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.
On the whole, I think we may conclude that habit, use, and disuse, have, in
some cases, played a considerable part in the modification of the
constitution, and of the structure of various organs; but that the effects
of use and disuse have often been largely combined with, and sometimes
overmastered by, the natural selection of innate differences.
Correlation of Growth. -- I mean by this expression that the whole
organisation 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 become modified. This is a very important
subject, most imperfectly understood. The most obvious case is, that
modifications accumulated solely for the good of the young or larva, will,
it may safely be concluded, affect the structure of the adult; in the same
manner as any malconformation affecting the early embryo, seriously affects
the whole organisation of the adult. The several parts of the body which
are homologous, and which, at an early embryonic period, are alike, seem
liable to vary in an allied 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 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 natural 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 the union of the petals
of the corolla into a tube. Hard parts seem to affect the form of
adjoining soft parts; it is believed by some authors that the diversity in
the shape of the pelvis in birds 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, according to Schlegel, the shape of the body and the manner of
swallowing determine the position of several of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure. M.
Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist, without
our being able to assign an
y reason. What can be more singular than the
relation between blue eyes and deafness in cats, and the tortoise-shell
colour with the female sex; the feathered feet and skin between the outer
toes in pigeons, and the presence of more or less down on the young birds
when first hatched, with the future colour of their plumage; or, again, the
relation between the hair and teeth in the naked Turkish dog, though here
probably homology comes into play? With respect to this latter case of
correlation, I think it can hardly be accidental, that if we pick out the
two orders of mammalia which are most abnormal in their dermal coverings,
viz. Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, &c.),
that these are likewise the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, 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 knows the difference in the ray and central florets of, for
instance, the daisy, and this difference is often accompanied with the
abortion of parts of the flower. But, in some Compositous plants, the
seeds also differ in shape and sculpture; and even the ovary itself, with
its accessory parts, differs, as has been described by Cassini. These
differences have been attributed by some authors to pressure, and the shape
of the seeds in the ray-florets in some Compositae countenances this idea;
but, in the case of the corolla of the Umbelliferae, it is by no means, as
Dr. Hooker informs me, in species with the densest heads that the inner and
outer flowers most frequently differ. It might have been thought that the
development of the ray-petals by drawing nourishment from certain other
parts of the flower had caused their abortion; but in some Compositae there
is a difference in the seeds of the outer and inner florets without any
difference in the corolla. Possibly, these several differences may be
connected with some difference in the flow of nutriment towards the central
and external flowers: we know, at least, that in irregular flowers, those
nearest to the axis are oftenest subject to peloria, and become regular. I
may add, as an instance of this, and of a striking case of correlation,
that I have recently observed in some garden pelargoniums, that the central
flower of the truss often loses the patches of darker colour in the two
upper petals; and that when this occurs, the adherent nectary is quite
aborted; when the colour is absent from only one of the two upper petals,
the nectary is only much shortened.
With respect to the difference in the corolla of the central and exterior
flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's
idea that the ray-florets serve to attract insects, whose agency is highly
advantageous in the fertilisation of plants of these two orders, is so
far-fetched, as it may at first appear: and if it be advantageous, natural
selection may have come into play. But in regard to the differences both
in the internal and external structure of the seeds, which are not always
correlated with any differences in the flowers, it seems impossible that
they can be in any way advantageous to the plant: yet in the Umbelliferae
these differences are of such apparent importance--the seeds being in some
cases, according to Tausch, orthospermous in the exterior flowers and
coelospermous in the central flowers,--that the elder De Candolle founded
his main divisions of the order on analogous differences. Hence we see
that modifications of structure, viewed by systematists as of high value,
may be wholly due to unknown laws of correlated growth, and without being,
as far as we can see, of the slightest service to the species.