The nature of the season has an especial influence on certain varieties

of the Dahlia: in 1841 two varieties were pre-eminently good, and the

next year these same two were pre-eminently bad. A famous amateur[659]

asserts that in 1861 many varieties of the Rose came so untrue in

character, "that it was hardly possible to recognise them, and the

thought was not seldom entertained that the grower had lost his tally."

The same amateur[660] states that in 1862 two-thirds of his Auriculas

produced central trusses of flowers, and these are remarkable from not

keeping true; {274} and he adds that in some seasons certain varieties

of this plant all prove good, and the next season all prove bad; whilst

exactly the reverse happens with other varieties. In 1845 the editor of

the 'Gardener's Chronicle'[661] remarked how singular it was that this

year many Calceolarias tended to assume a tubular form. With

Heartsease[662] the blotched sorts do not acquire their proper

character until hot weather sets in; whilst other varieties lose their

beautiful marks as soon as this occurs.

Analogous facts have been observed with leaves: Mr. Beaton asserts[663]

that he raised at Shrubland, during six years, twenty thousand

seedlings from the Punch Pelargonium, and not one had variegated

leaves; but at Surbiton, in Surrey, one-third, or even a greater

proportion, of the seedlings from this same variety were more or less

variegated. The soil of another district in Surrey has a strong

tendency to cause variegation, as appears from information given me by

Sir F. Pollock. Verlot[664] states that the variegated strawberry

retains its character as long as grown in a dryish soil, but soon loses

it when planted in fresh and humid soil. Mr. Salter, who is well known

for his success in cultivating variegated plants, informs me that rows

of strawberries were planted in his garden in 1859, in the usual way;

and at various distances in one row, several plants simultaneously

became variegated, and what made the case more extraordinary, all were

variegated in precisely the same manner. These plants were removed, but

during the three succeeding years other plants in the same row became

variegated, and in no instance were the plants in any adjoining row

affected.

The chemical qualities, odours, and tissues of plants are often

modified by a change which seems to us slight. The Hemlock is said not

to yield conicine in Scotland. The root of the _Aconitum napellus_

becomes innocuous in frigid climates. The medicinal properties of the

Digitalis are easily affected by culture. The Rhubarb flourishes in

England, but does not produce the medicinal substance which makes the

plant so valuable in Chinese Tartary. As the _Pistacia lentiscus_ grows

abundantly in the South of France, the climate must suit it, but it

yields no mastic. The _Laurus sassafras_ in Europe loses the odour

proper to it in North America.[665] Many similar facts could be given,

and they are remarkable because it might have been thought that

definite chemical compounds would have been little liable to change

either in quality or quantity.

The wood of the American Locust-tree (_Robinia_) when grown in England

is nearly worthless, as is that of the Oak-tree when grown at the Cape

of Good Hope.[666] Hemp and flax, as I hear from Dr. Falconer, flourish

and yield plenty of seed on the plains of India, but their fibres are

brittle {275} and useless. Hemp, on the other hand, fails to produce in

England that resinous matter which is so largely used in India as an

intoxicating drug.

The fruit of the Melon is greatly influenced by slight differences in

culture and climate. Hence it is generally a better plan, according to

Naudin, to improve an old kind than to introduce a new one into any

locality. The seed of the Persian Melon produces near Paris fruit

inferior to the poorest market kinds, but at Bordeaux yields delicious

fruit.[667] Seed is annually brought from Thibet to Kashmir,[668] and

produces fruit weighing from four to ten pounds, but plants raised from

seed saved in Kashmir next year give fruit weighing only from two to

three pounds. It is well known that American varieties of the Apple

produce in their native land magnificent and brightly-coloured fruit,

but in England of poor quality and a dull colour. In Hungary there are

many varieties of the Kidney-bean, remarkable for the beauty of their

seeds, but the Rev. M. J. Berkeley[669] found that their beauty could

hardly ever be preserved in England, and in some cases the colour was

greatly changed. We have seen in the ninth chapter, with respect to

wheat, what a remarkable effect transportal from the North to the South

of France, and reversely, produced on the weight of the grain.

When man can perceive no change in plants or animals which have been

exposed to a new climate or to different treatment, insects can sometimes

perceive a marked change. The same species of cactus has been carried to

India from Canton, Manilla, Mauritius, and from the hot-houses of Kew, and

there is likewise a so-called native kind, formerly introduced from South

America; all these plants are alike in appearance, but the cochineal insect

flourishes only on the native kind, on which it thrives prodigiously.[670]

Humboldt remarks[671] that white men "born in the torrid zone walk barefoot

with impunity in the same apartment where a European, recently landed, is

exposed to the attacks of the _Pulex penetrans_." This insect, the too

well-known chigoe, must therefore be able to distinguish what the most

delicate chemical analysis fails to distinguish, namely, a difference

between the blood or tissues of a European and those of a white man born in

the country. But the discernment of the chigoe is not so surprising as it

at first appears; for {276} according to Liebig[672] the blood of men with

different complexions, though inhabiting the same country, emits a

different odour.

Diseases peculiar to certain localities, heights, or climates, may be

here briefly noticed, as showing the influence of external

circumstances on the human body. Diseases confined to certain races of

man do not concern us, for the constitution of the race may play the

more important part, and this may have been determined by unknown

causes. The Plica Polonica stands, in this respect, in a nearly

intermediate position; for it rarely affects Germans, who inhabit the

neighbourhood of the Vistula, where so many Poles are grievously

affected; and on the other hand, it does not affect Russians, who are

said to belong to the same original stock with the Poles.[673] The

elevation of a district often governs the appearance of diseases; in

Mexico the yellow fever does not extend above 924 metres; and in Peru,

people are affected with the _verugas_ only between 600 and 1600 metres

above the sea; many other such cases could be given. A peculiar

cutaneous complaint, called the _Bouton d'Alep_, affects in Aleppo and

some neighbouring districts almost every native infant, and some few

strangers; and it seems fairly well established that this singular

complaint depends on drinking certain waters. In the healthy little

island of St. Helena the scarlet-fever is dreaded like the Plague;

analogous facts have been observed in Chili and Mexico.[674] Even in

the different departments of France it is found that the various

infirmities which render the conscript unfit for serving in the army,

prevail with remarkable inequality, revealing, as Boudin observes, that

many of them are endemic, which otherwise would never have been

suspected.[675] Any one who will study the distribution of disease will

be struck with surprise at what slight differences in the surrounding

circumstances govern the nature and severity of the complaints by which

man is at least temporarily affected.

The modifications as yet referred to have been extremely slight, and in

most cases have been caused, as far as we can judge, by equally slight

changes in the conditions. But can it be safely maintained that such

changed conditions, if acting during a long series of generations, would

not produce a marked effect? It is commonly believed that the people of the

United States differ in appearance from the parent Anglo-Saxon race; and

selection cannot have come into action within so short a period. A good

observer[676] states that a general absence of fat, {277} a thin and

elongated neck, stiff and lank hair, are the chief characteristics. The

change in the nature of the hair is supposed to be caused by the dryness of

the atmosphere. If immigration into the United States were now stopped, who

can say that the character of the whole people would not be greatly

modified in the course of two or three thousand years?

The direct and definite action of changed conditions, in

contradistinction to the accumulation of indefinite variations, seems

to me so important that I will give a large additional body of

miscellaneous facts. With plants, a considerable change of climate

sometimes produces a conspicuous result. I have given in detail in the

ninth chapter the most remarkable case known to me, namely, that in

Germany several varieties of maize brought from the hotter parts of

America were transformed in the course of only two or three

generations. Dr. Falconer informs me that he has seen the English

Ribston-pippin apple, a Himalayan oak, Prunus and Pyrus, all assume in

the hotter parts of India a fastigate or pyramidal habit; and this fact

is the more interesting, as a Chinese tropical species of Pyrus

naturally has this habit of growth. Although in these cases the changed

manner of growth seems to have been directly caused by the great heat,

we know that many fastigate trees have originated in their temperate

homes. In the Botanic Gardens of Ceylon the apple-tree[677] "sends out

numerous runners under ground, which continually rise into small stems,

and form a growth around the parent-tree." The varieties of the cabbage

which produce heads in Europe fail to do so in certain tropical

countries.[678] The _Rhododendron ciliatum_ produced at Kew flowers so

much larger and paler-coloured than those which it bears on its native

Himalayan mountain, that Dr. Hooker[679] would hardly have recognised

the species by the flowers alone. Many similar facts with respect to

the colour and size of flowers could be given.

