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, &c., 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, Schiödte 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
Schiödte 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 Schiödte’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 antennæ 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,
&c., 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 Encyclopædias
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 larvæ 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, &c.), 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 Compositæ countenances this idea;
but with the Umbelliferæ 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 Compositæ 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
Umbelliferæ these differences are of such apparent importance—the seeds
being sometimes orthospermous in the exterior flowers and cœlospermous
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 antennæ. 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 vertebræ 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,
&c., 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 Engidæ, 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 antennæ 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, &c., 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.