- Effects
of external conditions
- Use
and disuse, combined with
natural selection; organs of
flight
and of vision
- Acclimatisation
- Correlation
of growth
- Compensation
and economy of growth
- False
correlations
- Multiple,
rudimentary, and
lowly organised structures
variable
- Parts
developed in
an unusual manner
are highly
variable: specific
character more
variable than
generic: secondary
sexual
characters variable
- Species
of the same genus vary in
an analogous
manner
- Reversions
to long-lost
characters
- Summary
I HAVE hitherto sometimes spoken
as if the variations so common
and multiform in organic beings
under domestication, and in a
lesser degree in those in a state
of nature had been due to chance.
This, of course, is a wholly
incorrect expression, but it
serves to acknowledge plainly
our ignorance of the cause of
each particular variation. Some
authors believe it to be as much
the function of the reproductive
system to produce individual
differences, or very slight deviations
of structure, as to make the
child like its parents. But the
much greater variability, as
well as the greater frequency
of monstrosities, under domestication
or cultivation, than under nature,
leads me to believe that deviations
of structure are in some way
due to the nature of the conditions
of life, to which the parents
and their more remote ancestors
have been exposed during several
generations. I have remarked
in the first chapter but a long
catalogue of facts which cannot
be here given would be necessary
to show the truth of the remark
that the reproductive system
is eminently susceptible to changes
in the conditions of life; and
to this system being functionally
disturbed in the parents, I chiefly
attribute the varying or plastic
condition of the offspring. The
male and female sexual elements
seem to be affected before that
union takes place which is to
form a new being. In the case
of 'sporting' plants, the bud,
which in its earliest condition
does not apparently differ essentially
from an ovule, is alone affected.
But why, because the reproductive
system is disturbed, this or
that part should vary more or
less, we are profoundly ignorant.
Nevertheless, we can here and
there dimly catch a faint ray
of light, and we may feel sure
that there must be some cause
for each deviation of structure,
however slight.
How much direct
effect difference of climate,
food, &c., produces
on any being is extremely doubtful.
My impression is, that the effect
is extremely small in the case
of animals, but perhaps rather
more in that of plants. We may,
at least, safely conclude that
such influences cannot have produced
the many striking and complex
co-adaptations of structure between
one organic being and another,
which we see everywhere throughout
nature. Some little influence
may be attributed to climate,
food, &c.: thus, E. Forbes
speaks confidently that shells
at their southern limit, and
when living in shallow water,
are more brightly coloured than
those of the same species further
north or from greater depths.
Gould believes that birds of
the same species are more brightly
coloured under a clear atmosphere,
than when living on islands or
near the coast. So with insects,
Wollaston is convinced that residence
near the sea affects their colours.
Moquin-Tandon gives a list of
plants which when growing near
the sea-shore have their leaves
in some degree fleshy, though
not elsewhere fleshy. Several
other such cases could be given.
The fact of varieties of one
species, when they range into
the zone of habitation of other
species, often acquiring in a
very slight degree some of the
characters of such species, accords
with our view that species of
all kinds are only well-marked
and permanent varieties. Thus
the species of shells which are
confined to tropical and shallow
seas are generally brighter-coloured
than those confined to cold and
deeper seas. The birds which
are confined to continents are,
according to Mr Gould, brighter-coloured
than those of islands. The insect-species
confined to sea-coasts, as every
collector knows, are often brassy
or lurid. Plants which live exclusively
on the sea-side are very apt
to have fleshy leaves. He who
believes in the creation of each
species, will have to say that
this shell, for instance, was
created with bright colours for
a warm sea; but that this other
shell became bright-coloured
by variation when it ranged into
warmer or shallower waters.
When a variation is of the
slightest use to a being, we
cannot tell how much of it to
attribute to the accumulative
action of natural selection,
and how much to the conditions
of life. Thus, it is well known
to furriers that animals of the
same species have thicker and
better fur the more severe the
climate is under which they have
lived; but who can tell how much
of this difference may be due
to the warmest-clad individuals
having been favoured and preserved
during many generations, and
how much to the direct action
of the severe climate? for it
would appear that climate has
some direct action on the hair
of our domestic quadrupeds.
Instances could be given of
the same variety being produced
under conditions of life as different
as can well be conceived; and,
on the other hand, of different
varieties being produced from
the same species under the same
conditions. Such facts show how
indirectly the conditions of
life must act. Again, innumerable
instances are known to every
naturalist of species keeping
true, or not varying at all,
although living under the most
opposite climates. Such considerations
as these incline me to lay very
little weight on the direct action
of the conditions of life. Indirectly,
as already remarked, they seem
to play an important part in
affecting the reproductive system,
and in thus inducing variability;
and natural selection will then
accumulate all profitable variations,
however slight, until they become
plainly developed and appreciable
by us.