The experiments of Vilmorin and Buckman on carrots and parsnips prove

that abundant nutriment produces a definite and inheritable effect on

the so-called roots, with scarcely any change in other parts of the

plant. Alum directly influences the colour of the flowers of the

Hydrangea.[680] Dryness seems generally to favour the hairyness or

villosity of plants. Gaertner found that hybrid Verbascums became

extremely woolly when grown in pots. Mr. Masters, on the other hand,

states that the _Opuntia leucotricha_ "is well clothed with beautiful

white hairs when grown in a damp heat; but in a dry heat exhibits none

of this peculiarity."[681] Slight variations of many kinds, not worth

specifying in detail, are retained only as {278} long as plants are

grown in certain soils, of which Sageret[682] gives from his own

experience some instances. Odart, who insists strongly on the

permanence of the varieties of the grape, admits[683] that some

varieties, when grown under a different climate or treated differently,

vary in an extremely slight degree, as in the tint of the fruit and in

the period of ripening. Some authors have denied that grafting causes

even the slightest difference in the scion; but there is sufficient

evidence that the fruit is sometimes slightly affected in size and

flavour, the leaves in duration, and the flowers in appearance.[684]

With animals there can be no doubt, from the facts given in the first

chapter, that European dogs deteriorate in India, not only in their

instincts but in structure; but the changes which they undergo are of

such a nature, that they may be partly due to reversion to a primitive

form, as in the case of feral animals. In parts of India the turkey

becomes reduced in size, "with the pendulous appendage over the beak

enormously developed."[685] We have seen how soon the wild duck, when

domesticated, loses its true character, from the effects of abundant or

changed food, or from taking little exercise. From the direct action of

a humid climate and poor pasture the horse rapidly decreases in size in

the Falkland Islands. From information which I have received, this

seems likewise to be the case to a certain extent with sheep in

Australia.

Climate definitely influences the hairy covering of animals; in the

West Indies a great change is produced in the fleece of sheep, in about

three generations. Dr. Falconer states[686] that the Thibet mastiff and

goat, when brought down from the Himalaya to Kashmir, lose their fine

wool. At Angora not only goats, but shepherd-dogs and cats, have fine

fleecy hair, and Mr. Ainsworth[687] attributes the thickness of the

fleece to the severe winters, and its silky lustre to the hot summers.

Burnes states positively[688] that the Karakool sheep lose their

peculiar black curled fleeces when removed into any other country. Even

within the limits of England, I have been assured that with two breeds

of sheep the wool was slightly changed by the flocks being pastured in

different localities.[689] It has been asserted on good authority[690]

that horses kept during several years in the deep coal-mines of Belgium

become covered with velvety hair, almost like that on the mole. These

cases probably stand in close relation to the natural change of coat in

winter and summer. Naked varieties of several domestic animals have

occasionally appeared; but there is no reason to {279} believe that

this is in any way related to the nature of the climate to which they

have been exposed.[691]

It appears at first sight probable that the increased size, the

tendency to fatten, the early maturity and altered forms of our

improved cattle, sheep, and pigs, have directly resulted from their

abundant supply of food. This is the opinion of many competent judges,

and probably is to a great extent true. But as far as form is

concerned, we must not overlook the equal or more potent influence of

lessened use on the limbs and lungs. We see, moreover, as far as size

is concerned, that selection is apparently a more powerful agent than a

large supply of food, for we can thus only account for the existence,

as remarked to me by Mr. Blyth, of the largest and smallest breeds of

sheep in the same country, of Cochin-China fowls and Bantams, of small

Tumbler and large Runt pigeons, all kept together and supplied with

abundant nourishment. Nevertheless there can be little doubt that our

domesticated animals have been modified, independently of the increased

or lessened use of parts, by the conditions to which they have been

subjected, without the aid of selection. For instance, Prof.

Ruetimeyer[692] shows that the bones of all domesticated quadrupeds can

be distinguished from those of wild animals by the state of their

surface and general appearance. It is scarcely possible to read

Nathusius's excellent 'Vorstudien,'[693] and doubt that, with the

highly improved races of the pig, abundant food has produced a

conspicuous effect on the general form of the body, on the breadth of

the head and face, and even on the teeth. Nathusius rests much on the

case of a purely bred Berkshire pig, which when two months old became

diseased in its digestive organs, and was preserved for observation

until nineteen months old; at this age it had lost several

characteristic features of the breed, and had acquired a long, narrow

head, of large size relatively to its small body, and elongated legs.

But in this case and in some others we ought not to assume that,

because certain characters are lost, perhaps through reversion, under

one course of treatment, therefore that they had been at first directly

produced by an opposite course.

In the case of the rabbit, which has become feral on the island of

Porto Santo, we are at first strongly tempted to attribute the whole

change--the greatly reduced size, the altered tints of the fur, and the

loss of certain characteristic marks--to the definite action of the new

conditions to which it has been exposed. But in all such cases we have

to consider in addition the tendency to reversion to progenitors more

or less remote, and the natural selection of the finest shades of

difference.

The nature of the food sometimes either definitely induces certain

peculiarities, or stands in some close relation with them. Pallas long

ago asserted that the fat-tailed sheep of Siberia degenerated and lost

their enormous tails when removed from certain saline pastures; and

recently {280} Erman[694] states that this occurs with the Kirgisian

sheep when brought to Orenburgh.

It is well known that hemp-seed causes bullfinches and certain other

birds to become black. Mr. Wallace has communicated to me some much

more remarkable facts of the same nature. The natives of the Amazonian

region feed the common green parrot (_Chrysotis festiva_, Linn.) with

the fat of large Siluroid fishes, and the birds thus treated become

beautifully variegated with red and yellow feathers. In the Malayan

archipelago, the natives of Gilolo alter in an analogous manner the

colours of another parrot, namely, the _Lorius garrulus_, Linn., and

thus produce the _Lori rajah_ or King-Lory. These parrots in the Malay

Islands and South America, when fed by the natives on natural vegetable

food, such as rice and plantains, retain their proper colours. Mr.

Wallace has, also, recorded[695] a still more singular fact. "The

Indians (of S. America) have a curious art by which they change the

colours of the feathers of many birds. They pluck out those from the

part they wish to paint, and inoculate the fresh wound with the milky

secretion from the skin of a small toad. The feathers grow of a

brilliant yellow colour, and on being plucked out, it is said, grow

again of the same colour without any fresh operation."

Bechstein[696] does not entertain any doubt that seclusion from light

affects, at least temporarily, the colours of cage-birds.

It is well known that the shells of land-mollusca are affected by the

abundance of lime in different districts. Isidore Geoffroy St.

Hilaire[697] gives the case of _Helix lactea_, which has recently been

carried from Spain to the South of France and to the Rio Plata, and in

both these countries now presents a distinct appearance, but whether

this has resulted from food or climate is not known. With respect to

the common oyster, Mr. F. Buckland informs me that he can generally

distinguish the shells from different districts; young oysters brought

from Wales and laid down in beds where "_natives_" are indigenous, in

the short space of two months begin to assume the "native" character.

M. Costa[698] has recorded a much more remarkable case of the same

nature, namely, that young shells taken from the shores of England and

placed in the Mediterranean, at once altered their manner of growth and

formed prominent diverging rays, like those on the shells of the proper

Mediterranean oyster. The same individual shell, showing both forms of

growth, was exhibited before a society in Paris. Lastly, it is well

known that caterpillars fed on different food sometimes either

themselves acquire a different colour or produce moths different in

colour.[699]

{281}

It would be travelling beyond my proper limits here to discuss how far

organic beings in a state of nature are definitely modified by changed

conditions. In my 'Origin of Species' I have given a brief abstract of

the facts bearing on this point, and have shown the influence of light

on the colours of birds, and of residence near the sea on the lurid

tints of insects, and on the succulency of plants. Mr. Herbert

Spencer[700] has recently discussed with much ability this whole

subject on broad and general grounds. He argues, for instance, that

with all animals the external and internal tissues are differently

acted on by the surrounding conditions, and they invariably differ in

intimate structure. So again the upper and lower surfaces of true

leaves, as well as of stems and petioles, when these assume the

function and occupy the position of leaves, are differently

circumstanced with respect to light, &c., and apparently in consequence

differ in structure. But, as Mr. Herbert Spencer admits, it is most

difficult in all such cases to distinguish between the effects of the

definite action of physical conditions and the accumulation through

natural selection of inherited variations which are serviceable to the

organism, and which have arisen independently of the definite action of

these conditions.

