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under a clear atmosphere, than when living near the coast or on islands ; and
Wollaston is convinced that residence near the sea affects the colors of in-
sects. Moquin-Tandon gives a list of plants which, when growing near the
seashore, have their leaves in some degree fleshy, though not elsewhere

89



90 THE ORIGIN OF SPECIES

fleshy. These slightly varying organisms are interesting in as far as they pre-
sent characters analogous to those possessed by the species which are con-
fined to similar conditions.

When a variation is of the slightest use to any being, we cannot tell how
much to attribute to the accumulative action of natural selection, and how
much to the definite action of the conditions of life. Thus, it is well known
to furriers that animals of the same species have thicker and better fur the
farther north they live: but who can tell how much of this difference may be
due to the warmest clad individuals having been favored and preserved
during many generations, and how much to the action of the severe climate?
Tor it would appear that climate has some direct action on the hair of our
domestic quadrupeds.

Instances could be given of similar varieties being produced from the same
species under external conditions of life as different as can well be conceived ;
and, on the other hand, of dissimilar varieties being produced under appar-
ently the same external conditions. Again, innumerable instances are known
to every naturalist, of species keeping true, or not varying at all, although
living under the most opposite climates. Such considerations as these incline
me to lay less weight on the direct action of the surrounding conditions, than
on a tendency to vary, due to causes of which we are quite ignorant.

In one sense the conditions of life may be said, not only to cause variability,
either directly or indirectly, but likewise to include natural selection, for the
conditions determine whether this or that variety shall survive. But when
man is the selecting agent, we clearly see that the two elements of change
are distinct; variability is in some manner excited, but it is the will of man
which accumulates the variations in a certain direction; and it is this latter
agency which answers to the survival of the fittest under nature.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED

BY NATURAL SELECTION

From the facts alluded to in the first chapter, I think there can be no
doubt that use in our domestic animals has strengthened and enlarged cer-
tain parts, and disuse diminished them; and that such modifications are in-
herited. Under free nature we have no standard of comparison by which to
judge of the effects of long-continued use or disuse, for we know not the
parent-forms; but many animals possess structures which can be best ex-
plained by the effects of disuse. As Professor Owen has remarked, there is
no greater anomaly in nature than a bird that cannot fly; yet there are sev-
eral in this state. The logger-headed duck of South America can only flap
along the surface of the water, and has its wings in nearly the same condi-
tion as the domestic Aylesbury duck: it is a remarkable fact that the young
birds, according to Mr. Cunningham, can fly, while the adults have lost this
power. As the larger ground-feeding birds seldom take flight except to escape
danger, it is probable that the nearly wingless condition of several birds, now
inhabiting or which lately inhabited several oceanic islands, tenanted by no



LAWS OF VARIATION 91

beasts of prey, has been caused by disuse. The ostrich indeed inhabits con-
tinents, and is exposed to danger from which it cannot escape by flight, but
it can defend itself, by kicking its enemies, as efficiently as many quadrupeds.
We may believe that the progenitor of the ostrich genus had habits like those
of the bustard, and that, as the size and weight of its body were increased
during successive generations, its legs were used more and its wings less, until
they became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off; he
examined seventeen specimens in his own collection, and not one had even
a relic left. In the Onites apelles the tarsi are so habitually lost that the insect
has been described as not having them. In some other genera they are
present, but in a rudimentary condition. In the Ateuchus or sacred beetle of
the Egyptians, they are totally deficient. The evidence that accidental
mutilations can be inherited is at present not decisive; but the remarkable
cases observed by Brown- Sequard in guinea-pigs, of the inherited effects of
operations, should make us cautious in denying this tendency. Hence, it will
perhaps be safest to look at the entire absence of the anterior tarsi in
Ateuchus, and their rudimentary condition in some other genera, not as
cases of inherited mutilations, but as due to the effects of long-continued
disuse; for, as many dung-feeding beetles are generally found with their tarsi
lost, this must happen early in life; therefore the tarsi cannot be of much
importance or be much used by these insects.

