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Charles Darwin: The origin of species
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CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlated
variation -- 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 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 function 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 domestication than under nature, and the greater
variability of species having wide ranges than of those with restricted
ranges, lead to the conclusion that variability is generally related to the
conditions of life to which each species has been exposed during several
successive generations. In the first chapter I attempted to show that
changed conditions act in two ways, directly on the whole organisation or
on certain parts alone, and indirectly through the reproductive system. In
all cases there are two factors, the nature of the organism, 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 indefinite
results. In the latter case the organisation 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
conditions, 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 co-adaptations 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 coloured than
those of the same species from further 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 coloured under a clear atmosphere, than
when living near the coast or on islands; and Wollaston is convinced that
residence near the sea affects the colours of insects. 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. These slightly varying
organisms are interesting in as far as they present characters analogous to
those possessed by the species which are confined 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 further north they live; 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 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 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 apparently 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 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
certain parts, and disuse diminished them; and that such modifications are
inherited. 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
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: 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 beasts 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 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
discovered 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 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 Desertas
than in Madeira itself; and especially the extraordinary fact, so strongly
insisted on by Mr. Wollaston, 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 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 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 necessary to animals having
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 aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Carniola and 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, 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
limestone caverns under a nearly similar climate; so that, in accordance
with the old view of the blind animals having been separately created for
the American and European caverns, very close similarity in their
organisation and affinities might have been expected. This is certainly
not the case if we look at the two whole faunas; 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 Europe and of North America." On my view we must suppose that
American 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 themselves 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 surrounding 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 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. 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.
ACCLIMATISATION.
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 acclimatisation. As it is extremely
common 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
descended from a single parent-form, acclimatisation 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 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 different countries which are
here perfectly 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 acclimatised: 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
authentic instances could be adduced of species having largely extended,
within historical times, their range from warmer to colder latitudes, and
conversely; 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 acclimatised 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
uncivilised 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
probable origin of some of our domestic animals from several wild stocks:
the blood, 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 and 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 a 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
rhinoceros 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 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 an
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
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
constitutions best adapted to any country which they inhabited. In
treatises on many kinds of cultivated plants, certain varieties are said to
withstand certain 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 constitutional 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 acclimatisation 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 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 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.
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 constitution
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 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, and no doubt wholly different classes of facts may be here
easily confounded together. We shall presently see that simple inheritance
often gives the false appearance of correlation. One of the most obvious
real cases is, that variations of structure arising in the young or larvae
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 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 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 the
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 and form of several of the most important viscera.
The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.
Hilaire has forcibly remarked that certain malconformations frequently, and
that others rarely, coexist 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 colour
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 colour 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
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. Everyone 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 abortion 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 frequently differ in their inner and outer flowers. It might
have been thought that the development of the ray-petals, by drawing
nourishment from the reproductive organs causes their abortion; but this
can hardly be the sole case, for in some Compositae the seeds of the outer
and inner florets differ, without any difference in the corolla. Possibly
these several differences may be connected with the different flow of
nutriment towards the central and external flowers. We know, at least,
that with irregular flowers those nearest to the axis are most subject to
peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that in many
pelargoniums the two upper petals in the central flower of the truss often
lose their patches of darker colour; and when this occurs, the adherent
nectary is quite aborted, the central flower thus becoming peloric or
regular. When the colour is absent from only one of the two upper petals,
the nectary is not quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel's idea that the
ray-florets serve to attract insects, whose agency is highly advantageous,
or necessary for the fertilisation of these plants, is highly probable; and
if so, natural selection may have come into play. But with respect to the
seeds, it seems impossible that their differences in shape, which are not
always correlated with any difference in the corolla, can be in any way
beneficial; yet in the Umbelliferae these differences are of such apparent
importance--the seeds being sometimes orthospermous in the exterior flowers
and coelospermous in the central flowers--that the elder De Candolle
founded his main divisions in the order on such characters. Hence
modifications of structure, viewed by systematists as of high value, may be
wholly due to the laws of variation and correlation, without being, as far
as we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures which are
common to whole groups of species, and which in truth are simply due to
inheritance; for an ancient progenitor may have acquired through natural
selection some one modification in structure, and, after thousands of
generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would naturally be thought to be in some necessary manner
correlated. Some other correlations are apparently due to the manner in
which natural selection can alone act. For instance, Alph. De Candolle has
remarked that winged seeds are never found in fruits which do not open; I
should explain this rule by the impossibility of seeds gradually becoming
winged through natural selection, unless the capsules were open; for in
this case alone could the seeds, which were a little better adapted to be
wafted by the wind, gain an advantage over others less well fitted for wide
dispersal.