Although we are not here concerned with organic beings in a state of

nature, yet I may call attention to one case. Mr. Meehan,[701] in a

remarkable paper, compares twenty-nine kinds of American trees, belonging

to various orders, with their nearest European allies, all grown in close

proximity in the same garden and under as nearly as possible the same

conditions. In the American species Mr. Meehan finds, with the rarest

exceptions, that the leaves fall earlier in the season, and assume before

falling a brighter tint; that they are less deeply toothed or serrated;

that the buds are smaller; that the trees are more diffuse in growth and

have fewer branchlets; and, lastly, that the seeds are smaller--all in

comparison with the corresponding European species. Now, considering that

these trees belong to distinct orders, it is out of the question that the

peculiarities just specified should have been inherited in the one

continent from one progenitor, and in the other from another progenitor;

and considering that the trees inhabit widely different stations, these

peculiarities can hardly be supposed to be of any special {282} service to

the two series in the Old and New Worlds; therefore these peculiarities

cannot have been naturally selected. Hence we are led to infer that they

have been definitely caused by the long-continued action of the different

climate of the two continents on the trees.

_Galls._--Another class of facts, not relating to cultivated plants,

deserves attention. I allude to the production of galls. Every one knows

the curious, bright-red, hairy productions on the wild rose-tree, and the

various different galls produced by the oak. Some of the latter resemble

fruit, with one face as rosy as the rosiest apple. These bright colours can

be of no service either to the gall-forming insect or to the tree, and

probably are the direct result of the action of the light, in the same

manner as the apples of Nova Scotia or Canada are brighter coloured than

English apples. The strongest upholder of the doctrine that organic beings

are created beautiful to please mankind would not, I presume, extend this

view to galls. According to Osten Sacken's latest revision, no less than

fifty-eight kinds of galls are produced on the several species of oak, by

Cynips with its sub-genera; and Mr. B. D. Walsh[702] states that he can add

many others to the list. One American species of willow, the _Salix

humilis_, bears ten distinct kinds of galls. The leaves which spring from

the galls of various English willows differ completely in shape from the

natural leaves. The young shoots of junipers and firs, when punctured by

certain insects, yield monstrous growths like flowers and cones; and the

flowers of some plants become from the same cause wholly changed in

appearance. Galls are produced in every quarter of the world; of several

sent to me by Mr. Thwaites from Ceylon, some were as symmetrical as a

composite flower when in bud, others smooth and spherical like a berry;

some protected by long spines, others clothed with yellow wool formed of

long cellular hairs, others with regularly tufted hairs. In some galls the

internal structure is simple, but in others it is highly complex; thus M.

Lucaze-Duthiers[703] has figured in the common ink-gall no less than seven

concentric layers, composed of distinct tissue, {283} namely, the

epidermic, sub-epidermic, spongy, intermediate, and the hard protective

layer formed of curiously thickened woody cells, and, lastly, the central

mass abounding with starch-granules on which the larvae feed.

Galls are produced by insects of various orders, but the greater number by

species of Cynips. It is impossible to read M. Lucaze-Duthier's discussion

and doubt that the poisonous secretion of the insect causes the growth of

the gall, and every one knows how virulent is the poison secreted by wasps

and bees, which belong to the same order with Cynips. Galls grow with

extraordinary rapidity, and it is said that they attain their full size in

a few days;[704] it is certain that they are almost completely developed

before the larvae are hatched. Considering that many gall-insects are

extremely small, the drop of secreted poison must be excessively minute; it

probably acts on one or two cells alone, which, being abnormally

stimulated, rapidly increase by a process of self-division. Galls, as Mr.

Walsh[705] remarks, afford good, constant, and definite characters, each

kind keeping as true to form as does any independent organic being. This

fact becomes still more remarkable when we hear that, for instance, seven

out of the ten different kinds of galls produced on _Salix humilis_ are

formed by gall-gnats (_Cecidomyidae_) which, "though essentially distinct

species, yet resemble one another so closely that in almost all cases it is

difficult, and in some cases impossible, to distinguish the full-grown

insects one from the other."[706] For in accordance with a wide-spread

analogy we may safely infer that the poison secreted by insects so closely

allied would not differ much in nature; yet this slight difference is

sufficient to induce widely different results. In some few cases the same

species of gall-gnat produces on distinct species of willows galls which

cannot be distinguished; the _Cynips fecundatrix_, also, has been known to

produce on the Turkish oak, to which it is not properly attached, exactly

the same kind of gall as on the European oak.[707] These latter facts

apparently prove that the nature of the poison is a much more powerful

{284} agent in determining the form of the gall than the specific character

of the tree which is acted on.

As the poisonous secretion of insects belonging to various orders has the

special power of affecting the growth of various plants;--as a slight

difference in the nature of the poison suffices to produce widely different

results;--and lastly, as we know that the chemical compounds secreted by

plants are eminently liable to be modified by changed conditions of life,

we may believe it possible that various parts of a plant might be modified

through the agency of its own altered secretions. Compare, for instance,

the mossy and viscid calyx of a moss-rose, which suddenly appears through

bud-variation on a Provence-rose, with the gall of red moss growing from

the inoculated leaf of a wild rose, with each filament symmetrically

branched like a microscopical spruce-fir, bearing a glandular tip and

secreting odoriferous gummy matter.[708] Or compare, on the one hand, the

fruit of the peach, with its hairy skin, fleshy covering, hard shell and

kernel, and on the other hand one of the more complex galls with its

epidermic, spongy, and woody layers, surrounding tissue loaded with starch

granules. These normal and abnormal structures manifestly present a certain

degree of resemblance. Or, again, reflect on the cases above given of

parrots which have had their plumage brightly decorated through some change

in their blood, caused by having been fed on certain fishes, or locally

inoculated with the poison of a toad. I am far from wishing to maintain

that the moss-rose or the hard shell of the peach-stone or the bright

colours of birds are actually due to any chemical change in the sap or

blood; but these cases of galls and of parrots are excellently adapted to

show us how powerfully and singularly external agencies may affect

structure. With such facts before us, we need feel no surprise at the

appearance of any modification in any organic being.

I may, also, here allude to the remarkable effects which parasitic

fungi sometimes produce on plants. Reissek[709] has described a

Thesium, affected by an Oecidium, which was greatly modified, and

assumed some of the {285} characteristic features of certain allied

species, or even genera. Suppose, says Reissek, "the condition

originally caused by the fungus to become constant in the course of

time, the plant would, if found growing wild, be considered as a

distinct species or even as belonging to a new genus." I quote this

remark to show how profoundly, yet in how natural a manner, this plant

must have been modified by the parasitic fungus.

_Facts and Considerations opposed to the belief that the Conditions of Life

act in a potent manner in causing definite Modifications of Structure._

I have alluded to the slight differences in species when naturally living

in distinct countries under different conditions; and such differences we

feel at first inclined, probably to a limited extent with justice, to

attribute to the definite action of the surrounding conditions. But it must

be borne in mind that there are a far greater number of animals and plants

which range widely and have been exposed to great diversities of

conditions, yet remain nearly uniform in character. Some authors, as

previously remarked, account for the varieties of our culinary and

agricultural plants by the definite action of the conditions to which they

have been exposed in the different parts of Great Britain; but there are

about 200 plants[710] which are found in every single English county; these

plants must have been exposed for an immense period to considerable

differences of climate and soil, yet do not differ. So, again, some birds,

insects, other animals, and plants range over large portions of the world,

yet retain the same character.

Notwithstanding the facts previously given on the occurrence of highly

peculiar local diseases and on the strange modifications of structure

in plants caused by the inoculated poison of insects, and other

analogous cases; still there are a multitude of variations--such as the

modified skull of the niata ox and bulldog, the long horns of Caffre

cattle, the conjoined toes of the solid-hoofed swine, the immense crest

and protuberant skull of Polish fowls, the crop of the pouter-pigeon,

and a host of other such cases--which we can hardly attribute to the

definite action, in the sense before specified, of the external

conditions of life. No doubt in every case there must have been some

exciting cause; but as we see innumerable individuals exposed to nearly

the same conditions, and one alone is affected, we may conclude that

the constitution of the individual is of far higher {286} importance

than the conditions to which it has been exposed. It seems, indeed, to

be a general rule that conspicuous variations occur rarely, and in one

individual alone out of many thousands, though all may have been

exposed, as far as we can judge, to nearly the same conditions. As the

most strongly marked variations graduate insensibly into the most

trifling, we are led by the same train of thought to attribute each

slight variation much more to innate differences of constitution,

however caused, than to the definite action of the surrounding

conditions.