Effects of Use and Disuse
From the facts alluded to in
the first chapter, I think there
can be little doubt that use
in our domestic animals strengthens
and enlarges certain parts, and
disuse diminishes them; and that
such modifications are inherited.
Under free nature, we can have
no standard of comparison, by
which to judge of the effects
of long-continued use or disuse,
for we know not the parent-forms;
but many animals have structures
which can be explained by the
effects of disuse. As Professor
Owen has remarked, there is no
greater anomaly in nature than
a bird that cannot fly; yet there
are several in this state. The
logger-headed duck of South America
can only flap along the surface
of the water, and has its wings
in nearly the same condition
as the domestic Aylesbury duck.
As the larger ground-feeding
birds seldom take flight except
to escape danger, I believe that
the nearly wingless condition
of several birds, which now inhabit
or have lately inhabited several
oceanic islands, tenanted by
no beast of prey, has been caused
by disuse. The ostrich indeed
inhabits continents and is exposed
to danger from which it cannot
escape by flight, but by kicking
it can defend itself from enemies,
as well as any of the smaller
quadrupeds. We may imagine that
the early progenitor of the ostrich
had habits like those of a bustard,
and that as natural selection
increased in successive generations
the size and weight of its body,
its legs were used more, and
its wings less, until they became
incapable of flight.
Kirby has remarked (and I have
observed the same fact) that
the anterior tarsi, or feet,
of many male dung-feeding beetles
are very often broken off; he
examined seventeen specimens
in his own collection, and not
one had even a relic left. In
the Onites apelles the tarsi
are so habitually lost, that
the insect has been described
as not having them. In some other
genera they are present, but
in a rudimentary condition. In
the Ateuchus or sacred beetle
of the Egyptians, they are totally
deficient. There is not sufficient
evidence to induce us to believe
that mutilations are ever inherited;
and I should prefer explaining
the entire absence of the anterior
tarsi in Ateuchus, and their
rudimentary condition in some
other genera, by the long-continued
effects of disuse in their progenitors;
for as the tarsi are almost always
lost in many dung-feeding beetles,
they must be lost early in life,
and therefore cannot be much
used by these insects.
In some cases we might easily
put down to disuse modifications
of structure which are wholly,
or mainly, due to natural selection.
Mr. Wollaston has discovered
the remarkable fact that 200
beetles, out of the 550 species
inhabiting Madeira, are so far
deficient in wings that they
cannot fly; and that of the twenty-nine
endemic genera, no less than
twenty-three genera have all
their species in this condition!
Several facts, namely, that beetles
in many parts of the world are
very frequently blown to sea
and perish; that the beetles
in Madeira, as observed by Mr
Wollaston, lie much concealed,
until the wind lulls and the
sun shines; that the proportion
of wingless beetles is larger
on the exposed Dezertas than
in Madeira itself; and especially
the extraordinary fact, so strongly
insisted on by Mr. Wollaston,
of the almost entire absence
of certain large groups of beetles,
elsewhere excessively numerous,
and which groups have habits
of life almost necessitating
frequent flight; these several
considerations have made me believe
that the wingless condition of
so many Madeira beetles is mainly
due to the action of natural
selection, but combined probably
with disuse. For during thousands
of successive generations each
individual beetle which flew
least, either from its wings
having been ever so little less
perfectly developed or from indolent
habit, will have had the best
chance of surviving from not
being blown out to sea; and,
on the other hand, those beetles
which most readily took to flight
will oftenest have been blown
to sea and thus have been destroyed.
The insects in Madeira which
are not ground-feeders, and which,
as the flower-feeding coleoptera
and lepidoptera, must habitually
use their wings to gain their
subsistence, have, as Mr. Wollaston
suspects, their wings not at
all reduced, but even enlarged.
This is quite compatible with
the action of natural selection.
For when a new insect first arrived
on the island, the tendency of
natural selection to enlarge
or to reduce the wings, would
depend on whether a greater number
of individuals were saved by
successfully battling with the
winds, or by giving up the attempt
and rarely or never flying. As
with mariners ship-wrecked near
a coast, it would have been better
for the good swimmers if they
had been able to swim still further,
whereas it would have been better
for the bad swimmers if they
had not been able to swim at
all and had stuck to the wreck.