In some cases we might easily put down to disuse modifications of structure
which are wholly or mainly due to natural selection. Mr. Wollaston has dis-
covered the remarkable fact that 200 beetles, out of the 550 species (but
more are now known) inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that, of the twenty-nine endemic genera, no less than
twenty- three have all their species in this condition! Several facts, — namely,
that beetles in many parts of the world are frequently blown to sea and
perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much
concealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Desertas than in Madeira itself;
and especially the extraordinary fact, so strongly insisted on by Mr. Wollas-
ton, that certain large groups of beetles, elsewhere excessively numerous,
which absolutely require the use of their wings, are here almost entirely
absent. These several considerations make me believe that the wingless
condition of so many Madeira beetles is mainly due to the action of natural
selection, combined probably with disuse. For during many successive
generations each individual beetle which flew least, either from its wings
having been ever so little less perfectly developed or from indolent habit,
will have had the best chance of surviving from not being blown out to sea;
and, on the other hand, those beetles which most readily took to flight would
oftenest have been blown to sea, and thus destroyed.

The insects in Madeira which are not ground-feeders, and which, as
certain flower-feeding coleoptera and lepidoptera, must habitually use their



92 THE ORIGIN OF SPECIES

wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite compatible with the
action of natural selection. For when a new insect first arrived on the island,
the tendency of natural selection to enlarge or to reduce the wings, would
depend on whether a greater number of individuals were saved by success-
fully battling with the winds, or by giving up the attempt and rarely or never
flying. As with mariners shipwrecked near a coast, it would have been better
for the good swimmers if they had been able to swim still further, whereas it
would have been better for the swimmers if they had not been able to swim
at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered by skin and fur. This state of the eyes
is probably due to gradual reduction from disuse, but aided perhaps by
natural selection. In South America, a burrowing rodent, the tucotuco, or
Ctenomys, is even more subterranean in its habits than the mole; and I
was assured by a Spaniard, who had often caught them, that they were
frequently blind. One which I kept alive was certainly in this condition, the
cause, as appeared on dissection, having been inflammation of the nictitating
membrane. As frequent inflammation of the eyes must be injurious to any
animal, and as eyes are certainly not necessary to animals having subter-
ranean habits, a reduction in their size, with the adhesion of the eyelids and
growth of fur over them, might in such case be an advantage; and if so,
natural selection would aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Camiola. and Kentucky, are blind. In
some of the crabs the footstalk for the eyes remains, though the eye is gone;
the stand for the telescope is there, though the telescope with its glasses has
been lost. As it is difficult to imagine that eyes, though useless, could be in
any way injurious to animals living in darkness, their loss may be attributed
to disuse. In one of the blind animals, namely, the cave-rat (Neotoma),
two of which were captured by Professor Silliman at above half a mile
distance from the mouth of the cave, and therefore not in the profoundest
depths, the eyes were lustrous and of large size; and these animals, as I am
informed by Professor Silliman, after having been exposed for about a
month to a graduated light, acquired a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep lime-
stone caverns under a nearly similar climate; so that, in accordance with the
old view of the blind animals having been separately created for the Ameri-
can and European caverns, very close similarity in their organization and
affinities might have been expected. This is certainly not the case if we look
at the two whole faunas; and with respect to the insects alone, Schiodte has
remarked: "We are accordingly prevented from considering the entire
phenomenon in any other light than something purely local, and the
similarity which is exhibited in a few forms between the Mammoth Cave
(in Kentucky) and the caves in Carniola, otherwise than as a very plain
expression of that analogy which subsists generally between the fauna of



LAWS OF VARIATION 93

Europe and of North America." On my view we must suppose that Ameri-
can animals, having in most cases ordinary powers of vision, slowly migrated
by successive generations from the outer world into the deeper and deeper
recesses of the Kentucky caves, as did European animals into the caves of
Europe. We have some evidence of this gradation of habit; for, as Schiodte
remarks: "We accordingly look upon the subterranean faunas as small
ramifications which have penetrated into the earth from the geographically
limited faunas of the adjacent tracts, and which, as they extended them-
selves into darkness, have been accommodated to surrounding circumstances.
Animals not far remote from ordinary forms, prepare the transition from
light to darkness. Next follow those that are constructed for twilight; and,
last of all, those destined for total darkness, and whose formation is quite
peculiar." These remarks of Schiodte' s, it should be understood, apply not
to the same, but to distinct species. By the time that an animal had reached,
after numberless generations, the deepest recesses, disuse will on this view
have more or less perfectly obliterated its eyes, and natural selection will
often have effected other changes, such as an increase in the length of the
antennae or palpi, as a compensation for blindness. Notwithstanding such
modifications, we might expect still to see in the cave-animals of America,
affinities to the other inhabitants of that continent, and in those of Europe
to the inhabitants of the European continent. And this is the case with some
of the American cave-animals, as I hear from Professor Dana; and some
of the European cave-insects are very closely allied to those of the surround-
ing country. It would be difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabitants of the two con-
tinents on the ordinary view of their independent creation. That several of
the inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of their
other productions. As a blind species of Bathyscia is found in abundance on
shady rocks far from caves, the loss of vision in the cave species of this one
genus has probably had no relation to its dark habitation; for it is natural
that an insect already deprived of vision should readily become adapted to
dark caverns. Another blind genus (Anophthalmus) offers this remarkable
peculiarity, that the species, as Mr. Murray observes, have not as yet been
found anywhere except in caves; yet those which inhabit the several caves
of Europe and America are distinct; but it is possible that the progenitors
of these several species, while they were furnished with eyes, may formerly
have ranged over both continents, and then have become extinct, excepting
in their present secluded abodes. Far from feeling surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in regard
to the blind fish, the Amblyopsis, and as is the case with the blind Proteus,
with reference to the reptiles of Europe, I am only surprised that more
wrecks of ancient life have not been preserved, owing to the less severe
competition to which the scanty inhabitants of these dark abodes will have
been exposed.