We are led to the same conclusion by considering the cases, formerly

alluded to, of fowls and pigeons, which have varied and will no doubt

go on varying in directly opposite ways, though kept during many

generations under nearly the same conditions. Some, for instance, are

born with their beaks, wings, tails, legs, &c., a little longer, and

others with these same parts a little shorter. By the long-continued

selection of such slight individual differences, which occur in birds

kept in the same aviary, widely different races could certainly be

formed; and long-continued selection, important as is the result, does

nothing but preserve the variations which appear to us to arise

spontaneously.

In these cases we see that domesticated animals vary in an indefinite

number of particulars, though treated as uniformly as is possible. On

the other hand, there are instances of animals and plants, which,

though exposed to very different conditions, both under nature and

domestication, have varied in nearly the same manner. Mr. Layard

informs me that he has observed amongst the Caffres of South Africa a

dog singularly like an arctic Esquimaux dog. Pigeons in India present

nearly the same wide diversities of colour as in Europe; and I have

seen chequered and simply barred pigeons, and pigeons with blue and

white loins, from Sierra Leone, Madeira, England, and India. New

varieties of flowers are continually raised in different parts of Great

Britain, but many of these are found by the judges at our exhibitions

to be almost identical with old varieties. A vast number of new

fruit-trees and culinary vegetables have been produced in North

America: these differ from European varieties in the same general

manner as the several varieties raised in Europe differ from each

other; and no one has ever pretended that the climate of America has

given to the many American varieties any general character by which

they can be recognised. Nevertheless, from the facts previously

advanced on the authority of Mr. Meehan with respect to American and

European forest-trees, it would be rash to affirm that varieties raised

in the two countries would not in the course of ages assume a

distinctive character. Mr. Masters has recorded a striking fact[711]

bearing on this subject: he raised numerous plants of _Hybiscus

Syriacus_ from seed collected in South Carolina and the Holy Land,

where the parent-plants must have been exposed to considerably

different conditions; yet the seedlings from both localities broke into

two similar strains, one with obtuse leaves and purple or crimson

flowers, and the other with elongated leaves and more or less pink

flowers.

{287}

We may, also, infer the prepotent influence of the constitution of the

organism over the definite action of the conditions of life, from the

several cases given in the earlier chapters of parallel series of

varieties,--an important subject, hereafter to be more fully discussed.

Sub-varieties of the several kinds of wheat, gourds, peaches, and other

plants, and to a certain limited extent sub-varieties of the fowl,

pigeon, and dog, have been shown either to resemble or to differ from

each other in a closely corresponding and parallel manner. In other

cases, a variety of one species resembles a distinct species; or the

varieties of two distinct species resemble each other. Although these

parallel resemblances no doubt often result from reversion to the

former characters of a common progenitor; yet in other cases, when new

characters first appear, the resemblance must be attributed to the

inheritance of a similar constitution, and consequently to a tendency

to vary in the same manner. We see something of a similar kind in the

same monstrosity appearing and reappearing many times in the same

animal, and, as Dr. Maxwell Masters has remarked to me, in the same

plant.

We may at least conclude thus far, that the amount of modification which

animals and plants have undergone under domestication, does not correspond

with the degree to which they have been subjected to changed circumstances.

As we know the parentage of domesticated birds far better than of most

quadrupeds, we will glance through the list. The pigeon has varied in

Europe more than almost any other bird; yet it is a native species, and has

not been exposed to any extraordinary change of conditions. The fowl has

varied equally, or almost equally, with the pigeon, and is a native of the

hot jungles of India. Neither the peacock, a native of the same country,

nor the guinea-fowl, an inhabitant of the dry deserts of Africa, has varied

at all, or only in colour. The turkey, from Mexico, has varied but little.

The duck, on the other hand, a native of Europe, has yielded some

well-marked races; and as this is an aquatic bird, it must have been

subjected to a far more serious change in its habits than the pigeon or

even the fowl, which nevertheless have varied in a much higher degree. The

goose, a native of Europe and aquatic like the duck, has varied less than

any other domesticated bird, except the peacock.

Bud-variation is, also, important under our present point of view. In some

few cases, as when all the eyes or buds on the same tuber of the potato, or

all the fruit on the same plum-tree, or all the flowers on the same plant,

have suddenly varied in the same manner, it might be argued that the {288}

variation had been definitely caused by some change in the conditions to

which the plants had been exposed; yet, in other cases, such an admission

is extremely difficult. As new characters sometimes appear by

bud-variation, which do not occur in the parent-species or in any allied

species, we may reject, at least in these cases, the idea that they are due

to reversion. Now it is well worth while to reflect maturely on some

striking case of bud-variation, for instance that of the peach. This tree

has been cultivated by the million in various parts of the world, has been

treated differently, grown on its own roots and grafted on various stocks,

planted as a standard, against a wall, and under glass; yet each bud of

each sub-variety keeps true to its kind. But occasionally, at long

intervals of time, a tree in England, or under the widely-different climate

of Virginia, produces a single bud, and this yields a branch which ever

afterwards bears nectarines. Nectarines differ, as every one knows, from

peaches in their smoothness, size, and flavour; and the difference is so

great, that some botanists have maintained that they are specifically

distinct. So permanent are the characters thus suddenly acquired, that a

nectarine produced by bud-variation has propagated itself by seed. To guard

against the supposition that there is some fundamental distinction between

bud and seminal variation, it is well to bear in mind that nectarines have

likewise been produced from the stone of the peach; and, reversely, peaches

from the stone of the nectarine. Now is it possible to conceive external

conditions more closely alike than those to which the buds on the same tree

are exposed? Yet one bud alone, out of the many thousands borne by the same

tree, has suddenly without any apparent cause produced a nectarine. But the

case is even stronger than this, for the same flower-bud has yielded a

fruit, one-half or one-quarter a nectarine, and the other half or

three-quarters a peach. Again, seven or eight varieties of the peach have

yielded by bud-variation nectarines: the nectarines thus produced, no

doubt, differ a little from each other; but still they are nectarines. Of

course there must be some cause, internal or external, to excite the

peach-bud to change its nature; but I cannot imagine a class of facts

better adapted to force on our minds the conviction that what we call the

external conditions of life are quite insignificant in {289} relation to

any particular variation, in comparison with the organisation or

constitution of the being which varies.

It is known from the labours of Geoffroy St. Hilaire, and recently from

those of Dareste and others, that eggs of the fowl, if shaken, placed

upright, perforated, covered in part with varnish, &c., produce monstrous

chickens. Now these monstrosities may be said to be directly caused by such

unnatural conditions, but the modifications thus induced are not of a

definite nature. An excellent observer, M. Camille Dareste,[712] remarks

"that the various species of monstrosities are not determined by specific

causes; the external agencies which modify the development of the embryo

act solely in causing a perturbation--a perversion in the normal course of

development." He compares the result to what we see in illness: a sudden

chill, for instance, affects one individual alone out of many, causing

either a cold, or sore-throat, rheumatism, or inflammation of the lungs or

pleura. Contagious matter acts in an analogous manner.[713] We may take a

still more specific instance: seven pigeons were struck by

rattle-snakes;[714] some suffered from convulsions; some had their blood

coagulated, in others it was perfectly fluid; some showed ecchymosed spots

on the heart, others on the intestines, &c.; others again showed no visible

lesion in any organ. It is well known that excess in drinking causes

different diseases in different men; but men living under a cold and

tropical climate are differently affected:[715] and in this case we see the

definite influence of opposite conditions. The foregoing facts apparently

give us as good an idea as we are likely for a long time to obtain, how in

many cases external conditions act directly, though not definitely, in

causing modifications of structure.