The eyes of moles and of some
burrowing rodents are rudimentary
in size, and in some cases are
quite covered up by skin and
fur. This state of the eyes is
probably due to gradual reduction
from disuse, but aided perhaps
by natural selection. In South
America, a burrowing rodent,
the tuco-tuco, or Ctenomys, is
even more subterranean in its
habits than the mole; and I was
assured by a Spaniard, who had
often caught them, that they
were frequently blind; one which
I kept alive was certainly in
this condition, the cause, as
appeared on dissection, having
been inflammation of the nictitating
membrane. As frequent inflammation
of the eyes must be injurious
to any animal, and as eyes are
certainly not indispensable to
animals with subterranean habits,
a reduction in their size with
the adhesion of the eyelids and
growth of fur over them, might
in such case be an advantage;
and if so, natural selection
would constantly aid the effects
of disuse.
It is well known that several
animals, belonging to the most
different classes, which inhabit
the caves of Styria and of Kentucky,
are blind. In some of the crabs
the foot-stalk for the eye remains,
though the eye is gone; the stand
for the telescope is there, though
the telescope with its glasses
has been lost. As it is difficult
to imagine that eyes, though
useless, could be in any way
injurious to animals living in
darkness, I attribute their loss
wholly to disuse. In one of the
blind animals, namely, the cave-rat,
the eyes are of immense size;
and Professor Silliman thought
that it regained, after living
some days in the light, some
slight power of vision. In the
same manner as in Madeira the
wings of some of the insects
have been enlarged, and the wings
of others have been reduced by
natural selection aided by use
and disuse, so in the case of
the cave-rat natural selection
seems to have struggled with
the loss of light and to have
increased the size of the eyes;
whereas with all the other inhabitants
of the caves, disuse by itself
seems to have done its work.
It is difficult
to imagine conditions of life
more similar
than deep limestone caverns under
a nearly similar climate; so
that on the common view of the
blind animals having been separately
created for the American and
European caverns, close similarity
in their organisation and affinities
might have been expected; but,
as Schiödte and others have
remarked, this is not the case,
and the cave-insects of the two
continents are not more closely
allied than might have been anticipated
from the general resemblance
of the other inhabitants of North
America and Europe. On my view
we must suppose that American
animals, having ordinary powers
of vision, slowly migrated by
successive generations from the
outer world into the deeper and
deeper recesses of the Kentucky
caves, as did European animals
into the caves of Europe. We
have some evidence of this gradation
of habit; for, as Schiödte
remarks, 'animals not far remote
from ordinary forms, prepare
the transition from light to
darkness. Next follow those that
are constructed for twilight;
and, last of all, those destined
for total darkness.' By the time
that an animal had reached, after
numberless generations, the deepest
recesses, disuse will on this
view have more or less perfectly
obliterated its eyes, and natural
selection will often have effected
other changes, such as an increase
in the length of the antennae
or palpi, as a compensation for
blindness. Notwithstanding such
modifications, we might expect
still to see in the cave-animals
of America, affinities to the
other inhabitants of that continent,
and in those of Europe, to the
inhabitants of the European continent.
And this is the case with some
of the American cave-animals,
as I hear from Professor Dana;
and some of the European cave-insects
are very closely allied to those
of the surrounding country. It
would be most difficult to give
any rational explanation of the
affinities of the blind cave-animals
to the other inhabitants of the
two continents on the ordinary
view of their independent creation.
That several of the inhabitants
of the caves of the Old and New
Worlds should be closely related,
we might expect from the well-known
relationship of most of their
other productions. Far from feeling
any surprise that some of the
cave-animals should be very anomalous,
as Agassiz has remarked in regard
to the blind fish, the Amblyopsis,
and as is the case with the blind
Proteus with reference to the
reptiles of Europe, I am only
surprised that more wrecks of
ancient life have not been preserved,
owing to the less severe competition
to which the inhabitants of these
dark abodes will probably have
been exposed.
Acclimatisation
Habit is hereditary
with plants, as in the period
of flowering,
in the amount of rain requisite
for seeds to germinate, in the
time of sleep, &c., and this
leads me to say a few words on
acclimatisation. As it is extremely
common for species of the same
genus to inhabit very hot and
very cold countries, and as I
believe that all the species
of the same genus have descended
from a single parent, if this
view be correct, acclimatisation
must be readily effected during
long-continued descent. It is
notorious that each species is
adapted to the climate of its
own home: species from an arctic
or even from a temperate region
cannot endure a tropical climate,
or conversely. So again, many
succulent plants cannot endure
a damp climate. But the degree
of adaptation of species to the
climates under which they live
is often overrated. We may infer
this from our frequent inability
to predict whether or not an
imported plant will endure our
climate, and from the number
of plants and animals brought
from warmer countries which here
enjoy good health. We have reason
to believe that species in a
state of nature are limited in
their ranges by the competition
of other organic beings quite
as much as, or more than, by
adaptation to particular climates.
