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its conditions. This improvement inevitably leads to the gradual advance-
ment of the organization of the greater number of living beings throughout
the world. But here we enter on a very intricate subject, for naturalists have
not defined to each other's satisfaction what is meant by an advance in or-
ganization. Among the vertebrata the degree of intellect and an approach in
structure to man clearly come into play. It might be thought that the amount
of change which the various parts and organs pass through in their develop-
ment from embryo to maturity would suffice as a standard of comparison;
but there are cases, as with certain parasitic crustaceans, in which several
parts of the structure become less perfect, so that the mature animal cannot
be called higher than its larva. Von Baer's standard seems the most widely
applicable and the best, namely, the amount of differentiation of the parts of
the same organic being, in the adult state, as I should be inclined to add,
and their specialization for different functions; or, as Milne Edwards would
express it, the completeness of the division of physiological labor. But we
shall see how obscure this subject is if we look, for instance, to fishes, among
which some naturalists rank those as highest which, like the sharks, approach
nearest to amphibians; while other naturalists rank the common bony or
teleostean fishes as the highest, inasmuch as they are most strictly fish-like,
and differ most from the other vertebrate classes. We see still more plainly
the obscurity of the subject by turning to plants, among which the standard
of intellect is of course quite excluded; and here some botanists rank those
plants as highest which have every organ, as sepals, petals, stamens, and pis-
tils, fully developed in each flower; whereas other botanists, probably with



NATURAL SELECTION 83

more truth, look at the plants which have their several organs much modi-
fied and reduced in number, as the highest.

If we take as the standard of high organization, the amount of differentia-
tion and specialization of the several organs in each being when adult (and
this will include the advancem.ent of the brain for intellectual purposes),
natural selection clearly leads toward this standard : for all physiologists ad-
mit that the specialization of organs, inasmuch as in this state they perform
their functions better, is an advantage to each being; and hence the accumu-
lation of variations tending toward specialization is within the scope of natu-
ral selection. On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every unoc-
cupied or less well occupied place in the economy of nature, that it is quite
possible for natural selection gradually to fit a being to a situation in which
several organs would be superfluous or useless: in such cases there would be
retrogression in the scale of organization. Whether organization on the whole
has actually advanced from the remotest geological periods to the present
day, will be more conveniently discussed in our chapter on Geological Suc-
cession.

But it may be objected that if all organic beings thus tend to rise in the
scale, how is it that throughout the world a multitude of the lowest forms still
exist; and how is it that in each great class some forms are far more highly
developed than others? Why have not the more highly developed forms every-
where supplanted and exterminated the lower? Lamarck, who believed in an
innate and inevitable tendency toward perfection in all .organic beings, seems
to have felt this difficulty so strongly that he was led to suppose that new
and simple forms are continually being produced by spontaneous generation.
Science has not as yet proved the truth of this belief, whatever the future
may reveal. On our theory the continued existence of lowly organisms offers
no difficulty; for natural selection, or the survival of the fittest, does not
necessarily include progressive development — it only takes advantage of such
variations as arise and are beneficial to each creature under its complex rela-
tions of life. And it may be asked what advantage, as far as we can see, would
it be to an infusorian animalcule — to an intestinal worm — or even to an
earth-worm, to be highly organized. If it were no advantage, these forms
would be left, by natural selection, unimproved or but little improved, and
might remain for indefinite ages in their present lowly condition. And geology
tells us that some of the lowest forms, as the infusoria and rhizopods, have
remained for an enormous period in nearly their present state. But to suppose
that most of the many now existing low forms have not in the least advanced
since the first dawn of life would be extremely rash; for every naturalist who
has dissected some of the beings now ranked as very low in the scale, must
have been struck with their really wondrous and beautiful organization.

Nearly the same remarks are applicable, if we look to the different
grades of organization within the same great group ; for instance, in the ver-
tebrata, to the co-existence of mammals and fish — among mammalia, to the