* * * * *

_Summary._--There can be no doubt, from the facts given in the early part

of this chapter, that extremely slight changes in {290} the conditions of

life sometimes act in a definite manner on our already variable

domesticated productions; and, as the action of changed conditions in

causing general or indefinite variability is accumulative, so it may be

with their definite action. Hence it is possible that great and definite

modifications of structure may result from altered conditions acting during

a long series of generations. In some few instances a marked effect has

been produced quickly on all, or nearly all, the individuals which have

been exposed to some considerable change of climate, food, or other

circumstance. This has occurred, and is now occurring, with European men in

the United States, with European dogs in India, with horses in the Falkland

Islands, apparently with various animals at Angora, with foreign oysters in

the Mediterranean, and with maize grown in Europe from tropical seed. We

have seen that the chemical compounds secreted by plants and the state of

their tissues are readily affected by changed conditions. In some cases a

relation apparently exists between certain characters and certain

conditions, so that if the latter be changed the character is lost--as with

cultivated flowers, with some few culinary plants, with the fruit of the

melon, with fat-tailed sheep, and other sheep having peculiar fleeces.

The production of galls, and the change of plumage in parrots when fed on

peculiar food or when inoculated by the poison of a toad, prove to us what

great and mysterious changes in structure and colour may be the definite

result of chemical changes in the nutrient fluids or tissues.

We have also reason to believe that organic beings in a state of nature may

be modified in various definite ways by the conditions to which they have

been long exposed, as in the case of American trees in comparison with

their representatives in Europe. But in all such cases it is most difficult

to distinguish between the definite results of changed conditions, and the

accumulation through natural selection of serviceable variations which have

arisen independently of the nature of the conditions. If, for instance, a

plant had to be modified so as to become fitted to inhabit a humid instead

of an arid station, we have no reason to believe that variations of the

right kind would occur more frequently if the parent-plant inhabited a

station a little more {291} humid than usual. Whether the station was

unusually dry or humid, variations adapting the plant in a slight degree

for directly opposite habits of life would occasionally arise, as we have

reason to believe from what we know in other cases.

In most, perhaps in all cases, the organisation or constitution of the

being which is acted on, is a much more important element than the nature

of the changed conditions, in determining the nature of the variation. We

have evidence of this in the appearance of nearly similar modifications

under different conditions, and of different modifications under apparently

nearly the same conditions. We have still better evidence of this in

closely parallel varieties being frequently produced from distinct races,

or even distinct species, and in the frequent recurrence of the same

monstrosity in the same species. We have also seen that the degree to which

domesticated birds have varied, does not stand in any close relation with

the amount of change to which they have been subjected.

To recur once again to bud-variations. When we reflect on the millions of

buds which many trees have produced, before some one bud has varied, we are

lost in wonder what the precise cause of each variation can be. Let us

recall the case given by Andrew Knight of the forty-year-old tree of the

yellow magnum bonum plum, an old variety which has been propagated by

grafts on various stocks for a very long period throughout Europe and North

America, and on which a single bud suddenly produced the red magnum bonum.

We should also bear in mind that distinct varieties, and even distinct

species,--as in the case of peaches, nectarines, and apricots,--of certain

roses and camellias,--although separated by a vast number of generations

from any progenitor in common, and although cultivated under diversified

conditions, have yielded by bud-variation closely analogous varieties. When

we reflect on these facts we become deeply impressed with the conviction

that in such cases the nature of the variation depends but little on the

conditions to which the plant has been exposed, and not in any especial

manner on its individual character, but much more on the general nature or

constitution, inherited from some remote progenitor, of the whole group of

allied beings to which the plant belongs. We are thus driven to conclude

that in most {292} cases the conditions of life play a subordinate part in

causing any particular modification; like that which a spark plays, when a

mass of combustibles bursts into flame--the nature of the flame depending

on the combustible matter, and not on the spark.

No doubt each slight variation must have its efficient cause; but it is as

hopeless an attempt to discover the cause of each as to say why a chill or

a poison affects one man differently from another. Even with modifications

resulting from the definite action of the conditions of life, when all or

nearly all the individuals, which have been similarly exposed, are

similarly affected, we can rarely see the precise relation between cause

and effect. In the next chapter it will be shown that the increased use or

disuse of various organs, produces an inherited effect. It will further be

seen that certain variations are bound together by correlation and other

laws. Beyond this we cannot at present explain either the causes or manner

of action of Variation.

Finally, as indefinite and almost illimitable variability is the usual

result of domestication and cultivation, with the same part or organ

varying in different individuals in different or even in directly opposite

ways; and as the same variation, if strongly pronounced, usually recurs

only after long intervals of time, any particular variation would generally

be lost by crossing, reversion, and the accidental destruction of the

varying individuals, unless carefully preserved by man. Hence, although it

must be admitted that new conditions of life do sometimes definitely affect

organic beings, it may be doubted whether well-marked races have often been

produced by the direct action of changed conditions without the aid of

selection either by man or nature.

* * * * *

{293}

CHAPTER XXIV.

LAWS OF VARIATION--USE AND DISUSE, ETC.

NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION--ON

THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS--CHANGED HABITS

OF LIFE--ACCLIMATISATION WITH ANIMALS AND PLANTS--VARIOUS METHODS BY

WHICH THIS CAN BE EFFECTED--ARRESTS OF DEVELOPMENT--RUDIMENTARY ORGANS.

In this and the two following chapters I shall discuss, as well as the

difficulty of the subject permits, the several laws which govern

Variability. These may be grouped under the effects of use and disuse,

including changed habits and acclimatisation--arrests of

development--correlated variation--the cohesion of homologous parts--the

variability of multiple parts--compensation of growth--the position of buds

with respect to the axis of the plant--and lastly, analogous variation.

These several subjects so graduate into each other that their distinction

is often arbitrary.

It may be convenient first briefly to discuss that co-ordinating and

reparative power which is common, in a higher or lower degree, to all

organic beings, and which was formerly designated by physiologists as the

_nisus formativus_.

Blumenbach and others[716] have insisted that the principle which

permits a Hydra, when cut into fragments, to develop itself into two or

more perfect animals, is the same with that which causes a wound in the

higher animals to heal by a cicatrice. Such cases as that of the Hydra

are evidently analogous with the spontaneous division or fissiparous

generation of the lowest animals, and likewise with the budding of

plants. Between these extreme cases and that of a mere cicatrice we

have every gradation. Spallanzani,[717] by cutting off the legs and

tail of a Salamander, got in the course of three months six crops of

these members; so that 687 perfect bones were reproduced by one animal

during one season. At whatever {294} point the limb was cut off, the

deficient part, and no more, was exactly reproduced. Even with man, as

we have seen in the twelfth chapter, when treating of polydactylism,

the entire limb whilst in an embryonic state, and supernumerary digits,

are occasionally, though imperfectly, reproduced after amputation. When

a diseased bone has been removed, a new one sometimes "gradually

assumes the regular form, and all the attachments of muscles,

ligaments, &c., become as complete as before."[718]

This power of regrowth does not, however, always act perfectly: the

reproduced tail of a lizard differs in the forms of the scales from the

normal tail: with certain Orthopterous insects the large hind legs are

reproduced of smaller size:[719] the white cicatrice which in the

higher animals unites the edges of a deep wound is not formed of

perfect skin, for elastic tissue is not produced till long

afterwards.[720] "The activity of the _nisus formativus_," says

Blumenbach, "is in an inverse ratio to the age of the organised body."

To this may be added that its power is greater in animals the lower

they are in the scale of organisation; and animals low in the scale

correspond with the embryos of higher animals belonging to the same

class. Newport's observations[721] afford a good illustration of this

fact, for he found that "myriapods, whose highest development scarcely

carries them beyond the larvae of perfect insects, can regenerate limbs

and antennae up to the time of their last moult;" and so can the larvae

of true insects, but not the mature insect. Salamanders correspond in

development with the tadpoles or larvae of the tailless Batrachians,

and both possess to a large extent the power of regrowth; but not so

the mature tailless Batrachians.

Absorption often plays an important part in the repairs of injuries.

When a bone is broken, and does not unite, the ends are absorbed and

rounded, so that a false joint is formed; or if the ends unite, but

overlap, the projecting parts are removed.[722] But absorption comes

into action, as Virchow remarks, during the normal growth of bones;

parts which are solid during youth become hollowed out for the

medullary tissue as the bone increases in size. In trying to understand

the many well-adapted cases of regrowth when aided by absorption, we

should remember that most parts of the organisation, even whilst

retaining the same form, undergo constant renewal; so that a part which

was not renewed would naturally be liable to complete absorption.