But whether or not the adaptation
be generally very close, we have
evidence, in the case of some
few plants, of their becoming,
to a certain extent, naturally
habituated to different temperatures,
or becoming acclimatised: thus
the pines and rhododendrons,
raised from seed collected by
Dr Hooker from trees growing
at different heights on the Himalaya
were found in this country to
possess different constitutional
powers of resisting cold. Mr
Thwaites informs me that he has
observed similar facts in Ceylon,
and analogous observations have
been made by Mr H. C. Watson
on European species of plants
brought from the Azores to England.
In regard to animals, several
authentic cases could be given
of species within historical
times having largely extended
their range from warmer to cooler
latitudes, and conversely; but
we do not positively know that
these animals were strictly adapted
to their native climate, but
in all ordinary cases we assume
such to be the case; nor do we
know that they have subsequently
become acclimatised to their
new homes.
As I believe that our domestic
animals were originally chosen
by uncivilised man because they
were useful and bred readily
under confinement, and not because
they were subsequently found
capable of far-extended transportation,
I think the common and extraordinary
capacity in our domestic animals
of not only withstanding the
most different climates but of
being perfectly fertile (a far
severer test) under them, may
be used as an argument that a
large proportion of other animals,
now in a state of nature, could
easily be brought to bear widely
different climates. We must not,
however, push the foregoing argument
too far, on account of the probable
origin of some of our domestic
animals from several wild stocks:
the blood, for instance, of a
tropical and arctic wolf or wild
dog may perhaps be mingled in
our domestic breeds. The rat
and mouse cannot be considered
as domestic animals, but they
have been transported by man
to many parts of the world, and
now have a far wider range than
any other rodent, living free
under the cold climate of Faroe
in the north and of the Falklands
in the south, and on many islands
in the torrid zones. Hence I
am inclined to look at adaptation
to any special climate as a quality
readily grafted on an innate
wide flexibility of constitution,
which is common to most animals.
On this view, the capacity of
enduring the most different climates
by man himself and by his domestic
animals, and such facts as that
former species of the elephant
and rhinoceros were capable of
enduring a glacial climate, whereas
the living species are now all
tropical or sub-tropical in their
habits, ought not to be looked
at as anomalies, but merely as
examples of a very common flexibility
of constitution, brought, under
peculiar circumstances, into
play.
How much of the acclimatisation
of species to any peculiar climate
is due to mere habit, and how
much to the natural selection
of varieties having different
innate constitutions, and how
much to means combined, is a
very obscure question. That habit
or custom has some influence
I must believe, both from analogy,
and from the incessant advice
given in agricultural works,
even in the ancient Encyclopaedias
of China, to be very cautious
in transposing animals from one
district to another; for it is
not likely that man should have
succeeded in selecting so many
breeds and sub-breeds with constitutions
specially fitted for their own
districts: the result must, I
think, be due to habit. On the
other hand, I can see no reason
to doubt that natural selection
will continually tend to preserve
those individuals which are born
with constitutions best adapted
to their native countries. In
treatises on many kinds of cultivated
plants, certain varieties are
said to withstand certain climates
better than others: this is very
strikingly shown in works on
fruit trees published in the
United States, in which certain
varieties are habitually recommended
for the northern, and others
for the southern States; and
as most of these varieties are
of recent origin, they cannot
owe their constitutional differences
to habit. The case of the Jerusalem
artichoke, which is never propagated
by seed, and of which consequently
new varieties have not been produced,
has even been advanced for it
is now as tender as ever it was
-- as proving that acclimatisation
cannot be effected! The case,
also, of the kidney-bean has
been often cited for a similar
purpose, and with much greater
weight; but until some one will
sow, during a score of generations,
his kidney-beans so early that
a very large proportion are destroyed
by frost, and then collect seed
from the few survivors, with
care to prevent accidental crosses,
and then again get seed from
these seedlings, with the same
precautions, the experiment cannot
be said to have been even tried.
Nor let it be supposed that no
differences in the constitution
of seedling kidney-beans ever
appear, for an account has been
published how much more hardy
some seedlings appeared to be
than others.
On the whole, I think we may
conclude that habit, use, and
disuse, have, in some cases,
played a considerable part in
the modification of the constitution,
and of the structure of various
organs; but that the effects
of use and disuse have often
been largely combined with, and
sometimes overmastered by, the
natural selection of innate differences.
Correlation of Growth
I mean by this expression that
the whole organisation is so
tied together during its growth
and development, that when slight
variations in any one part occur,
and are accumulated through natural
selection, other parts become
modified. This is a very important
subject, most imperfectly understood.