COMPENSATION AND ECONOMY OF GROWTH.
The elder Geoffroy and Goethe propounded, at about the same time, their law
of compensation or balancement of growth; or, as Goethe expressed it, "in
order to spend on one side, nature is forced to economise on the other
side." I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it
rarely flows, at least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties of
the cabbage do not yield abundant and nutritious foliage and a copious
supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our
poultry, a large tuft of feathers on the head is generally accompanied by a
diminished comb, and a large beard by diminished wattles. With species in
a state of nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists, believe in
its truth. I will not, however, here give any instances, for I see hardly
any way of distinguishing between the effects, on the one hand, of a part
being largely developed through natural selection and another and adjoining
part being reduced by the same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the excess
of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more general
principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions
of life a structure, before useful, becomes less useful, its diminution
will be favoured, for it will profit the individual not to have its
nutriment wasted in building up a useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes,
and of which many other instances could be given: namely, that when a
cirripede is parasitic within another cirripede and is thus protected, it
loses more or less completely its own shell or carapace. This is the case
with the male Ibla, and in a truly extraordinary manner with the
Proteolepas: for the carapace in all other cirripedes consists of the
three highly important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic and
protected Proteolepas, the whole anterior part of the head is reduced to
the merest rudiment attached to the bases of the prehensile antennae. Now
the saving of a large and complex structure, when rendered superfluous,
would be a decided advantage to each successive individual of the species;
for in the struggle for life to which every animal is exposed, each would
have a better chance of supporting itself, by less nutriment being wasted.
Thus, as I believe, natural selection will tend in the long run to reduce
any part of the organisation, as soon as it becomes, through changed
habits, superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And conversely, that natural
selection may perfectly well succeed in largely developing an organ without
requiring as a necessary compensation the reduction of some adjoining part.
MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with
varieties and species, that when any part or organ is repeated many times
in the same individual (as the vertebrae in snakes, and the stamens in
polyandrous flowers) the number is variable; whereas the number of the same
part or organ, when it occurs in lesser numbers, is constant. The same
author as well as some botanists, have further remarked that multiple parts
are extremely liable to vary in structure. As "vegetative repetition," to
use Professor Owen's expression, is a sign of low organisation; the
foregoing statements accord with the common opinion of naturalists, that
beings which stand low in the scale of nature are more variable than those
which are higher. I presume that lowness here means that the several parts
of the organisation have been but little specialised for particular
functions; and as long as the same part has to perform diversified work, we
can perhaps see why it should remain variable, that is, why natural
selection should not have preserved or rejected each little deviation of
form so carefully as when the part has to serve for some one special
purpose. In the same way that a knife which has to cut all sorts of things
may be of almost any shape; whilst a tool for some particular purpose must
be of some particular shape. Natural selection, it should never be
forgotten, can act solely through and for the advantage of each being.
Rudimentary parts, as is generally admitted, are apt to be highly variable.
We shall have to recur to this subject; and I will here only add that their
variability seems to result from their uselessness, and consequently from
natural selection having had no power to check deviations in their
structure.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.