Some cases, usually classed under the so-called _nisus formativus_, at

first appear to come under a distinct head; for not only are old

structures reproduced, but structures which appear new are formed.

Thus, after inflammation "false membranes," furnished with

blood-vessels, lymphatics, and nerves, are developed; or a foetus

escapes from the Fallopian tubes, and falls into the abdomen, "nature

pours out a quantity of plastic lymph, which forms itself into

organised membrane, richly supplied with blood-vessels," and the foetus

is nourished for a time. In certain cases of {295} hydrocephalus the

open and dangerous spaces in the skull are filled up with new bones,

which interlock by perfect serrated sutures.[723] But most

physiologists, especially on the Continent, have now given up the

belief in plastic lymph or blastema, and Virchow[724] maintains that

every structure, new or old, is formed by the proliferation of

pre-existing cells. On this view false membranes, like cancerous or

other tumours, are merely abnormal developments of normal growths; and

we can thus understand how it is that they resemble adjoining

structures; for instance, that "false membrane in the serous cavities

acquires a covering of epithelium exactly like that which covers the

original serous membrane; adhesions of the iris may become black

apparently from the production of pigment-cells like those of the

uvea."[725]

No doubt the power of reparation, though not always quite perfect, is

an admirable provision, ready for various emergencies, even for those

which occur only at long intervals of time.[726] Yet this power is not

more wonderful than the growth and development of every single

creature, more especially of those which are propagated by fissiparous

generation. This subject has been here noticed, because we may infer

that, when any part or organ is either greatly increased in size or

wholly suppressed through variation and continued selection, the

co-ordinating power of the organisation will continually tend to bring

all the parts again into harmony with each other.

_On the Effects of the Increased Use and Disuse of Organs._

It is notorious, and we shall immediately adduce proofs, that increased use

or action strengthens muscles, glands, sense-organs, &c.; and that disuse,

on the other hand, weakens them. I have not met with any clear explanation

of this fact in works on Physiology. Mr. Herbert Spencer[727] maintains

that when muscles are much used, or when intermittent pressure is applied

to the epidermis, an excess of nutritive matter exudes from the vessels,

and that this gives additional development to the adjoining parts. That an

increased flow of blood towards an organ leads to its greater development

is probable, if not certain. Mr. Paget[728] thus accounts for the long,

thick, and dark-coloured hair which occasionally grows, even in young

children, near old-standing inflamed surfaces or fractured bones. When

Hunter {296} inserted the spur of a cock into the comb, which is well

supplied with blood-vessels, it grew in one case in a spiral direction to a

length of six inches, and in another case forward, like a horn, so that the

bird could not touch the ground with its beak. But whether Mr. Herbert

Spencer's view of the exudation of nutritive matter due to increased

movement and pressure, will fully account for the augmented size of bones,

ligaments, and especially of internal glands and nerves, seems doubtful.

According to the interesting observations of M. Sedillot,[729] when a

portion of one bone of the leg or fore-arm of an animal is removed and is

not replaced by growth, the associated bone enlarges till it attains a bulk

equal to that of the two bones, of which it has to perform the functions.

This is best exhibited in dogs in which the tibia has been removed; the

companion bone, which is naturally almost filiform and not one-fifth the

size of the other, soon acquires a size equal to or greater than the tibia.

Now, it is at first difficult to believe that increased weight acting on a

straight bone could, by alternately increased and diminished pressure,

cause nutritive matter to exude from the vessels which permeate the

periosteum. Nevertheless, the observations adduced by Mr. Spencer,[730] on

the strengthening of the bowed bones of rickety children, along their

concave sides, leads to the belief that this is possible.

Mr. H. Spencer has also shown that the ascent of the sap in trees is aided

by the rocking movement caused by the wind; and the sap strengthens the

trunk "in proportion to the stress to be borne; since the more severe and

the more repeated the strains, the greater must be the exudation from the

vessels into the surrounding tissue, and the greater the thickening of this

tissue by secondary deposits."[731] But woody trunks may be formed of hard

tissue without their having been subjected to any movement, as we see with

ivy closely attached to old walls. In all these cases, it is very difficult

to disentangle the effects of long-continued selection from those

consequent on the increased action or movement of the part. Mr. H.

Spencer[732] acknowledges this difficulty, and gives as an instance the

spines {297} or thorns of trees, and the shells of nuts. Here we have

extremely hard woody tissue without the possibility of any movement to

cause exudation, and without, as far as we can see, any other directly

exciting cause; and as the hardness of these parts is of manifest service

to the plant, we may look at the result as probably due to the selection of

so-called spontaneous variations. Every one knows that hard work thickens

the epidermis on the hands; and when we hear that with infants long before

their birth the epidermis is thicker on the palms and soles of the feet

than on any other part of the body, as was observed with admiration by

Albinus,[733] we are naturally inclined to attribute this to the inherited

effects of long-continued use or pressure. We are tempted to extend the

same view even to the hoofs of quadrupeds; but who will pretend to

determine how far natural selection may have aided in the formation of

structures of such obvious importance to the animal?

That use strengthens the muscles may be seen in the limbs of artisans

who follow different trades; and when a muscle is strengthened, the

tendons, and the crests of bone to which they are attached, become

enlarged; and this must likewise be the case with the blood-vessels and

nerves. On the other hand, when a limb is not used, as by Eastern

fanatics, or when the nerve supplying it with nervous power is

effectually destroyed, the muscles wither. So again, when the eye is

destroyed the optic nerve becomes atrophied, sometimes even in the

course of a few months.[734] The Proteus is furnished with branchiae as

well as with lungs: and Schreibers[735] found that when the animal was

compelled to live in deep water the branchiae were developed to thrice

their ordinary size, and the lungs were partially atrophied. When, on

the other hand, the animal was compelled to live in shallow water, the

lungs became larger and more vascular, whilst the branchiae disappeared

in a more or less complete degree. Such modifications as these are,

however, of comparatively little value for us, as we do not actually

know that they tend to be inherited.

In many cases there is reason to believe that the lessened use of

various organs has affected the corresponding parts in the offspring.

But there is no good evidence that this ever follows in the course of a

single generation. It appears, as in the case of general or indefinite

variability, that several generations must be subjected to changed

habits for any appreciable result. Our domestic fowls, ducks, and geese

have almost lost, not {298} only in the individual but in the race,

their power of flight; for we do not see a chicken, when frightened,

take flight like a young pheasant. Hence I was led carefully to compare

the limb-bones of fowls, ducks, pigeons, and rabbits, with the same

bones in the wild parent-species. As the measurements and weights were

fully given in the earlier chapters, I need here only recapitulate the

results. With domestic pigeons, the length of the sternum, the

prominence of its crest, the length of the scapulae and furcula, the

length of the wings as measured from tip to tip of the radius, are all

reduced relatively to the same parts in the wild pigeon. The wing and

tail feathers, however, are increased in length, but this may have as

little connection with the use of the wings or tail, as the lengthened

hair on a dog with the amount of exercise which the breed has

habitually taken. The feet of pigeons, except in the long-beaked races,

are reduced in size. With fowls the crest of the sternum is less

prominent, and is often distorted or monstrous; the wing-bones have

become lighter relatively to the leg-bones, and are apparently a little

shorter in comparison with those of the parent-form, the _Gallus

bankiva_. With ducks, the crest of the sternum is affected in the same

manner as in the foregoing cases: the furcula, coracoids, and scapulae

are all reduced in weight relatively to the whole skeleton: the bones

of the wings are shorter and lighter, and the bones of the legs longer

and heavier, relatively to each other, and relatively to the whole

skeleton, in comparison with the same bones in the wild-duck. The

decreased weight and size of the bones, in the foregoing cases, is

probably the indirect result of the reaction of the weakened muscles on

the bones. I failed to compare the feathers of the wings of the tame

and wild duck; but Gloger[736] asserts that in the wild duck the tips

of the wing-feathers reach almost to the end of the tail, whilst in the

domestic duck they often hardly reach to its base. He remarks, also, on

the greater thickness of the legs, and says that the swimming membrane

between the toes is reduced; but I was not able to detect this latter

difference.

