Chapter V

Laws of Variation

Effects of external conditions -- Use and disuse, combined with natural

selection; organs of flight and of vision -- Acclimatisation -- Correlation

of growth -- Compensation and economy of growth -- False correlations --

Multiple, rudimentary, and lowly organised structures variable -- Parts

developed in an unusual manner are highly variable: specific characters

more variable than generic: secondary sexual characters variable --

Species of the same genus vary in an analogous manner -- Reversions to long

lost characters -- Summary.

I have hitherto sometimes spoken as if the variations--so common and

multiform in organic beings under domestication, and in a lesser degree in

those in a state of nature--had been due to chance. This, of course, is a

wholly incorrect expression, but it serves to acknowledge plainly our

ignorance of the cause of each particular variation. Some authors believe

it to be as much the function of the reproductive system to produce

individual differences, or very slight deviations of structure, as to make

the child like its parents. But the much greater variability, as well as

the greater frequency of monstrosities, under domestication or cultivation,

than under nature, leads me to believe that deviations of structure are in

some way due to the nature of the conditions of life, to which the parents

and their more remote ancestors have been exposed during several

generations. I have remarked in the first chapter--but a long catalogue of

facts which cannot be here given would be necessary to show the truth of

the remark--that the reproductive system is eminently susceptible to

changes in the conditions of life; and to this system being functionally

disturbed in the parents, I chiefly attribute the varying or plastic

condition of the offspring. The male and female sexual elements seem to be

affected before that union takes place which is to form a new being. In

the case of 'sporting' plants, the bud, which in its earliest condition

does not apparently differ essentially from an ovule, is alone affected.

But why, because the reproductive system is disturbed, this or that part

should vary more or less, we are profoundly ignorant. Nevertheless, we can

here and there dimly catch a faint ray of light, and we may feel sure that

there must be some cause for each deviation of structure, however slight.

How much direct effect difference of climate, food, &c., produces on any

being is extremely doubtful. My impression is, that the effect is

extremely small in the case of animals, but perhaps rather more in that of

plants. We may, at least, safely conclude that such influences cannot have

produced the many striking and complex co-adaptations of structure between

one organic being and another, which we see everywhere throughout nature.

Some little influence may be attributed to climate, food, &c.: thus, E.

Forbes speaks confidently that shells at their southern limit, and when

living in shallow water, are more brightly coloured than those of the same

species further north or from greater depths. Gould believes that birds of

the same species are more brightly coloured under a clear atmosphere, than

when living on islands or near the coast. So with insects, Wollaston is

convinced that residence near the sea affects their colours. Moquin-Tandon

gives a list of plants which when growing near the sea-shore have their

leaves in some degree fleshy, though not elsewhere fleshy. Several other

such cases could be given.

The fact of varieties of one species, when they range into the zone of

habitation of other species, often acquiring in a very slight degree some

of the characters of such species, accords with our view that species of

all kinds are only well-marked and permanent varieties. Thus the species

of shells which are confined to tropical and shallow seas are generally

brighter-coloured than those confined to cold and deeper seas. The birds

which are confined to continents are, according to Mr. Gould,

brighter-coloured than those of islands. The insect-species confined to

sea-coasts, as every collector knows, are often brassy or lurid. Plants

which live exclusively on the sea-side are very apt to have fleshy leaves.

He who believes in the creation of each species, will have to say that this

shell, for instance, was created with bright colours for a warm sea; but

that this other shell became bright-coloured by variation when it ranged

into warmer or shallower waters.

When a variation is of the slightest use to a being, we cannot tell how

much of it to attribute to the accumulative action of natural selection,

and how much to the conditions of life. Thus, it is well known to furriers

that animals of the same species have thicker and better fur the more

severe the climate is under which they have lived; but who can tell how

much of this difference may be due to the warmest-clad individuals having

been favoured and preserved during many generations, and how much to the

direct action of the severe climate? for it would appear that climate has

some direct action on the hair of our domestic quadrupeds.

Instances could be given of the same variety being produced under

conditions of life as different as can well be conceived; and, on the other

hand, of different varieties being produced from the same species under the

same conditions. Such facts show how indirectly the conditions of life

must act. Again, innumerable instances are known to every naturalist of

species keeping true, or not varying at all, although living under the most

opposite climates. Such considerations as these incline me to lay very

little weight on the direct action of the conditions of life. Indirectly,

as already remarked, they seem to play an important part in affecting the

reproductive system, and in thus inducing variability; and natural

selection will then accumulate all profitable variations, however slight,

until they become plainly developed and appreciable by us.