The most obvious case is, that
modifications accumulated solely
for the good of the young or
larva, will, it may safely be
concluded, affect the structure
of the adult; in the same manner
as any malconformation affecting
the early embryo, seriously affects
the whole organisation of the
adult. The several parts of the
body which are homologous, and
which, at an early embryonic
period, are alike, seem liable
to vary in an allied manner:
we see this in the right and
left sides of the body varying
in the same manner; in the front
and hind legs, and even in the
jaws and limbs, varying together,
for the lower jaw is believed
to be homologous with the limbs.
These tendencies, I do not doubt,
may be mastered more or less
completely by natural selection:
thus a family of stags once existed
with an antler only on one side;
and if this had been of any great
use to the breed it might probably
have been rendered permanent
by natural selection.
Homologous parts, as has been
remarked by some authors, tend
to cohere; this is often seen
in monstrous plants; and nothing
is more common than the union
of homologous parts in normal
structures, as the union of the
petals of the corolla into a
tube. Hard parts seem to affect
the form of adjoining soft parts;
it is believed by some authors
that the diversity in the shape
of the pelvis in birds causes
the remarkable diversity in the
shape of their kidneys. Others
believe that the shape of the
pelvis in the human mother influences
by pressure the shape of the
head of the child. In snakes,
according to Schlegel, the shape
of the body and the manner of
swallowing determine the position
of several of the most important
viscera.
The nature
of the bond of correlation
is very frequently quite obscure.
M. Is. Geoffroy St Hilaire has
forcibly remarked, that certain
malconformations very frequently,
and that others rarely coexist,
without our being able to assign
any reason. What can be more
singular than the relation between
blue eyes and deafness in cats,
and the tortoise-shell colour
with the female sex; the feathered
feet and skin between the outer
toes in pigeons, and the presence
of more or less down on the young
birds when first hatched, with
the future colour of their plumage;
or, again, the relation between
the hair and teeth in the naked
Turkish dog, though here probably
homology comes into play? With
respect to this latter case of
correlation, I think it can hardly
be accidental, that if we pick
out the two orders of mammalia
which are most abnormal in their
dermal coverings, viz. Cetacea
(whales) and Edentata (armadilloes,
scaly ant-eaters, &c.), that
these are likewise the most abnormal
in their teeth.
I know of no case better adapted
to show the importance of the
laws of correlation in modifying
important structures, independently
of utility and, therefore, of
natural selection, than that
of the difference between the
outer and inner flowers in some
Compositous and Umbelliferous
plants. Every one knows the difference
in the ray and central florets
of, for instance, the daisy,
and this difference is often
accompanied with the abortion
of parts of the flower. But,
in some Compositous plants, the
seeds also differ in shape and
sculpture; and even the ovary
itself, with its accessory parts,
differs, as has been described
by Cassini. These differences
have been attributed by some
authors to pressure, and the
shape of the seeds in the ray-florets
in some Compositae countenances
this idea; but, in the case of
the corolla of the Umbelliferae,
it is by no means, as Dr Hooker
informs me, in species with the
densest heads that the inner
and outer flowers most frequently
differ. It might have been thought
that the development of the ray-petals
by drawing nourishment from certain
other parts of the flower had
caused their abortion; but in
some Compositae there is a difference
in the seeds of the outer and
inner florets without any difference
in the corolla. Possibly, these
several differences may be connected
with some difference in the flow
of nutriment towards the central
and external flowers: we know,
at least, that in irregular flowers,
those nearest to the axis are
oftenest subject to peloria,
and become regular. I may add,
as an instance of this, and of
a striking case of correlation,
that I have recently observed
in some garden pelargoniums,
that the central flower of the
truss often loses the patches
of darker colour in the two upper
petals; and that when this occurs,
the adherent nectary is quite
aborted; when the colour is absent
from only one of the two upper
petals, the nectary is only much
shortened.
With respect to the difference
in the corolla of the central
and exterior flowers of a head
or umbel, I do not feel at all
sure that C. C. Sprengel's idea
that the ray-florets serve to
attract insects, whose agency
is highly advantageous in the
fertilisation of plants of these
two orders, is so far-fetched,
as it may at first appear: and
if it be advantageous, natural
selection may have come into
play. But in regard to the differences
both in the internal and external
structure of the seeds, which
are not always correlated with
any differences in the flowers,
it seems impossible that they
can be in any way advantageous
to the plant: yet in the Umbelliferae
these differences are of such
apparent importance the seeds
being in some cases, according
to Tausch, orthospermous in the
exterior flowers and coelospermous
in the central flowers, that
the elder De Candolle founded
his main divisions of the order
on analogous differences. Hence
we see that modifications of
structure, viewed by systematists
as of high value, may be wholly
due to unknown laws of correlated
growth, and without being, as
far as we can see, of the slightest
service to the species.