84 THE ORIGIN OF SPECIES

co-existence of man and the Ornithorhynchus — among fishes, to the co-exist•^
ence of the shark and the lancelet (Amphioxus), which latter fish in the ex-
treme simplicity of its structure approaches the invertebrate classes. But
mammals and fish hardly come into competition with each other; the ad-
vancement of the whole class of mammals, or of certain members in this class,
to the highest grade, would not lead to their taking the place of fishes. Physi-
ologists believe that the brain must be bathed by warm blood to be highly
active, and this requires aerial respiration; so that warm-blooded mammals
when inhabiting the water lie under a disadvantage in having to come con-
tinually to the surface to breathe. With fishes, members of the shark family
would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz
Miiller, has as sole companion and competitor on the barren sandy shore of
South Brazil, an anomalous annelid. The three lowest orders of mammals,
namely, marsupials, edentata, and rodents, co-exist in South America in the
same region with numerous monkeys, and probably interfere little with each
other. Although organization, on the whole, may have advanced and be
still advancing throughout the world, yet the scale will always present many
degrees of perfection; for the high advancement of certain whole classes, or
of certain m^embers of each class, does not at all necessarily lead to the ex-
tinction of those groups with which they do not enter into close competition.
In some cases, as we shall hereafter see, lowly organized forms appear to
have been preserved to the present day, from inhabiting confined or peculiar
stations, where they have been subjected to less severe competition, and
where their scanty numbers have retarded the chance of favorable varia-
tions arising.

Finally, I believe that many lowly organized forms now exist throughout
the world, from various causes. In some cases, variations or individual dif-
ferences of a favorable nature may never have arisen for natural selection
to act on and accumulate. In no case, probably, has time sufficed for the
utmost possible amount of development. In some few cases there has been
what we must call retrogression of organization. But the main cause lies in
the fact that under very simple conditions of life a high organization would
be of no service, — possibly would be of actual disservice, as being of a more
delicate nature, and more liable to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may be-
lieve, presented the simplest structure, how, it has been asked, could the first
step in the advancement or differentiation of parts have arisen? Mr. Herbert
Spencer would probably answer, that, as soon as simple unicellular organism
came by growth or division to be compounded of several cells, or became
attached to any supporting surface, his law "that homologous units of any
order become differentiated in proportion as their relations to incident forces
become different" would come into action. But as we have no facts to guide
us, speculation on the subject is almost useless. It is, however, an error to sup-
pose that there would be no struggle for existence, and consequently no natu-
ral selection, until many forms had been produced: variations in a single
species inhabiting an isolated station might be beneficial, and thus the whole



NATURAL SELECTION 85

mass of individuals might be modified, or two distinct forms might arise.
But, as I remarked toward the close of the introduction, no one ought to feel
surprise at much remaining as yet unexplained on the origin of species, if we
make due allowance for our profound ignorance on the mutual relations of
the inhabitants of the world at the present time, and still more so during
past ages.

CONVERGENCE OF CHARACTER

Mr. H. G. Watson thinks that I have overrated the importance of diver-
gence of character (in which, however, he apparently believes) , and that con-
vergence, as it may be called, has likewise played a part. If two species be-
longing to two distinct though allied genera, had both produced a large
number of new and divergent forms, it is conceivable that these might ap-
proach each other so closely that they would have all to be classed under the
same genus ; and thus the descendants of two distinct genera would converge
into one. But it would in most cases be extremely rash to attribute to con-
vergence a close and general similarity of structure in the modified descend-
ants of widely distinct forms. The shape of a crystal is determined solely by
the molecular forces, and it is not surprising that dissimilar substances should
sometimes assume the same form; but with organic beings we should bear in
mind that the form of each depends on an infinitude of complex relations,
namely on the variations which have arisen, these being due to causes far too
intricate to be followed out — on the nature of the variations which have been
preserved or selected, and this depends on the surrounding physical condi-
tions, and in a still higher degree on the surrounding organisms with which
each being has come into competition — and lastly, on inheritance (in itself
a fluctuating element) from innumerable progenitors, all of which have had
their forms determined through equally complex relations. It is incredible
that the descendants of two organisms, which had originally differed in a
marked manner, should ever afterward converge so closely as to lead to a
near approach to identity throughout their whole organization. If this had
occurred, we should meet with the same form, independently of genetic con-
nection, recurring in widely separated geological formations; and the bal-
ance of evidence is opposed to any such admission.

Mr. Watson has also objected that the continued action of natural selec-
tion, together with divergence of character, would tend to make an indefi-
nite number of specific forms. As far as mere inorganic conditions are con-
cerned, it seems probable that a sufficient number of species would soon be-
come adapted to all considerable diversities of heat, moisture, etc.; but I
fully admit that the mutual relations of organic beings are more important;
and as the number of species in any country goes on increasing, the organic
conditions of life must become more and more complex. Consequently there
seems at first no limit to the amount of profitable diversification of structure,
and therefore no limit to the number of species which might be produced.
We do not know that even the most prolific area is fully stocked with specific
forms: at the Cape of Good Hope and in Australia, which support such an



