On the Origin of Species by Means of Natural Selection
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Chapter V
Laws of Variation
Effects of external conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlation
of growth -- Compensation and economy of growth -- False correlations --
Multiple, rudimentary, and lowly organised structures variable -- Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable --
Species of the same genus vary in an analogous manner -- Reversions to long
lost characters -- Summary.
I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser degree in
those in a state of nature--had been 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 function of the reproductive system to produce
individual differences, or very slight deviations of structure, as to make
the child like its parents. But the much greater variability, as well as
the greater frequency of monstrosities, under domestication or cultivation,
than under nature, leads me to believe that deviations of structure are in
some way due to the nature of the conditions of life, to which the parents
and their more remote ancestors have been exposed during several
generations. I have remarked in the first chapter--but a long catalogue of
facts which cannot be here given would be necessary to show the truth of
the remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being functionally
disturbed in the parents, I chiefly attribute the varying or plastic
condition of the offspring. The male and female sexual elements seem to be
affected before that union takes place which is to form a new being. In
the case of 'sporting' plants, the bud, which in its earliest condition
does not apparently differ essentially from an ovule, is alone affected.
But why, because the reproductive system is disturbed, this or that part
should vary more or less, we are profoundly ignorant. Nevertheless, we can
here and there dimly catch a faint ray of light, and we may feel sure that
there must be some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, &c., produces on any
being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that of
plants. We may, at least, safely conclude that such influences cannot have
produced the many striking and complex co-adaptations of structure between
one organic being and another, which we see everywhere throughout nature.
Some little influence may be attributed to climate, food, &c.: thus, E.
Forbes speaks confidently that shells at their southern limit, and when
living in shallow water, are more brightly coloured than those of the same
species further north or from greater depths. Gould believes that birds of
the same species are more brightly coloured under a clear atmosphere, than
when living on islands or near the coast. So with insects, Wollaston is
convinced that residence near the sea affects their colours. Moquin-Tandon
gives a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several other
such cases could be given.
The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree some
of the characters of such species, accords with our view that species of
all kinds are only well-marked and permanent varieties. Thus the species
of shells which are confined to tropical and shallow seas are generally
brighter-coloured than those confined to cold and deeper seas. The birds
which are confined to continents are, according to Mr. Gould,
brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants
which live exclusively on the sea-side are very apt to have fleshy leaves.
He who believes in the creation of each species, will have to say that this
shell, for instance, was created with bright colours for a warm sea; but
that this other shell became bright-coloured by variation when it ranged
into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural selection,
and how much to the conditions of life. Thus, it is well known to furriers
that animals of the same species have thicker and better fur the more
severe the climate is under which they have lived; but who can tell how
much of this difference may be due to the warmest-clad individuals having
been favoured and preserved during many generations, and how much to the
direct action of the severe climate? for it would appear that climate has
some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the other
hand, of different varieties being produced from the same species under the
same conditions. Such facts show how indirectly the conditions of life
must act. 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 very
little weight on the direct action of the conditions of life. Indirectly,
as already remarked, they seem to play an important part in affecting the
reproductive system, and in thus inducing variability; and natural
selection will then accumulate all profitable variations, however slight,
until they become plainly developed and appreciable by us.
Effects of Use and Disuse. -- From the facts alluded to in the first
chapter, I think there can be little doubt that use in our domestic animals
strengthens and enlarges certain parts, and disuse diminishes them; and
that such modifications are inherited. Under free nature, we can 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 have
structures which can be explained 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 several 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 condition as the domestic Aylesbury duck. As the
larger ground-feeding birds seldom take flight except to escape danger, I
believe that the nearly wingless condition of several birds, which now
inhabit or have lately inhabited several oceanic islands, tenanted by no
beast of prey, has been caused by disuse. The ostrich indeed inhabits
continents and is exposed to danger from which it cannot escape by flight,
but by kicking it can defend itself from enemies, as well as any of the
smaller quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural selection
increased in successive generations the size and weight of its body, 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 very 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. There is
not sufficient evidence to induce us to believe that mutilations are ever
inherited; and I should prefer explaining the entire absence of the
anterior tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors; for
as the tarsi are almost always lost in many dung-feeding beetles, they must
be lost early in life, and therefore cannot 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
discovered the remarkable fact that 200 beetles, out of the 550 species
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 genera
have all their species in this condition! Several facts, namely, that
beetles in many parts of the world are very 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 Dezertas than in Madeira itself;
and especially the extraordinary fact, so strongly insisted on by Mr.