We may often falsely attribute
to correlation of growth, 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 correlated in some necessary
manner. So, again, I do not doubt
that some apparent correlations,
occurring throughout whole orders,
are entirely due to the manner
alone in which natural selection
can act. For instance, Alph.
De Candolle has remarked that
winged seeds are never found
in fruits which do not open:
I should explain the rule by
the fact that seeds could not
gradually become winged through
natural selection, except in
fruits which opened; so that
the individual plants producing
seeds which were a little better
fitted to be wafted further,
might get an advantage over those
producing seed less fitted for
dispersal; and this process could
not possibly go on in fruit which
did not open.
The elder Geoffroy and Goethe
propounded, at about the same
period, 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 this 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,
any diminution, however slight,
in its development, will be seized
on by natural selection, for
it will profit the individual
not to have its nutriment wasted
in building up an 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 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
by the parasitic habits of the
Proteolepas, though effected
by slow steps, 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
individual Proteolepas would
have a better chance of supporting
itself, by less nutriment being
wasted in developing a structure
now become useless.
Thus, as I believe, natural
selection will always succeed
in the long run in reducing and
saving every part of the organisation,
as soon as it is rendered 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
any organ, without requiring
as a necessary compensation the
reduction of some adjoining part.
It seems to be a rule, as remarked
by Is. Geoffroy St Hilaire, both
in varieties and in species,
that when any part or organ is
repeated many times in the structure
of 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 and some botanists have
further remarked that multiple
parts are also very liable to
variation in structure. Inasmuch
as this 'vegetative repetition,'
to use Prof. Owen's expression,
seems to be a sign of low organisation;
the foregoing remark seems connected
with the very general opinion
of naturalists, that beings low
in the scale of nature are more
variable than those which are
higher. I presume that lowness
in this case 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 have
preserved or rejected each little
deviation of form less carefully
than when the part has to serve
for one special purpose alone.
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
object had better be of some
particular shape. Natural selection,
it should never be forgotten,
can act on each part of each
being, solely through and for
its advantage.
Rudimentary parts, it has been
stated by some authors, and I
believe with truth, are apt to
be highly variable. We shall
have to recur to the general
subject of rudimentary and aborted
organs; and I will here only
add that their variability seems
to be owing to their uselessness,
and therefore to natural selection
having no power to check deviations
in their structure. Thus rudimentary
parts are left to the free play
of the various laws of growth,
to the effects of long-continued
disuse, and to the tendency to
reversion.
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 with a remark, nearly
to the above effect, published
by Mr Waterhouse. I infer also
from an observation made by Professor
Owen, with respect to the length
of the arms of the ourang-outang,
that he has come to a nearly
similar conclusion. It is hopeless
to attempt to convince any one
of the truth of this 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 allowance
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 comparison with
the same part in closely allied
species. Thus, the bat's wing
is a most abnormal structure
in the class mammalia; but the
rule would not here apply, because
there is a whole group of bats
having wings; it would apply
only if some one species of bat
had its wings developed in some
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, applies
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 as females more
rarely offer remarkable secondary
sexual characters, it applies
more rarely to them. 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;
and I may here add, that I particularly
attended to Mr. Waterhouse's
remark, whilst investigating
this Order, and I am fully convinced
that the rule almost invariably
holds good with cirripedes. I
shall, in my future work, give
a list of the more remarkable
cases; I will here only briefly
give 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 different 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 several of the species is
so great, that it is no exaggeration
to state that the varieties differ
more from each other in the characters
of these important valves than
do other species of distinct
genera.
As birds within the same country
vary in a remarkably small degree,
I have particularly attended
to them, and the rule seems to
me certainly to hold good in
this class. I cannot make out
that it applies to plants, and
this would seriously have 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 any species, the
fair presumption is that it is
of high importance to that species;
nevertheless the part in this
case is 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
have descended from other species,
and have been modified through
natural selection, I think we
can obtain some light. In our
domestic animals, if 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 nearly uniform character.
The breed will then be said to
have degenerated. In rudimentary
organs, and in those which have
been but little specialized for
any particular purpose, and perhaps
in polymorphic groups, we see
a nearly parallel natural 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
especially concerns us is, that
in our domestic animals those
points, which at the present
time are undergoing rapid change
by continued selection, are also
eminently liable to variation.
Look at the breeds of the pigeon;
see what a prodigious amount
of difference there is in the
beak of the different tumblers,
in the beak and wattle of the
different carriers, in the carriage
and tail of our fantails, &c.,
these being the points now mainly
attended to by English fanciers.