86 THE ORIGIN OF SPECIES

astonishing number of species, many European plants have become natural-
ized. But geology shows us, that from an early part of the tertiary period the
number of species of shells, and that from the middle part of this same period
the number of mammals, has not greatly or at all increased. What then checks
an indefinite increase in the number of species? The amount of life ( I do
not mean the number of specific forms) supported on an area must have a
limit, depending so largely as it does on physical conditions ; therefore, if an
area be inhabited by very many species, each or nearly each species will be
represented by few individuals; and such species will be liable to extermina-
tion from accidental fluctuations in the nature of the seasons or in the num-
ber of their enemies. The process of extermination in such cases would be
rapid, whereas the production of new species must always be slow. Imagine
the extreme case of as many species as individuals in England, and the first
severe winter or very dry summer would exterminate thousands on thousands
of species. Rare species, and each species will become rare if the number of
species in any country becomes indefinitely increased, will, on the principle
often explained, present within a given period few favorable variations; con-
sequently, the process of giving birth to new specific forms would thus be re-
tarded. When any species becomes very rare, close interbreeding will help to
exterminate it; authors have thought that this comes into play in accounting
for the deterioration of the aurochs in Lithuania, of red deer in Scotland,
and of bears in Norway, etc. Lastly, and this I am inclined to think is the
most important element, a dominant species, which has already beaten many
competitors in its own home, will tend to spread and supplant many others.
Alph. de CandoUe has shown that those species which spread widely tend
generally to spread very widely, consequently they will tend to supplant and
exterminate several species in several areas, and thus check the inordinate
increase of specific forms throughout the world. Dr. Hooker has recently
shown that in the south-east corner of Australia, where, apparently, there are
many invaders from diff"erent quarters of the globe, the endemic Australian
species have been greatly reduced in number. How much weight to attribute
to these several considerations I will not pretend to say; but conjointly they
must limit in each country the tendency to an indefinite augmentation of
specific forms.

SUMMARY OF CHAPTER

If under changing conditions of life organic beings present individual dif-
ferences in almost every part of their structure, and this cannot be disputed;
if there be, owing to their geometrical rate of increase, a severe struggle for
life at some age, season, or year, and this certainly cannot be disputed ; then,
considering the infinite complexity of the relations of all organic beings to
each other and to their conditions of life, causing an infinite diversity in
structure, constitution, and habits, to be advantageous to them, it would be
a most extraordinary fact if no variations had ever occurred useful to each
being's own welfare, in the same manner as so many variations have occurred
useful to man.^ut if variations useful to any organic being ever do occur,



NATURAL SELECTION 87

ssuredlv individuals thus characterized will have the best chance of being
ireserved in the struggle for life; and from the strong principle of inheritance,
hese will tend to produce offspring similarly characterized. This principle
,f preservation, or the survival of the fittest, I have called natural selection.:'
t leads to the improvement of each creature in relation to its organic and
nor^anic conditions of life; and consequently, in most cases, to what must be
•e^arded as an advance in organization. Nevertheless, low and simple forms
Yill lonff endure if well fitted for their simple conditions of life.^

Natural selection, on the principle of qualities being inherited at corre-
;ponding ages, can modify the egg, seed, or young, as easily as the adult.
\mon^ many animals sexual selection will have given its aid to ordinary
;election by assuring to the most vigorous and best adapted males the great-
est number of offspring. Sexual selection will also give characters useful to
■he males alone in their struggles or rivalry with other males; and these char-
acters will be transmitted to one sex or to both sexes, according to the lorm
Df inheritance which prevails. , . , . i

Whether natural selection has really thus acted m adapting the various
forms of life to their several conditions and stations, must be judged by the
general tenor and balance of evidence given in the following chapters. But
we have aheady seen how it entails extinction; and how largely extinction
has acted in the world's history, geology plainly declares. Natural selection,
also leads to divergence of character; for the more organic beings diverge
in structure, habits, and constitution, by so much the more can a large num-
ber be supported on the area, of which we see proof by looking to the inhabit-
ants of any small spot, and to the productions naturaHzed m foreign lands.
Therefore during the modification of the descendants of any one species,
and during the incessant struggle of all species to increase in numbers, the
more diversified the descendants become, the better will be their chance of
success nTthTbatde for life:\Thus the small differences distinguishing varie-
ties of the same species, steadily tend to increase, till they equal the greater
differences between species of the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused, and widely rang-
ing species, belonging to the larger genera within each class, which vary
most, and these tend to transmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection,
as has just been remarked, leads to divergence of character and to much
extinction of the less improved and intermediate forms of hfe. On these prin-
ciples, the nature of the affinities, and the generally well defined distinctions
between the innumerable organic beings in each class throughout the world,
may be explained. It is a truly wonderful fact— the wonder of which we are
apt to overlook from familiarity— that all animals and all plants throughout
all time and space, should be related to each other in groups, subordinate to
groups, in the manner which we everywhere behold— namely, varieties of the
same species most closely related, species of the same genus less closely and
unequally related, forming sections and sub-genera, species of distinct genera
much less closely related, and genera related in different degrees, forming