Wollaston, of the almost entire absence of certain large groups of beetles,
elsewhere excessively numerous, and which groups have habits of life almost
necessitating frequent flight;--these several considerations have made me
believe that the wingless condition of so many Madeira beetles is mainly
due to the action of natural selection, but combined probably with disuse.
For during thousands of 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 will oftenest have been blown to
sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their 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 successfully 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 bad 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 up 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 tuco-tuco, 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 indispensable to animals with
subterranean 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 constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the foot-stalk for the eye 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, I
attribute their loss wholly to disuse. In one of the blind animals,
namely, the cave-rat, the eyes are of immense size; and Professor Silliman
thought that it regained, after living some days in the light, some slight
power of vision. In the same manner as in Madeira the wings of some of the
insects have been enlarged, and the wings of others have been reduced by
natural selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and to
have increased the size of the eyes; whereas with all the other inhabitants
of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common
view of the blind animals having been separately created for the American
and European caverns, close similarity in their organisation and affinities
might have been expected; but, as Schiodte and others have remarked, this
is not the case, and the cave-insects of the two continents are not more
closely allied than might have been anticipated from the general
resemblance of the other inhabitants of North America and Europe. On my
view we must suppose that American animals, having 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, '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.' 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
surrounding country. It would be most difficult to give any rational
explanation of the affinities of the blind cave-animals to the other
inhabitants of the two continents 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. Far from feeling any
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 inhabitants of these dark
abodes will probably have been exposed.
Acclimatisation. -- Habit is hereditary with plants, as in the period of
flowering, in the amount of rain requisite for seeds to germinate, in the
time of sleep, &c., and this leads me to say a few words on
acclimatisation. As it is extremely common for species of the same genus
to inhabit very hot and very cold countries, and as I believe that all the
species of the same genus have descended from a single parent, if this view
be correct, acclimatisation must be readily effected during long-continued
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 overrated. 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 warmer countries which here enjoy good health. We
have reason to believe that species in a state of nature are 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 the
adaptation be generally very close, we have evidence, in the case of some
few plants, of their becoming, to a certain extent, naturally habituated to
different temperatures, or becoming acclimatised: thus the pines and
rhododendrons, raised from seed collected by Dr. Hooker from trees growing
at different heights on the Himalaya, were found in this country to possess
different constitutional powers of resisting cold. Mr. Thwaites informs me
that he has observed similar facts in Ceylon, and analogous observations
have been made by Mr. H. C. Watson on European species of plants brought
from the Azores to England. In regard to animals, several authentic cases
could be given of species within historical times having largely extended
their range from warmer to cooler latitudes, and conversely; but we do not
positively know that these animals were strictly adapted to their native
climate, but in all ordinary cases we assume such to be the case; nor do we
know that they have subsequently become acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think 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 probable origin of some of our domestic animals from several
wild stocks: the blood, for instance, of a tropical and arctic wolf or
wild dog 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, living free under the cold climate of Faroe in the north and
of the Falklands in the south, and on many islands in the torrid zones.
Hence I am inclined to look at adaptation to any special climate as a
quality readily grafted on an innate wide flexibility of constitution,
which is common to most animals. On this view, the capacity of enduring
the most different climates by man himself and by his domestic animals, and
such facts as that former species of the elephant and rhinoceros were
capable of enduring 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 merely as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is due
to mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to both means combined, is a
very obscure question. That habit or custom has some influence I must
believe, both from analogy, and from the incessant advice given in
agricultural works, even in the ancient Encyclopaedias of China, to be very
cautious in transposing animals from one district to another; for 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, I can see no
reason to doubt that natural selection will continually tend to preserve
those individuals which are born with constitutions best adapted to their
native countries. In treatises on many kinds of cultivated plants, certain
varieties are said to withstand certain climates better than others: this
is very 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 constitutional
differences to habit. The case of the Jerusalem artichoke, which is never
propagated by seed, and of which consequently new varieties have not been
produced, has even been advanced--for it is now as tender as ever it
was--as proving that acclimatisation cannot be effected! The case, also,
of the kidney-bean has been often cited for a similar purpose, 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 proportion 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 even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has been
published how much more hardy some seedlings appeared to be than others.
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