Several years ago I was much struck by a remark to the above effect made by
Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of the above proposition without giving the long array of facts which
I have collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowances for
them. It should be understood that the rule by no means applies to any
part, however unusually developed, unless it be unusually developed in one
species or in a few species in comparison with the same part in many
closely allied species. Thus, the wing of the bat is a most abnormal
structure in the class of mammals; but the rule would not apply here,
because the whole group of bats possesses wings; it would apply only if
some one species had wings developed in a remarkable manner in comparison
with the other species of the same genus. The rule applies very strongly
in the case of secondary sexual characters, when displayed in any unusual
manner. The term, secondary sexual characters, used by Hunter, relates to
characters which are attached to one sex, but are not directly connected
with the act of reproduction. The rule applies to males and females; but
more rarely to females, as they seldom offer remarkable secondary sexual
characters. The rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner--of which fact I think there
can be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes; I
particularly attended to Mr. Waterhouse's remark, whilst investigating this
order, and I am fully convinced that the rule almost always holds good. I
shall, in a future work, give a list of all the more remarkable cases. I
will here give only one, as it illustrates the rule in its largest
application. The opercular valves of sessile cirripedes (rock barnacles)
are, in every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species of one
genus, Pyrgoma, these valves present a marvellous amount of
diversification; the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the
individuals of the same species is so great that it is no exaggeration to
state that the varieties of the same species differ more from each other in
the characters derived from these important organs, than do the species
belonging to other distinct genera.
As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them; and
the rule certainly seems to hold good in this class. I cannot make out
that it applies to plants, and this would have seriously shaken my belief
in its truth, had not the great variability in plants made it particularly
difficult to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner in
a species, the fair presumption is that it is of high importance to that
species: nevertheless it is in this case eminently liable to variation.
Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species are descended from
some other species, and have been modified through natural selection, I
think we can obtain some light. First let me make some preliminary
remarks. If, in our domestic animals, any part or the whole animal be
neglected, and no selection be applied, that part (for instance, the comb
in the Dorking fowl) or the whole breed will cease to have a uniform
character: and the breed may be said to be degenerating. In rudimentary
organs, and in those which have been but little specialised for any
particular purpose, and perhaps in polymorphic groups, we see a nearly
parallel case; for in such cases natural selection either has not or cannot
come into full play, and thus the organisation is left in a fluctuating
condition. But what here more particularly concerns us is, that those
points in our domestic animals, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the individuals of the same breed of the pigeon; and
see what a prodigious amount of difference there is in the beak of
tumblers, in the beak and wattle of carriers, in the carriage and tail of
fantails, etc., these being the points now mainly attended to by English
fanciers. Even in the same sub-breed, as in that of the short-faced
tumbler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be a
constant struggle going on between, on the one hand, the tendency to
reversion to a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady selection to keep
the breed true. In the long run selection gains the day, and we do not
expect to fail so completely as to breed a bird as coarse as a common
tumbler pigeon from a good short-faced strain. But as long as selection is
rapidly going on, much variability in the parts undergoing modification may
always be expected.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species of
the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus. This period
will seldom be remote in any extreme degree, as species rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability, which
has continually been accumulated by natural selection for the benefit of
the species. But as the variability of the extraordinarily developed part
or organ has been so great and long-continued within a period not
excessively remote, we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organisation which
have remained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection on the
one hand, and the tendency to reversion and variability on the other hand,
will in the course of time cease; and that the most abnormally developed
organs may be made constant, I see no reason to doubt. Hence, when an
organ, however abnormal it may be, has been transmitted in approximately
the same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to our theory, for an immense
period in nearly the same state; and thus it has come not to be more
variable than any other structure. It is only in those cases in which the
modification has been comparatively recent and extraordinarily great that
we ought to find the GENERATIVE VARIABILITY, as it may be called, still
present in a high degree. For in this case the variability will seldom as
yet have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of those
tending to revert to a former and less modified condition.
SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.