With the domesticated rabbit the body, together with the whole

skeleton, is generally larger and heavier than in the wild animal, and

the leg-bones are heavier in due proportion; but whatever standard of

comparison be taken, neither the leg-bones nor the scapulae have

increased in length proportionally with the increased dimensions of the

rest of the skeleton. The skull has become in a marked manner narrower,

and, from the measurements of its capacity formerly given, we may

conclude, that this narrowness results from the decreased size of the

brain, consequent on the mentally inactive life led by these

closely-confined animals.

We have seen in the eighth chapter that silk-moths, which have been

kept during many centuries closely confined, emerge from their cocoons

with their wings distorted, incapable of flight, often greatly reduced

in size, or even, according to Quatrefages, quite rudimentary. This

condition of the wings may be largely owing to the same kind of

monstrosity which often affects wild Lepidoptera when artificially

reared from the cocoon; or it may {299} be in part due to an inherent

tendency, which is common to the females of many Bombycidae, to have

their wings in a more or less rudimentary state; but part of the effect

may probably be attributed to long-continued disuse.

From the foregoing facts there can be no doubt that certain parts of the

skeleton in our anciently domesticated animals, have been modified in

length and weight by the effects of decreased or increased use; but they

have not been modified, as shown in the earlier chapters, in shape or

structure. We must, however, be cautious in extending this latter

conclusion to animals living a free life; for these will occasionally be

exposed during successive generations to the severest competition. With

wild animals it would be an advantage in the struggle for life that every

superfluous and useless detail of structure should be removed or absorbed;

and thus the reduced bones might ultimately become changed in structure.

With highly-fed domesticated animals, on the other hand, there is no

economy of growth; nor any tendency to the elimination of trifling and

superfluous details of structure.

Turning now to more general observations, Nathusius has shown that, with

the improved races of the pig, the shortened legs and snout, the form of

the articular condyles of the occiput, and the position of the jaws with

the upper canine teeth projecting in a most anomalous manner in front of

the lower canines, may be attributed to these parts not having been fully

exercised. For the highly-cultivated races do not travel in search of food,

nor root up the ground with their ringed muzzles. These modifications of

structure, which are all strictly inherited, characterise several improved

breeds, so that they cannot have been derived from any single domestic or

wild stock.[737] With respect to cattle, Professor Tanner has remarked that

the lungs and liver in the improved breeds "are found to be considerably

reduced in size when compared with those possessed by animals having

perfect liberty;"[738] and the reduction of these organs affects the

general shape of the body. The cause of the reduced lungs in highly-bred

animals which take little exercise is {300} obvious; and perhaps the liver

may be affected by the nutritious and artificial food on which they largely

subsist.

It is well known that, when an artery is tied, the anastomosing

branches, from being forced to transmit more blood, increase in

diameter; and this increase cannot be accounted for by mere extension,

as their coats gain in strength. Mr. Herbert Spencer[739] has argued

that with plants the flow of sap from the point of supply to the

growing part first elongates the cells in this line; and that the cells

then become confluent, thus forming the ducts; so that, on this view,

the vessels in plants are formed by the mutual reaction of the flowing

sap and cellular tissue. Dr. W. Turner has remarked,[740] with respect

to the branches of arteries, and likewise to a certain extent with

nerves, that the great principle of compensation frequently comes into

play; for "when two nerves pass to adjacent cutaneous areas, an inverse

relation as regards size may subsist between them; a deficiency in one

may be supplied by an increase in the other, and thus the area of the

former may be trespassed on by the latter nerve." But how far in these

cases the difference in size in the nerves and arteries is due to

original variation, and how far to increased use or action, is not

clear.

In reference to glands, Mr. Paget observes that "when one kidney is

destroyed the other often becomes much larger, and does double

work."[741] If we compare the size of the udders and their power of

secretion in cows which have been long domesticated, and in certain

goats in which the udders nearly touch the ground, with the size and

power of secretion of these organs in wild or half-domesticated

animals, the difference is great. A good cow with us daily yields more

than five gallons, or forty pints of milk, whilst a first-rate animal,

kept, for instance, by the Damaras of South Africa,[742] "rarely gives

more than two or three pints of milk daily, and, should her calf be

taken from her, she absolutely refuses to give any." We may attribute

the excellence of our cows, and of certain goats, partly to the

continued selection of the best milking animals, and partly to the

inherited effects of the increased action, through man's art, of the

secreting glands.

It is notorious, as was remarked in the twelfth chapter, that

short-sight is inherited; and if we compare watchmakers or engravers

with, for instance, sailors, we can hardly doubt that vision

continually directed towards a near object permanently affects the

structure of the eye.

Veterinarians are unanimous that horses become affected with spavins,

splints, ringbones, &c., from being shod, and from travelling on hard

roads, and they are almost equally unanimous that these injuries are

transmitted. Formerly horses were not shod in North Carolina, and it

has been asserted that they did not then suffer from these diseases of

the legs and feet.[743]

{301}

Our domesticated quadrupeds are all descended, as far as is known, from

species having erect ears; yet few kinds can be named, of which at least

one race has not drooping ears. Cats in China, horses in parts of Russia,

sheep in Italy and elsewhere, the guinea-pig in Germany, goats and cattle

in India, rabbits, pigs, and dogs in all long-civilised countries, have

dependent ears. With wild animals, which constantly use their ears like

funnels to catch every passing sound, and especially to ascertain the

direction whence it comes, there is not, as Mr. Blyth has remarked, any

species with drooping ears except the elephant. Hence the incapacity to

erect the ears is certainly in some manner the result of domestication; and

this incapacity has been attributed by various authors[744] to disuse, for

animals protected by man are not compelled habitually to use their ears.

Col. Hamilton Smith[745] states that in ancient effigies of the dog, "with

the exception of one Egyptian instance, no sculpture of the earlier Grecian

era produces representations of hounds with completely drooping ears; those

with them half pendulous are missing in the most ancient; and this

character increases, by degrees, in the works of the Roman period." Godron

also has remarked that "the pigs of the ancient Egyptians had not their

ears enlarged and pendent."[746] But it is remarkable that the drooping of

the ears, though probably the effect of disuse, is not accompanied by any

decrease in size; on the contrary, when we remember that animals so

different as fancy rabbits, certain Indian breeds of the goat, our petted

spaniels, bloodhounds, and other dogs, have enormously elongated ears, it

would appear as if disuse actually caused an increase in length. With

rabbits, the drooping of the much elongated ears has affected even the

structure of the skull.

The tail of no wild animal, as remarked to me by Mr. Blyth, is curled;

whereas pigs and some races of dogs have their tails much curled. This

deformity, therefore, appears to be the result of domestication, but

whether in any way connected with the lessened use of the tail is doubtful.

{302}

The epidermis on our hands is easily thickened, as every one knows, by hard

work. In a district of Ceylon the sheep have "horny callosities that defend

their knees, and which arise from their habit of kneeling down to crop the

short herbage, and this distinguishes the Jaffna flocks from those of other

portions of the island;" but it is not stated whether this peculiarity is

inherited.[747]

The mucous membrane which lines the stomach is continuous with the external

skin of the body; therefore it is not surprising that its texture should be

affected by the nature of the food consumed, but other and more interesting

changes likewise follow. Hunter long ago observed that the muscular coat of

the stomach of a gull (_Larus tridactylus_) which had been fed for a year

chiefly on grain was thickened; and, according to Dr. Edmondston, a similar

change periodically occurs in the Shetland Islands in the stomach of the

_Larus argentatus_, which in the spring frequents the corn-fields and feeds

on the seed. The same careful observer has noticed a great change in the

stomach of a raven which had been long fed on vegetable food. In the case

of an owl (_Strix grallaria_) similarly treated, Menetries states that the

form of the stomach was changed, the inner coat became leathery, and the

liver increased in size. Whether these modifications in the digestive

organs would in the course of generations become inherited is not

known.[748]

The increased or diminished length of the intestines, which apparently

results from changed diet, is a more remarkable case, because it is

characteristic of certain animals in their domesticated condition, and

therefore must be inherited. The complex absorbent system, the

blood-vessels, nerves, and muscles, are necessarily all modified together

with the intestines. According to Daubenton, the intestines of the domestic

cat are one-third longer than those of the wild cat of Europe; and although

this species is not the parent-stock of the domestic animal, yet, as

Isidore Geoffroy has remarked, the several species {303} of cats are so

closely allied that the comparison is probably a fair one. The increased

length appears to be due to the domestic cat being less strictly

carnivorous in its diet than any wild feline species; I have seen a French

kitten eating vegetables as readily as meat. According to Cuvier, the

intestines of the domesticated pig exceed greatly in proportionate length

those of the wild boar. In the tame and wild rabbit the change is of an

opposite nature, and probably results from the nutritious food given to the

tame rabbit.[749]

* * * * *

_Changed Habits of Life, independently of the Use or Disuse of particular

Organs._--This subject, as far as the mental powers of animals are

concerned, so blends into instinct, on which I shall treat in a future

work, that I will here only remind the reader of the many cases which occur

under domestication, and which are familiar to every one--for instance the

tameness of our animals--the pointing or retrieving of dogs--their not

attacking the smaller animals kept by man--and so forth. How much of these

changes ought to be attributed to inherited habit, and how much to the

selection of individuals which have varied in the desired manner,

irrespectively of the special circumstances under which they have been

kept, can seldom be told. We have already seen that animals may be

habituated to a changed diet; but a few additional instances may here be

given.