94 THE ORIGIN OF SPECIES



ACCLIMATIZATION

Habit is hereditary with plants, as in the period of flowering, in the time
of sleep, in the amount of rain requisite for seeds to germinate, etc., and
this leads me to say a few words on acclimatization. As it is extremely com-
mon for distinct species belonging to the same genus to inhabit hot and'
cold countries, if it be true that all the species of the same genus are de-
scended from a single parent-form, acclimatization must be readily effected
during a long course of descent. It is notorious that each species is adapted
to the climate of its own home: species from an arctic or even from a
temperate region cannot endure a tropical climate, or conversely. So again,
many succulent plants cannot endure a damp climate. But the degree of
adaptation of species to the climates under which they live is often over-
rated. We may infer this from our frequent inability to predict whether or
not an imported plant will endure our climate, and from the number of
plants and animals brought from different countries which are here per-
fectly healthy. We have reason to believe that species in a state of nature
are closely limited in their ranges by the competition of other organic
beings quite as much as, or more than, by adaptation to particular climates.
But whether or not this adaptation is in most cases very close, we have
evidence with some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures; that is, they become ac-
climatized; thus the pines and rhododendrons, raised from seed collected
by Dr. Hooker from the same species growing at different heights on the
Himalayas, were found to possess in this country different constitutional
powers of resisting cold. Mr. Thwaites informs me that he has observed
similar facts in Ceylon; analogous observations have been made by Mr.
H. C. Watson on European species of plants brought from the Azores to
England ; and I could give other cases. In regard to animals, several authen-
tic instances could be adduced of species having largely extended, within
historical times, their range from warmer to colder latitudes, and con-
versely; but we do not positively know that these animals were strictly
adapted to their native climate, though in all ordinary cases we assume
such to be the case; nor do we know that they have subsequently become
specially acclimated to their new homes, so as to be better fitted for them
than they were at first.

As we may infer that our domestic animals were originally chosen by
uncivilized man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable
of far-extended transportation, the common and extraordinary capacity in
our domestic animals of not only withstanding the most different climates,
but of being perfectly fertile (a far severer test) under them, may be used
as an argument that a large proportion of other animals now in a state of
nature could easily be brought to bear widely different climates. We must
not, however, push the foregoing argument too far, on account of the



LAWS OF VARIATION 95

probable origin of some of our domestic animals from several wild stocks;
the bloodj for instance, of a tropical and arctic wolf may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered
as domestic animals, but they have been transported by man to many parts
of the world, and now have a far wider range than any other rodent; for
they live under the cold climate of Faroe in the north, of the Falklands in
the south, and on many an island in the torrid zones. Hence adaptation to
any special climate may be looked at as quality readily grafted on an
innate wide flexibility of constitution, common to most animals. On this
view, the capacity of enduring the most different climates by man himself
and by his domestic animals, and the fact of the extinct elephant and rhi-
noceros having formerly endured a glacial climate, whereas the living
species are now all tropical or sub-tropical in their habits, ought not to be
looked at as anomalies, but as examples of a very common flexibility of
constitution brought, under peculiar circumstances, into action.