The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant: if
in a large genus of plants some species had blue flowers and some had red,
the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely; but
if all the species had blue flowers, the colour would become a generic
character, and its variation would be a more unusual circumstance. I have
chosen this example because the explanation which most naturalists would
advance is not here applicable, namely, that specific characters are more
variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera. I
believe this explanation is partly, yet only indirectly, true; I shall,
however, have to return to this point in the chapter on Classification. It
would be almost superfluous to adduce evidence in support of the statement,
that ordinary specific characters are more variable than generic; but with
respect to important characters, I have repeatedly noticed in works on
natural history, that when an author remarks with surprise that some
important organ or part, which is generally very constant throughout a
large group of species, DIFFERS considerably in closely-allied species, it
is often VARIABLE in the individuals of the same species. And this fact
shows that a character, which is generally of generic value, when it sinks
in value and becomes only of specific value, often becomes variable, though
its physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy St. Hilaire
apparently entertains no doubt, that the more an organ normally differs in
the different species of the same group, the more subject it is to
anomalies in the individuals.
On the ordinary view of each species having been independently created, why
should that part of the structure, which differs from the same part in
other independently created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not
see that any explanation can be given. But on the view that species are
only strongly marked and fixed varieties, we might expect often to find
them still continuing to vary in those parts of their structure which have
varied within a moderately recent period, and which have thus come to
differ. Or to state the case in another manner: the points in which all
the species of a genus resemble each other, and in which they differ from
allied genera, are called generic characters; and these characters may be
attributed to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several distinct
species, fitted to more or less widely different habits, in exactly the
same manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come to differ
in any degree, or only in a slight degree, it is not probable that they
should vary at the present day. On the other hand, the points in which
species differ from other species of the same genus are called specific
characters; and as these specific characters have varied and come to differ
since the period when the species branched off from a common progenitor, it
is probable that they should still often be in some degree variable--at
least more variable than those parts of the organisation which have for a
very long period remained constant.
SECONDARY SEXUAL CHARACTERS VARIABLE.
I think it will be admitted by naturalists, without my entering on details,
that secondary sexual characters are highly variable. It will also be
admitted that species of the same group differ from each other more widely
in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the
males of gallinaceous birds, in which secondary sexual characters are
strongly displayed, with the amount of difference between the females. The
cause of the original variability of these characters is not manifest; but
we can see why they should not have been rendered as constant and uniform
as others, for they are accumulated by sexual selection, which is less
rigid in its action than ordinary selection, as it does not entail death,
but only gives fewer offspring to the less favoured males. Whatever the
cause may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the same group a
greater amount of difference in these than in other respects.
It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts of
the organisation in which the species of the same genus differ from each
other. Of this fact I will give in illustration the first two instances
which happen to stand on my list; and as the differences in these cases are
of a very unusual nature, the relation can hardly be accidental. The same
number of joints in the tarsi is a character common to very large groups of
beetles, but in the Engidae, as Westwood has remarked, the number varies
greatly and the number likewise differs in the two sexes of the same
species. Again in the fossorial hymenoptera, the neuration of the wings is
a character of the highest importance, because common to large groups; but
in certain genera the neuration differs in the different species, and
likewise in the two sexes of the same species. Sir J. Lubbock has recently
remarked, that several minute crustaceans offer excellent illustrations of
this law. "In Pontella, for instance, the sexual characters are afforded
mainly by the anterior antennae and by the fifth pair of legs: the
specific differences also are principally given by these organs." This
relation has a clear meaning on my view: I look at all the species of the
same genus as having as certainly descended from the same progenitor, as
have the two sexes of any one species. Consequently, whatever part of the
structure of the common progenitor, or of its early descendants, became
variable; variations of this part would, it is highly probable, be taken
advantage of by natural and sexual selection, in order to fit the several
places in the economy of nature, and likewise to fit the two sexes of the
same species to each other, or to fit the males to struggle with other
males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species; that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in its
congeners; and the slight degree of variability in a part, however
extraordinarily it may be developed, if it be common to a whole group of
species; that the great variability of secondary sexual characters and
their great difference in closely allied species; that secondary sexual and
ordinary specific differences are generally displayed in the same parts of
the organisation, are all principles closely connected together. All being
mainly due to the species of the same group being the descendants of a
common progenitor, from whom they have inherited much in common, to parts
which have recently and largely varied being more likely still to go on
varying than parts which have long been inherited and have not varied, to
natural selection having more or less completely, according to the lapse of
time, overmastered the tendency to reversion and to further variability, to
sexual selection being less rigid than ordinary selection, and to
variations in the same parts having been accumulated by natural and sexual
selection, and thus having been adapted for secondary sexual, and for
ordinary purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF ONE
SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR REVERTS
TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of the pigeon, in countries
widely apart, present sub-varieties with reversed feathers on the head, and
with feathers on the feet, characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or as commonly called roots, of the Swedish turnip and ruta-baga,
plants which several botanists rank as varieties produced by cultivation
from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a third
may be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to attribute
this similarity in the enlarged stems of these three plants, not to the
vera causa of community of descent, and a consequent tendency to vary in a
like manner, but to three separate yet closely related acts of creation.