Effects of Use and Disuse. -- From the facts alluded to in the first

chapter, I think there can be little doubt that use in our domestic animals

strengthens and enlarges certain parts, and disuse diminishes them; and

that such modifications are inherited. Under free nature, we can have no

standard of comparison, by which to judge of the effects of long-continued

use or disuse, for we know not the parent-forms; but many animals have

structures which can be explained by the effects of disuse. As Professor

Owen has remarked, there is no greater anomaly in nature than a bird that

cannot fly; yet there are several in this state. The logger-headed duck of

South America can only flap along the surface of the water, and has its

wings in nearly the same condition as the domestic Aylesbury duck. As the

larger ground-feeding birds seldom take flight except to escape danger, I

believe that the nearly wingless condition of several birds, which now

inhabit or have lately inhabited several oceanic islands, tenanted by no

beast of prey, has been caused by disuse. The ostrich indeed inhabits

continents and is exposed to danger from which it cannot escape by flight,

but by kicking it can defend itself from enemies, as well as any of the

smaller quadrupeds. We may imagine that the early progenitor of the

ostrich had habits like those of a bustard, and that as natural selection

increased in successive generations the size and weight of its body, its

legs were used more, and its wings less, until they became incapable of

flight.

Kirby has remarked (and I have observed the same fact) that the anterior

tarsi, or feet, of many male dung-feeding beetles are very often broken

off; he examined seventeen specimens in his own collection, and not one had

even a relic left. In the Onites apelles the tarsi are so habitually lost,

that the insect has been described as not having them. In some other

genera they are present, but in a rudimentary condition. In the Ateuchus

or sacred beetle of the Egyptians, they are totally deficient. There is

not sufficient evidence to induce us to believe that mutilations are ever

inherited; and I should prefer explaining the entire absence of the

anterior tarsi in Ateuchus, and their rudimentary condition in some other

genera, by the long-continued effects of disuse in their progenitors; for

as the tarsi are almost always lost in many dung-feeding beetles, they must

be lost early in life, and therefore cannot be much used by these insects.

In some cases we might easily put down to disuse modifications of structure

which are wholly, or mainly, due to natural selection. Mr. Wollaston has

discovered the remarkable fact that 200 beetles, out of the 550 species

inhabiting Madeira, are so far deficient in wings that they cannot fly; and

that of the twenty-nine endemic genera, no less than twenty-three genera

have all their species in this condition! Several facts, namely, that

beetles in many parts of the world are very frequently blown to sea and

perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much

concealed, until the wind lulls and the sun shines; that the proportion of

wingless beetles is larger on the exposed Dezertas than in Madeira itself;

and especially the extraordinary fact, so strongly insisted on by Mr.

Wollaston, of the almost entire absence of certain large groups of beetles,

elsewhere excessively numerous, and which groups have habits of life almost

necessitating frequent flight;--these several considerations have made me

believe that the wingless condition of so many Madeira beetles is mainly

due to the action of natural selection, but combined probably with disuse.

For during thousands of successive generations each individual beetle which

flew least, either from its wings having been ever so little less perfectly

developed or from indolent habit, will have had the best chance of

surviving from not being blown out to sea; and, on the other hand, those

beetles which most readily took to flight will oftenest have been blown to

sea and thus have been destroyed.

The insects in Madeira which are not ground-feeders, and which, as the

flower-feeding coleoptera and lepidoptera, must habitually use their wings

to gain their subsistence, have, as Mr. Wollaston suspects, their wings not

at all reduced, but even enlarged. This is quite compatible with the

action of natural selection. For when a new insect first arrived on the

island, the tendency of natural selection to enlarge or to reduce the

wings, would depend on whether a greater number of individuals were saved

by successfully battling with the winds, or by giving up the attempt and

rarely or never flying. As with mariners shipwrecked near a coast, it

would have been better for the good swimmers if they had been able to swim

still further, whereas it would have been better for the bad swimmers if

they had not been able to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,

and in some cases are quite covered up by skin and fur. This state of the

eyes is probably due to gradual reduction from disuse, but aided perhaps by

natural selection. In South America, a burrowing rodent, the tuco-tuco, or

Ctenomys, is even more subterranean in its habits than the mole; and I was

assured by a Spaniard, who had often caught them, that they were frequently

blind; one which I kept alive was certainly in this condition, the cause,

as appeared on dissection, having been inflammation of the nictitating

membrane. As frequent inflammation of the eyes must be injurious to any

animal, and as eyes are certainly not indispensable to animals with

subterranean habits, a reduction in their size with the adhesion of the

eyelids and growth of fur over them, might in such case be an advantage;