88 THE ORIGIN OF SPECIES

sub-familiesj families, orders, sub-classes, and classes. The several subordinate
groups in any class cannot be ranked in a single file, but seem clustered round
points, and these round other points, and so on in almost endless cycles. If
species had been independently created, no explanation would have been
possible of this kind of classification; but it is explained through inheritance
and the complex action of natural selection, entailing extinction and diver-
gence of character, as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been rep-
resented by a great tree. I believe this simile largely speaks the truth. The
green and budding twigs may represent existing species ; and those produced
during former years may represent the long succession of extinct species. At
each period of growth all the growing twigs have tried to branch out on all
sides, and to overtop and kill the surrounding twigs and branches, in the
same manner as species and groups of species have at all times overmastered
other species in the great battle for life. The limbs divided into great branches,
and these into lesser and lesser branches, were themselves once, when the tree
was young, budding twigs; and this connection of the former and present
buds, by ramifying branches, may well represent the classification of all ex-
tinct and living species in groups subordinate to groups. Of the many twigs
which flourished when thc'tree was a mere bush, only two or three, now
grown into great branches, yet survive and bear the other branches; so with
the species which lived during long-past geological periods, very few have
left living and modified descendants. From the first growth of the free, many
a limb and branch has decayed and dropped off; and these fallen branches
of various sizes may represent those whole orders, families, and genera which
have now no living representatives, and which are known to us only in a fos-
sil state. As we here and there see a thin, straggling branch springing from
a fork low down in a tree, and which by some chance has been favored and
is still alive on its summit, so we occasionally see an animal like the Orni -
thorhynchus or Lepidosiren, which in some small degree connects by its af-
finities two large branches of life, and which has apparently been saved from
fatal competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and overtop on
all sides many a feebler branch, so by generation I believe it has been with
the great Tree of Life, which fills with its dead and broken branches the
crust of the eartfi, and covers the surface with its ever-branching and beauti-»
ful ramifications.



CHAPTER V

Laws of Variation

Effects of Changed Conditions — Use and Disuse, combined with Natural Selection;
Organs of Flight and of Vision — Acclimatization — Correlated Variation — Com-
pensation and Economy of Growth — False Correlations — Multiple, Rudimentary,
and Lowly Organized Structures Variable — Parts developed in an Unusual Man-
ner are highly Variable: Specific Characters more Variable than Generic; Sec-
ondary Sexual Characters Variable — Species of the Same Genus vary in an
Analogous Manner — Reversions to Long-lost Characters — Summary.

I HAVE hitherto sometimes spoken as if the variations — so common and mul-
tiform with organic beings under domestication, and in a lesser degree with
those under nature — were due to chance. This, of course, is a wholly incorrect
expression, but it serves to acknowledge plainly our ignorance of the cause
of each particular variation. Some authors believe it to be as much the func-
tion of the reproductive system to produce individual differences, or slight
deviations of structure, as to make the child like its parents. But the fact of
variations and monstrosities occurring much more frequently under domes-
tication than under nature, and the greater variability of species having wide
ranges than of those with restricted ranges, lead to the conclusion that vari-
ability is generally related to the conditions of life to which each species has
been exposed during several successive generations. In the first chapter I at-
tempted to show that changed conditions act in two ways, directly on the
whole organization or on certain parts alone, and indirectly through the re-
productive system. In all cases there are two factors, the nature of the organ-
ism, which is much the most important of the two, and the nature of the
conditions. The direct action of changed conditions leads to definite or in-
definite results. In the latter case the organization seems to become plastic,
and we have much fluctuating variability. In the former case the nature of
the organism is such that it yields readily, when subjected to certain condi-
tions, and all, or nearly all, the individuals become modified in the same way.
It is very difficult to decide how far changed conditions, such as of climate,
food, etc., have acted in a definite manner. There is reason to believe that
in the course of time the effects have been greater than can be proved by
clear evidence. But we may safely conclude that the innumerable complex
coadaptations of structure, which we see throughout nature between various
organic beings, cannot be attributed simply to such action. In the following
cases the conditions seem to have produced some slight definite effect: E.
Forbes asserts that shells at their southern limit, and when living in shallow
water, are more brightly colored than those of the same species from farther
north or from a greater depth; but this certainly does not always hold good.
Mr. Gould believes that birds of the same species are more brightly colored



Online LibraryCharles DarwinThe origin of species → online text (page 12 of 50)