Even in the sub-breeds, as in
the short-faced tumbler, it is
notoriously difficult to breed
them nearly to perfection, and
frequently individuals are born
which depart widely from the
standard. There may be truly
said to be a constant struggle
going on between, on the one
hand, the tendency to reversion
to a less modified state, as
well as an innate tendency to
further variability of all kinds,
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 far as to breed
a bird as coarse as a common
tumbler from a good short-faced
strain. But as long as selection
is rapidly going on, there may
always be expected to be much
variability in the structure
undergoing modification. It further
deserves notice that these variable
characters, produced by man's
selection, sometimes become attached,
from causes quite unknown to
us, more to one sex than to the
other, generally to the male
sex, as with the wattle of carriers
and the enlarged crop of pouters.
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 species branched
off from the common progenitor
of the genus. This period will
seldom be remote in any extreme
degree, as species very 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, expect still
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 can 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 my
theory, for an immense period
in nearly the same state; and
thus it comes to be no 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.
The principle included in these
remarks may be extended. It is
notorious that specific characters
are more variable than generic.
To explain by a simple example
what is meant. If some species
in a large genus of plants 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
an explanation is not in this
case applicable, which most naturalists
would advance, 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
subject in our chapter on Classification.
It would be almost superfluous
to adduce evidence in support
of the above statement, that
specific characters are more
variable than generic; but I
have repeatedly noticed in works
on natural history, that when
an author has remarked with surprise
that some important organ
or part, which is generally very
constant throughout large groups
of species, has differed considerably
in closely-allied species, that
it has, also, been variable in
the individuals of some of the
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 seems to
entertain no doubt, that the
more an organ normally differs
in the different species of the
same group, the more subject
it is to individual anomalies.
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 of species being
only strongly marked and fixed
varieties, we might surely expect
to find them still often 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 the species of some
other genus, are called generic
characters; and these characters
in common I attribute to inheritance
from a common progenitor, for
it can rarely have happened that
natural selection will have modified
several 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 a remote
period, since that period when
the 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 within the
period of the branching off of
the species 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.
In connexion with the present
subject, I will make only two
other remarks. I think it will
be admitted, without my entering
on details, that secondary sexual
characters are very variable;
I think it also will 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
their females; and the truth
of this proposition will be granted.
The cause of the original variability
of secondary sexual characters
is not manifest; but we can see
why these characters should not
have been rendered as constant
and uniform as other parts of
the organisation; for secondary
sexual characters have been 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 readily
have succeeded in giving to the
species of the same group a greater
amount of difference in their
sexual characters, than in other
parts of their structure.
It is a remarkable fact, that
the secondary sexual differences
between the two sexes of the
same species are generally displayed
in the very same parts of the
organisation in which the different
species of the same genus differ
from each other. Of this fact
I will give in illustration two
instances, the first 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 generally
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 fossorial hymenoptera,
the manner of 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. This relation
has a clear meaning on my view
of the subject: 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
of the 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 species to
their 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 and females to different
habits of life, or 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 the species possess in
common; 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 not great 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 the great amount of difference
in these same characters between
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 having descended
from 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 adapted for
secondary sexual, and for ordinary
specific purposes.
Distinct species present analogous
variations; and a variety of
one species often assumes some
of the characters of 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 pigeons, in
countries most widely apart,
present sub-varieties with reversed
feathers on the head and 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 roots
as commonly called, 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.
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, a white
rump, 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,
perhaps for hundreds of generations.
But when a breed has been crossed
only once by some other breed,
the offspring occasionally show
a tendency to revert in character
to the foreign breed for many
generations some say, for a dozen
or even a score of generations.
After twelve generations, the
proportion of blood, to use a
common expression, of any 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 very small
proportion 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 be, as was formerly remarked,
for all that we can see to the
contrary, 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 the offspring suddenly
takes after an ancestor some
hundred generations distant,
but that in each successive generation
there has been a tendency to
reproduce the character in question,
which at last, under unknown
favourable conditions, gains
an ascendancy. For instance,
it is probable that in each generation
of the barb-pigeon, which produces
most rarely a blue and black-barred
bird, there has been a tendency
in each generation in the plumage
to assume this colour. This view
is hypothetical, but could be
supported by some facts; and
I can see no more abstract improbability
in a tendency to produce any
character being inherited for
an endless number of generations,
than in quite useless or rudimentary
organs being, as we all know
them to be, thus inherited. Indeed,
we may sometimes observe a mere
tendency to produce a rudiment
inherited: for instance, in the
common snapdragon (Antirrhinum)
a rudiment of a fifth stamen
so often appears, that this plant
must have an inherited tendency
to produce it.
