate fact. As the first origin of life on this earth, as well as the
continued life of each individual, is at present quite beyond the scope of
science, I do not wish to lay much stress on the greater simplicity of the
view of a few forms or of only one form having been originally created,
instead of innumerable miraculous creations having been necessary at
innumerable periods; though this more simple view accords well with
Maupertuis's philosophical axiom of "least action."
In considering how far the theory of natural selection may be extended,
--that is, in determining from how many progenitors the inhabitants of the
world have descended,--we may conclude that at least all the members of the
same class have descended from a single ancestor. A number of organic
beings are included in the same class, because they present, independently
of their habits of life, the same fundamental type of structure, and
because they graduate into each other. Moreover, members of the same class
can in most cases be shown to be closely alike at an early embryonic age.
These facts can be explained on the belief of their descent from a common
form; therefore it may be safely admitted that all the members of the same
class are descended from one progenitor. But as the members of quite
distinct classes have something in common in structure and much in common
in constitution, analogy would lead us one step further, and to infer as
probable that all living creatures are descended from a single prototype.
I hope that the reader will pause before coming to any final and hostile
conclusion on the theory of natural selection. The reader may consult my
'Origin of Species' for a general sketch of the whole subject; but in that
work he has to take many statements on trust. In considering the theory of
natural selection, he will assuredly meet with weighty difficulties, but
these difficulties relate chiefly to subjects--such as the degree of
perfection of the geological record, the means of distribution, the
possibility of transitions in organs, etc.--on which we are confessedly
ignorant; nor do we know how ignorant we are. If we are much more ignorant
than is generally supposed, most of these difficulties wholly disappear.
Let the reader reflect on the difficulty of looking at whole classes of
facts from a new point of view. Let him observe how slowly, but surely, the
noble views of Lyell on the gradual changes now in progress on the earth's
surface have been accepted as sufficient to account for all that we see in
its past history. The present action of natural selection may seem more or
less probable; but I believe in the truth of the theory, because it
collects, under one point of view, and gives a rational explanation of,
many apparently independent classes of facts. (Introduction/4. In treating
the several subjects included in the present and my other works I have
continually been led to ask for information from many zoologists,
botanists, geologists, breeders of animals, and horticulturists, and I have
invariably received from them the most generous assistance. Without such
aid I could have effected little. I have repeatedly applied for information
and specimens to foreigners, and to British merchants and officers of the
Government residing in distant lands, and, with the rarest exceptions, I
have received prompt, open-handed, and valuable assistance. I cannot
express too strongly my obligations to the many persons who have assisted
me, and who, I am convinced, would be equally willing to assist others in
any scientific investigation.)