and if so, natural selection would constantly aid the effects of disuse.

It is well known that several animals, belonging to the most different

classes, which inhabit the caves of Styria and of Kentucky, are blind. In

some of the crabs the foot-stalk for the eye remains, though the eye is

gone; the stand for the telescope is there, though the telescope with its

glasses has been lost. As it is difficult to imagine that eyes, though

useless, could be in any way injurious to animals living in darkness, I

attribute their loss wholly to disuse. In one of the blind animals,

namely, the cave-rat, the eyes are of immense size; and Professor Silliman

thought that it regained, after living some days in the light, some slight

power of vision. In the same manner as in Madeira the wings of some of the

insects have been enlarged, and the wings of others have been reduced by

natural selection aided by use and disuse, so in the case of the cave-rat

natural selection seems to have struggled with the loss of light and to

have increased the size of the eyes; whereas with all the other inhabitants

of the caves, disuse by itself seems to have done its work.

It is difficult to imagine conditions of life more similar than deep

limestone caverns under a nearly similar climate; so that on the common

view of the blind animals having been separately created for the American

and European caverns, close similarity in their organisation and affinities

might have been expected; but, as Schiodte and others have remarked, this

is not the case, and the cave-insects of the two continents are not more

closely allied than might have been anticipated from the general

resemblance of the other inhabitants of North America and Europe. On my

view we must suppose that American animals, having ordinary powers of

vision, slowly migrated by successive generations from the outer world into

the deeper and deeper recesses of the Kentucky caves, as did European

animals into the caves of Europe. We have some evidence of this gradation

of habit; for, as Schiodte remarks, 'animals not far remote from ordinary

forms, prepare the transition from light to darkness. Next follow those

that are constructed for twilight; and, last of all, those destined for

total darkness.' By the time that an animal had reached, after numberless

generations, the deepest recesses, disuse will on this view have more or

less perfectly obliterated its eyes, and natural selection will often have

effected other changes, such as an increase in the length of the antennae

or palpi, as a compensation for blindness. Notwithstanding such

modifications, we might expect still to see in the cave-animals of America,

affinities to the other inhabitants of that continent, and in those of

Europe, to the inhabitants of the European continent. And this is the case

with some of the American cave-animals, as I hear from Professor Dana; and

some of the European cave-insects are very closely allied to those of the

surrounding country. It would be most difficult to give any rational

explanation of the affinities of the blind cave-animals to the other

inhabitants of the two continents on the ordinary view of their independent

creation. That several of the inhabitants of the caves of the Old and New

Worlds should be closely related, we might expect from the well-known

relationship of most of their other productions. Far from feeling any

surprise that some of the cave-animals should be very anomalous, as Agassiz

has remarked in regard to the blind fish, the Amblyopsis, and as is the

case with the blind Proteus with reference to the reptiles of Europe, I am

only surprised that more wrecks of ancient life have not been preserved,

owing to the less severe competition to which the inhabitants of these dark

abodes will probably have been exposed.