As all the species of the same
genus are supposed, on my theory,
to have descended from a common
parent, it might be expected
that they would occasionally
vary in an analogous manner;
so that a variety of one species
would resemble in some of its
characters another species; this
other species being on my view
only a well-marked and permanent
variety. But characters thus
gained would probably be of an
unimportant nature, for the presence
of all important characters will
be governed by natural selection,
in accordance with the diverse
habits of the species, and will
not be left to the mutual action
of the conditions of life and
of a similar inherited constitution.
It might further be expected
that the species of the same
genus would occasionally exhibit
reversions to lost ancestral
characters. As, however, we never
know the exact character of the
common ancestor of a group, we
could not distinguish these two
cases: if, for instance, we did
not know that the rock-pigeon
was not feather-footed or turn-crowned,
we could not have told, whether
these characters in our domestic
breeds were reversions or only
analogous variations; but we
might have inferred that the
blueness was a case of reversion,
from the number of the markings,
which are correlated with the
blue tint, and which it does
not appear probable would all
appear together from simple variation.
More especially we might have
inferred this, from the blue
colour and marks so often appearing
when distinct breeds of diverse
colours are crossed. Hence, though
under nature it must generally
be left doubtful, what cases
are reversions to an anciently
existing character, and what
are new but analogous variations,
yet we ought, on my theory, sometimes
to find the varying offspring
of a species assuming characters
(either from reversion or from
analogous variation) which already
occur in some members of the
same group. And this undoubtedly
is the case in nature.
A considerable part of the
difficulty in recognising a variable
species in our systematic works,
is due to its varieties mocking,
as it were, come of the other
species of the same genus. A
considerable catalogue, also,
could be given of forms intermediate
between two other forms, which
themselves must be doubtfully
ranked as either varieties or
species, that the one in varying
has assumed some of the characters
of the other, so as to produce
the intermediate form. But the
best evidence is afforded by
parts or organs of an important
and uniform nature occasionally
varying so as to acquire, in
some degree, the character of
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
a great disadvantage in not being
able to give them. I can only
repeat that such cases certainly
do 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 apparently
of reversion. The ass not rarely
has very distinct transverse
bars on its legs, like those
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. It has also been asserted
that the stripe on each shoulder
is sometimes double. The shoulder-stripe
is certainly 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;
but traces of it, as stated by
Mr Blyth and others, occasionally
appear: 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;
and a man, whom I can implicitly
trust, has examined for me a
small dun Welch pony with three short
parallel stripes on each shoulder.
In the north-west part of India
the Kattywar breed of horses
is so generally striped, that,
as I hear from Colonel Poole,
who examined the 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 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.
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 have 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
I am not at all satisfied with
this theory, and should be loth
to apply it to breeds so distinct
as the heavy Belgian cart-horse,
Welch ponies, cobs, the lanky
Kattywar race, &c., inhabiting
the most distant parts of the
world.
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.
I once saw a mule with its legs
so much striped that any one
at first would have thought that
it must have been the product
of a 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 Moreton's famous hybrid
from a chestnut mare and male
quagga, the hybrid, and even
the pure offspring subsequently
produced from the 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 seldom has stripes
on its 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 Welch pony,
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 would commonly be called
an accident, 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
occur 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
very 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
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 differs,
more or less, from the same part
in the parents. 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. The external
conditions of life, as climate
and food, &c., seem to have
induced some slight modifications.
Habit in producing constitutional
differences, and use in strengthening,
and disuse in weakening and diminishing
organs, seem to have been more
potent in their effects. Homologous
parts tend to vary in the same
way, 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
to the individual, will be saved.
Changes of structure at an early
age will generally affect parts
subsequently developed; and there
are very many other correlations
of growth, the nature of which
we are utterly unable to understand.
Multiple parts are variable in
number and in structure, perhaps
arising from such parts not having
been closely specialized to any
particular function, so that
their modifications have not
been closely checked by natural
selection. It is probably from
this same cause that organic
beings low in the scale of nature
are more variable than those
which have their whole organisation
more specialized, and are higher
in the scale. Rudimentary organs,
from being useless, will be disregarded
by natural selection, and hence
probably 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 any 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 there, on an average,
we now find most varieties or
incipient species. 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 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
any extraordinarily-developed
organ has become the parent of
many modified descendants which
on my view must be a very slow
process, requiring a long lapse
of time in this case, natural
selection may readily have succeeded
in giving a fixed character to
the organ, in however extraordinary
a manner it may be developed.
Species inheriting nearly the
same constitution from a common
parent and exposed to similar
influences will naturally tend
to present analogous variations,
and 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 in the
offspring from their parents
and a cause for each must exist
it is the steady accumulation,
through natural selection, of
such differences, when beneficial
to the individual, that gives
rise to all the more important
modifications of structure, by
which the innumerable beings
on the face of this earth are
enabled to struggle with each
other, and the best adapted to
survive. |