Many similar cases of analogous variation have been observed by Naudin in
the great gourd family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have lately been
discussed with much ability by Mr. Walsh, who has grouped them under his
law of equable variability.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may, I think, confidently
come to this conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on
the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after
having been lost for many, probably for hundreds of generations. But when
a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character to
the foreign breed--some say, for a dozen or even a score of generations.
After twelve generations, the proportion of blood, to use a common
expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is
generally believed that a tendency to reversion is retained by this remnant
of foreign blood. In a breed which has not been crossed, but in which BOTH
parents have lost some character which their progenitor possessed, the
tendency, whether strong or weak, to reproduce the lost character might, as
was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a character which
has been lost in a breed, reappears after a great number of generations,
the most probable hypothesis is, not that one individual suddenly takes
after an ancestor removed by some hundred generations, but that in each
successive generation the character in question has been lying latent, and
at last, under unknown favourable conditions, is developed. With the
barb-pigeon, for instance, which very rarely produces a blue bird, it is
probable that there is a latent tendency in each generation to produce blue
plumage. The abstract improbability of such a tendency being transmitted
through a vast number of generations, is not greater than that of quite
useless or rudimentary organs being similarly transmitted. A mere tendency
to produce a rudiment is indeed sometimes thus inherited.
As all the species of the same genus are supposed to be descended from a
common progenitor, it might be expected that they would occasionally vary
in an analogous manner; so that the varieties of two or more species would
resemble each other, or that a variety of one species would resemble in
certain characters another and distinct species, this other species being,
according to our view, only a well-marked and permanent variety. But
characters exclusively due to analogous variation would probably be of an
unimportant nature, for the preservation of all functionally important
characters will have been determined through natural selection, in
accordance with the different habits of the species. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to long-lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between reversionary
and analogous characters. If, for instance, we did not know that the
parent rock-pigeon was not feather-footed or turn-crowned, we could not
have told, whether such characters in our domestic breeds were reversions
or only analogous variations; but we might have inferred that the blue
colour was a case of reversion from the number of the markings, which are
correlated with this tint, and which would not probably have all appeared
together from simple variation. More especially we might have inferred
this from the blue colour and the several marks so often appearing when
differently coloured breeds are crossed. Hence, although under nature it
must generally be left doubtful, what cases are reversions to formerly
existing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a species
assuming characters which are already present in other members of the same
group. And this undoubtedly is the case.