How much of the acclimatization of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties hav-
ing different innate constitutions, and how much to both means combined,
is an obscure question. That habit or custom has some influence, I must
believe, both from analogy and from the incessant advice given in agricul-
tural works, even in the ancient Encyclopedias of China, to be very cautious
in transporting animals from one district to another. And as it is not likely
that man should have succeeded in selecting so many breeds and sub-
breeds with constitutions specially fitted for their own districts, the result
must, I think, be due to habit. On the other hand, natural selection would
inevitably tend to preserve those individuals which were born with con-
stitutions best adapted to any country which they inhabited. In treatises on
many kinds of cultivated plants, certain varieties are said to withstand cer-
tain climates better than others; this is strikingly shown in works on fruit-
trees published in the United States, in which certain varieties are habitually
recommended for the Northern and others for the Southern States; and
as most of these varieties are of recent origin, they cannot owe their con-
stitutional differences to habit. The case of the Jerusalem artichoke, which
is never propagated in England by seed, and of which, consequently, new
varieties have not been produced, has even been advanced, as proving that
acclimatization cannot be effected, for it is now as tender as ever it was!
The case, also, of the kidney-bean has been often cited for a similar pur-
pose, and with much greater weight; but until some one will sow, during
a score of generations, his kidney-beans so early that a very large propor-
tion are destroyed by frost, and then collect seed from the few survivors,
with care to prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to have
been tried. Nor let it be supposed that differences in the constitution of
seedling kidney-beans never appear, for an account has been published
how much more hardy some seedlings are than others; and of this fact I
have myself observed striking instances.



96 THE ORIGIN OF SPECIES

On the whole, we may conclude that habit, or use and disuse, have, in
some cases, played a considerable part in the modification of the constitu-
tion and structure; but that the effects have often been largely combined
with, and sometimes overmastered by, the natural selection of innate
variations.

CORRELATED VARIATION

I mean by this expression that the whole organization is so tied together,
during its growth and development, that when slight variations in any one
part occur and are accumulated through natural selection, other parts be-
come modified. This is a very important subject, most imperfectly under-
stood, and no doubt wholly different classes of facts may be here easily con-
founded together. We shall presently see that simple inheritance often gives
the false appearance of correlation. One of the most real obvious cases is
that variations of structure arising in the young or larvse naturally tend to
affect the structure of the mature animal. The several parts which are
homologous, and which, at an early embryonic period, are identical in
structure, and which are necessarily exposed to similar conditions, seem
eminently liable to vary in a like manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind legs,
and even in the jaws and limbs, varying together, for the lower jaw is
believed by some anatomists to be homologous with the limbs. These
tendencies, I do not doubt, may be mastered more or less completely by
natural selection; thus a family of stags once existed with an antler only on
one side; and if this had been of any great use to the breed, it might
probably have been rendered permanent by selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants: and nothing is more common than
the union of homologous parts in normal structures, as in the union of the
petals into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that with birds the diversity in the
shape of the pelvis causes the remarkable diversity in the shape of their
kidneys. Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes, ac-
cording to Schlegel, the form of the body and the manner of swallowing
determine the position and form of several of the most important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy Saint-
Hilaire has forcibly remarked that certain malconformations frequently, and
that others rarely, co-exist without our being able to assign any reason. What
can be more singular than the relation in cats between complete whiteness
and blue eyes with deafness, or between the tortoise-shell color and the
female sex; or in pigeons, between their feathered feet and skin betwixt
the outer toes, or between the presence of more or less down on the young
pigeon, when first hatched, with the future color of its plumage; or again,
the relation between the hair and the teeth in the naked Turkish dog,
though here no doubt homology comes into play? With respect to this



LAWS OF VARIATION 97

latter case of correlation, I think it can hardly be accidental that the two
orders of mammals which are most abnormal in their dermal covering, viz.,
cetacea (whales) and edentata (armadilloes, scaly ant-eaters, etc.), are
likewise on the whole the most abnormal in their teeth, but there are so
many exceptions to this rule, as Mr. Mivart has remarked, that it has
little value.

I know of no case better adapted to show the importance of the laws of
correlation and variation, independently of utility, and therefore of natural
selection, than that of the difference between the outer and inner flowers
in some compositous and umbelliferous plants. Every one is familiar with
the difference between the ray and central florets of, for instance, the daisy,
and this difference is often accompanied with the partial or complete abor-
tion of the reproductive organs. But in some of these plants the seeds also
differ in shape and sculpture. These differences have sometimes been
attributed to the pressure of the involucra on the florets, or to their mutual
pressure, and the shape of the seeds in the ray florets of some compositae
countenances this idea; but with the umbelliferae it is by no means, as Dr.
Hooker informs me, the species with the densest heads which most



Online LibraryCharles DarwinThe origin of species → online text (page 13 of 50)