Acclimatisation. -- Habit is hereditary with plants, as in the period of

flowering, in the amount of rain requisite for seeds to germinate, in the

time of sleep, &c., and this leads me to say a few words on

acclimatisation. As it is extremely common for species of the same genus

to inhabit very hot and very cold countries, and as I believe that all the

species of the same genus have descended from a single parent, if this view

be correct, acclimatisation must be readily effected during long-continued

descent. It is notorious that each species is adapted to the climate of

its own home: species from an arctic or even from a temperate region

cannot endure a tropical climate, or conversely. So again, many succulent

plants cannot endure a damp climate. But the degree of adaptation of

species to the climates under which they live is often overrated. We may

infer this from our frequent inability to predict whether or not an

imported plant will endure our climate, and from the number of plants and

animals brought from warmer countries which here enjoy good health. We

have reason to believe that species in a state of nature are limited in

their ranges by the competition of other organic beings quite as much as,

or more than, by adaptation to particular climates. But whether or not the

adaptation be generally very close, we have evidence, in the case of some

few plants, of their becoming, to a certain extent, naturally habituated to

different temperatures, or becoming acclimatised: thus the pines and

rhododendrons, raised from seed collected by Dr. Hooker from trees growing

at different heights on the Himalaya, were found in this country to possess

different constitutional powers of resisting cold. Mr. Thwaites informs me

that he has observed similar facts in Ceylon, and analogous observations

have been made by Mr. H. C. Watson on European species of plants brought

from the Azores to England. In regard to animals, several authentic cases

could be given of species within historical times having largely extended

their range from warmer to cooler latitudes, and conversely; but we do not

positively know that these animals were strictly adapted to their native

climate, but in all ordinary cases we assume such to be the case; nor do we

know that they have subsequently become acclimatised to their new homes.

As I believe that our domestic animals were originally chosen by

uncivilised man because they were useful and bred readily under

confinement, and not because they were subsequently found capable of

far-extended transportation, I think the common and extraordinary capacity

in our domestic animals of not only withstanding the most different

climates but of being perfectly fertile (a far severer test) under them,

may be used as an argument that a large proportion of other animals, now in

a state of nature, could easily be brought to bear widely different

climates. We must not, however, push the foregoing argument too far, on

account of the probable origin of some of our domestic animals from several

wild stocks: the blood, for instance, of a tropical and arctic wolf or

wild dog may perhaps be mingled in our domestic breeds. The rat and mouse

cannot be considered as domestic animals, but they have been transported by

man to many parts of the world, and now have a far wider range than any

other rodent, living free under the cold climate of Faroe in the north and

of the Falklands in the south, and on many islands in the torrid zones.

Hence I am inclined to look at adaptation to any special climate as a

quality readily grafted on an innate wide flexibility of constitution,

which is common to most animals. On this view, the capacity of enduring

the most different climates by man himself and by his domestic animals, and

such facts as that former species of the elephant and rhinoceros were

capable of enduring a glacial climate, whereas the living species are now

all tropical or sub-tropical in their habits, ought not to be looked at as

anomalies, but merely as examples of a very common flexibility of

constitution, brought, under peculiar circumstances, into play.

How much of the acclimatisation of species to any peculiar climate is due

to mere habit, and how much to the natural selection of varieties having

different innate constitutions, and how much to both means combined, is a

very obscure question. That habit or custom has some influence I must

believe, both from analogy, and from the incessant advice given in

agricultural works, even in the ancient Encyclopaedias of China, to be very

cautious in transposing animals from one district to another; for it is not

likely that man should have succeeded in selecting so many breeds and

sub-breeds with constitutions specially fitted for their own districts:

the result must, I think, be due to habit. On the other hand, I can see no

reason to doubt that natural selection will continually tend to preserve

those individuals which are born with constitutions best adapted to their

native countries. In treatises on many kinds of cultivated plants, certain

varieties are said to withstand certain climates better than others: this

is very strikingly shown in works on fruit trees published in the United

States, in which certain varieties are habitually recommended for the

northern, and others for the southern States; and as most of these

varieties are of recent origin, they cannot owe their constitutional

differences to habit. The case of the Jerusalem artichoke, which is never

propagated by seed, and of which consequently new varieties have not been

produced, has even been advanced--for it is now as tender as ever it

was--as proving that acclimatisation cannot be effected! The case, also,

of the kidney-bean has been often cited for a similar purpose, and with

much greater weight; but until some one will sow, during a score of

generations, his kidney-beans so early that a very large proportion are

destroyed by frost, and then collect seed from the few survivors, with care

to prevent accidental crosses, and then again get seed from these

seedlings, with the same precautions, the experiment cannot be said to have

been even tried. Nor let it be supposed that no differences in the

constitution of seedling kidney-beans ever appear, for an account has been

published how much more hardy some seedlings appeared to be than others.

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 specialised

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 other 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, some 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; and this shows,

unless all these forms be considered as independently created 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 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. 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. The hemionus has no 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 specialised 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 specialised, 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.