The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate between
two other forms, which themselves can only doubtfully be ranked as species;
and this shows, unless all these closely allied forms be considered as
independently created species, that they have in varying assumed some of
the characters of the others. But the best evidence of analogous
variations is afforded by parts or organs which are generally constant in
character, but which occasionally vary so as to resemble, in some degree,
the same part or organ in an allied species. I have collected a long list
of such cases; but here, as before, I lie under the great disadvantage of
not being able to give them. I can only repeat that such cases certainly
occur, and seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as affecting
any important character, but from occurring in several species of the same
genus, partly under domestication and partly under nature. It is a case
almost certainly of reversion. The ass sometimes has very distinct
transverse bars on its legs, like those on the legs of a zebra. It has
been asserted that these are plainest in the foal, and from inquiries which
I have made, I believe this to be true. The stripe on the shoulder is
sometimes double, and is very variable in length and outline. A white ass,
but NOT an albino, has been described without either spinal or shoulder
stripe; and these stripes are sometimes very obscure, or actually quite
lost, in dark-coloured asses. The koulan of Pallas is said to have been
seen with a double shoulder-stripe. Mr. Blyth has seen a specimen of the
hemionus with a distinct shoulder-stripe, though it properly has none; and
I have been informed by Colonel Poole that foals of this species are
generally striped on the legs and faintly on the shoulder. The quagga,
though so plainly barred like a zebra over the body, is without bars on the
legs; but Dr. Gray has figured one specimen with very distinct zebra-like
bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut; a faint shoulder-stripe may sometimes be seen in
duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes. I have myself seen a dun
Devonshire pony, and a small dun Welsh pony has been carefully described to
me, both with THREE parallel stripes on each shoulder.
In the northwest part of India the Kattywar breed of horses is so generally
striped, that, as I hear from Colonel Poole, who examined this breed for
the Indian Government, a horse without stripes is not considered as purely
bred. The spine is always striped; the legs are generally barred; and the
shoulder-stripe, which is sometimes double and sometimes treble, is common;
the side of the face, moreover, is sometimes striped. The stripes are
often plainest in the foal; and sometimes quite disappear in old horses.
Colonel Poole has seen both gray and bay Kattywar horses striped when first
foaled. I have also reason to suspect, from information given me by Mr.
W.W. Edwards, that with the English race-horse the spinal stripe is much
commoner in the foal than in the full-grown animal. I have myself recently
bred a foal from a bay mare (offspring of a Turkoman horse and a Flemish
mare) by a bay English race-horse. This foal, when a week old, was marked
on its hinder quarters and on its forehead with numerous very narrow, dark,
zebra-like bars, and its legs were feebly striped. All the stripes soon
disappeared completely. Without here entering on further details I may
state that I have collected cases of leg and shoulder stripes in horses of
very different breeds in various countries from Britain to Eastern China;
and from Norway in the north to the Malay Archipelago in the south. In all
parts of the world these stripes occur far oftenest in duns and mouse-duns;
by the term dun a large range of colour is included, from one between brown
and black to a close approach to cream colour.
I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse are descended from several
aboriginal species, one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But this view may be safely rejected, for it is highly improbable
that the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world, should
have all have been crossed with one supposed aboriginal stock.
Now let us turn to the effects of crossing the several species of the horse
genus. Rollin asserts that the common mule from the ass and horse is
particularly apt to have bars on its legs; according to Mr. Gosse, in
certain parts of the United States, about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any one
might have thought that it was a hybrid zebra; and Mr. W.C. Martin, in his
excellent treatise on the horse, has given a figure of a similar mule. In
four coloured drawings, which I have seen, of hybrids between the ass and
zebra, the legs were much more plainly barred than the rest of the body;
and in one of them there was a double shoulder-stripe. In Lord Morton's
famous hybrid, from a chestnut mare and male quagga, the hybrid and even
the pure offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is even
the pure quagga. Lastly, and this is another most remarkable case, a
hybrid has been figured by Dr. Gray (and he informs me that he knows of a
second case) from the ass and the hemionus; and this hybrid, though the ass
only occasionally has stripes on his legs and the hemionus has none and has
not even a shoulder-stripe, nevertheless had all four legs barred, and had
three short shoulder-stripes, like those on the dun Devonshire and Welsh
ponies, and even had some zebra-like stripes on the sides of its face.
With respect to this last fact, I was so convinced that not even a stripe
of colour appears from what is commonly called chance, that I was led
solely from the occurrence of the face-stripes on this hybrid from the ass
and hemionus to ask Colonel Poole whether such face-stripes ever occurred
in the eminently striped Kattywar breed of horses, and was, as we have
seen, answered in the affirmative.
What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on the
legs like a zebra, or striped on the shoulders like an ass. In the horse
we see this tendency strong whenever a dun tint appears--a tint which
approaches to that of the general colouring of the other species of the
genus. The appearance of the stripes is not accompanied by any change of
form, or by any other new character. We see this tendency to become
striped most strongly displayed in hybrids from between several of the most
distinct species. Now observe the case of the several breeds of pigeons:
they are descended from a pigeon (including two or three sub-species or
geographical races) of a bluish colour, with certain bars and other marks;
and when any breed assumes by simple variation a bluish tint, these bars
and other marks invariably reappear; but without any other change of form
or character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks to
reappear in the mongrels. I have stated that the most probable hypothesis
to account for the reappearance of very ancient characters, is--that there
is a TENDENCY in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse genus
the stripes are either plainer or appear more commonly in the young than in
the old. Call the breeds of pigeons, some of which have bred true for
centuries, species; and how exactly parallel is the case with that of the
species of the horse genus! For myself, I venture confidently to look back
thousands on thousands of generations, and I see an animal striped like a
zebra, but perhaps otherwise very differently constructed, the common
parent of our domestic horse (whether or not it be descended from one or
more wild stocks) of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I
presume, assert that each species has been created with a tendency to vary,
both under nature and under domestication, in this particular manner, so as
often to become striped like the other species of the genus; and that each
has been created with a strong tendency, when crossed with species
inhabiting distant quarters of the world, to produce hybrids resembling in
their stripes, not their own parents, but other species of the genus. To
admit this view is, as it seems to me, to reject a real for an unreal, or
at least for an unknown cause. It makes the works of God a mere mockery
and deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species
of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and
these may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional
peculiarities, and use in strengthening, and disuse in weakening and
diminishing organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining parts;
and every part of the structure which can be saved without detriment will
be saved. Changes of structure at an early age may affect parts
subsequently developed; and many cases of correlated variation, the nature
of which we are unable to understand, undoubtedly occur. Multiple parts
are variable in number and in structure, perhaps arising from such parts
not having been closely specialised for any particular function, so that
their modifications have not been closely checked by natural selection. It
follows probably from this same cause, that organic beings low in the scale
are more variable than those standing higher in the scale, and which have
their whole organisation more specialised. Rudimentary organs, from being
useless, are not regulated by natural selection, and hence are variable.
Specific characters--that is, the characters which have come to differ
since the several species of the same genus branched off from a common
parent--are more variable than generic characters, or those which have long
been inherited, and have not differed within this same period. In these
remarks we have referred to special parts or organs being still variable,
because they have recently varied and thus come to differ; but we have also
seen in the second chapter that the same principle applies to the whole
individual; for in a district where many species of a genus are found--that
is, where there has been much former variation and differentiation, or
where the manufactory of new specific forms has been actively at work--in
that district and among these species, we now find, on an average, most
varieties. Secondary sexual characters are highly variable, and such
characters differ much in the species of the same group. Variability in
the same parts of the organisation has generally been taken advantage of in
giving secondary sexual differences to the two sexes of the same species,
and specific differences to the several species of the same genus. Any
part or organ developed to an extraordinary size or in an extraordinary
manner, in comparison with the same part or organ in the allied species,
must have gone through an extraordinary amount of modification since the
genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases
not as yet have had time to overcome the tendency to further variability
and to reversion to a less modified state. But when a species with an
extraordinarily developed organ has become the parent of many modified
descendants--which on our view must be a very slow process, requiring a
long lapse of time--in this case, natural selection has succeeded in giving
a fixed character to the organ, in however extraordinary a manner it may
have been developed. Species inheriting nearly the same constitution from
a common parent, and exposed to similar influences, naturally tend to
present analogous variations, or these same species may occasionally revert
to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous
variation, such modifications will add to the beautiful and harmonious
diversity of nature.
Whatever the cause may be of each slight difference between the offspring
and their parents--and a cause for each must exist--we have reason to
believe that it is the steady accumulation of beneficial differences which
has given rise to all the more important modifications of structure in
relation to the habits of each species.
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