In the Polynesian Islands and in China the dog is fed exclusively on

vegetable matter, and the taste for this kind of food is to a certain

extent inherited.[750] Our sporting dogs will not touch the bones of game

birds, whilst other dogs devour them with greediness. In some parts of the

world sheep have been largely fed on fish. The domestic hog is fond of

barley, the wild boar is said to disdain it; and the disdain is partially

inherited, for some young wild pigs bred in captivity showed an aversion

for this grain, whilst others of the same brood relished it.[751] One of my

relations bred some young pigs from {304} a Chinese sow by a wild Alpine

boar; they lived free in the park, and were so tame that they came to the

house to be fed; but they would not touch swill, which was devoured by the

other pigs. An animal when once accustomed to an unnatural diet, which can

generally be effected only during youth, dislikes its proper food, as

Spallanzani found to be the case with a pigeon which had been long fed on

meat. Individuals of the same species take to new food with different

degrees of readiness; one horse, it is stated, soon learned to eat meat,

whilst another would have perished from hunger rather than have partaken of

it.[752]

The caterpillars of the _Bombyx hesperus_ feed in a state of nature on the

leaves of the _Cafe diable_, but, after having been reared on the

Ailanthus, they would not touch the _Cafe diable_, and actually died of

hunger.[753]

It has been found possible to accustom marine fish to live in fresh water;

but as such changes in fish, and other marine animals, have been chiefly

observed in a state of nature, they do not properly belong to our present

subject. The period of gestation and of maturity, as shown in the earlier

chapters,--the season and the frequency of the act of breeding,--have all

been greatly modified under domestication. With the Egyptian goose the rate

of change in the season has been recorded.[754] The wild drake pairs with

one female, the domestic drake is polygamous. Certain breeds of fowls have

lost the habit of incubation. The paces of the horse, and the manner of

flight in certain breeds of the pigeon, have been modified, and are

inherited. The voice differs much in certain fowls and pigeons. Some breeds

are clamorous and others silent, as in the Call and common duck, or in the

Spitz and pointer dog. Every one knows how dogs differ from each other in

their manner of hunting, and in their ardour after different kinds of game

or vermin.

With plants the period of vegetation is easily changed and is inherited, as

in the case of summer and winter wheat, barley, {305} and vetches; but to

this subject we shall immediately return under acclimatisation. Annual

plants sometimes become perennial under a new climate, as I hear from Dr.

Hooker is the case with the stock and mignonette in Tasmania. On the other

hand, perennials sometimes become annuals, as with the Ricinus in England,

and as, according to Captain Mangles, with many varieties of the

heartsease. Von Berg[755] raised from seed of _Verbascum phoenicium_, which

is usually a biennial, both annual and perennial varieties. Some deciduous

bushes become evergreen in hot countries.[756] Rice requires much water,

but there is one variety in India which can be grown without

irrigation.[757] Certain varieties of the oat and of our other cereals are

best fitted for certain soils.[758] Endless similar facts could be given in

the animal and vegetable kingdoms. They are noticed here because they

illustrate analogous differences in closely allied natural species, and

because such changed habits of life, whether due to use and disuse, or to

the direct action of external conditions, or to so-called spontaneous

variation, would be apt to lead to modifications of structure.

* * * * *

_Acclimatisation._--From the previous remarks we are naturally led to the

much disputed subject of acclimatisation. There are two distinct questions:

Do varieties descended from the same species differ in their power of

living under different climates? And secondly, if they so differ, how have

they become thus adapted? We have seen that European dogs do not succeed

well in India, and it is asserted,[759] that no one has succeeded in there

keeping the Newfoundland long alive; but then it may be argued, probably

with truth, that these northern breeds are specifically distinct from the

native dogs which flourish in India. The same remark may be made with

respect to different breeds of sheep, of which, according to Youatt,[760]

not one brought "from a torrid climate lasts out the second year," in the

Zoological Gardens. But sheep are capable of some degree of

acclimatisation, for Merino sheep bred at the Cape of Good Hope have been

found {306} far better adapted for India than those imported from

England.[761] It is almost certain that the breeds of the fowl are

descended from the same species; but the Spanish breed, which there is good

reason to believe originated near the Mediterranean,[762] though so fine

and vigorous in England, suffers more from frost than any other breed. The

Arrindy silk-moth introduced from Bengal, and the Ailanthus moth from the

temperate province of Shan Tung, in China, belong to the same species, as

we may infer from their identity in the caterpillar, cocoon, and mature

states;[763] yet they differ much in constitution: the Indian form "will

flourish only in warm latitudes," the other is quite hardy and withstands

cold and rain.

Plants are more strictly adapted to climate than are animals. The

latter when domesticated withstand such great diversities of climate,

that we find nearly the same species in tropical and temperate

countries; whilst the cultivated plants are widely dissimilar. Hence a

larger field is open for inquiry in regard to the acclimatisation of

plants than of animals. It is no exaggeration to say that with almost

every plant which has long been cultivated varieties exist, which are

endowed with constitutions fitted for very different climates; I will

select only a few of the more striking cases, as it would be tedious to

give all. In North America numerous fruit-trees have been raised, and

in horticultural publications,--for instance, in Downing,--lists are

given of the varieties which are best able to withstand the severe

climate of the northern States and Canada. Many American varieties of

the pear, plum, and peach are excellent in their own country, but until

recently hardly one was known that succeeded in England; and with

apples,[764] not one succeeds. Though the American varieties can

withstand a severer winter than ours, the summer here is not hot

enough. Fruit-trees have originated in Europe as in America with

different constitutions, but they are not here much noticed, as the

same nurserymen do not supply a wide area. The Forelle pear flowers

early, and when the flowers have just set, and this is the critical

period, they have been observed, both in France and England, to

withstand with complete impunity a frost of 18deg and even 14deg Fahr.,

which killed the flowers, whether fully expanded or in bud, of all

other kinds of pears.[765] This power in the flower of resisting cold

and afterwards producing fruit does not invariably depend, as we know

on good authority,[766] on general constitutional vigour.

{307}

In proceeding northward, the number of varieties which are enabled to

resist the climate rapidly decreases, as may be seen in the list of the

varieties of the cherry, apple, and pear, which can be cultivated in

the neighbourhood of Stockholm.[767] Near Moscow, Prince Troubetzkoy

planted for experiment in the open ground several varieties of the

pear, but one alone, the _Poire sans Pepins_, withstood the cold of

winter.[768] We thus see that our fruit-trees, like distinct species of

the same genus, certainly differ from each other in their

constitutional adaptation to different climates.

With the varieties of many plants, the adaptation to climate is often

very close. Thus it has been proved by repeated trials "that few if any

of the English varieties of wheat are adapted for cultivation in

Scotland;"[769] but the failure in this case is at first only in the

quantity, though ultimately in the quality, of the grain produced. The

Rev. J. M. Berkeley sowed wheat-seed from India, and got "the most

meagre ears," on land which would certainly have yielded a good crop

from English wheat.[770] In these cases varieties have been carried

from a warmer to a cooler climate; in the reverse case, as "when wheat

was imported directly from France into the West Indian Islands, it

produced either wholly barren spikes or furnished with only two or

three miserable seeds, while West Indian seed by its side yielded an

enormous harvest."[771] Here is another case of close adaptation to a

slightly cooler climate; a kind of wheat which in England may be used

indifferently either as a winter or summer variety, when sown under the

warmer climate of Grignan, in France, behaved exactly as if it had been

a true winter wheat.[772]

Botanists believe that all the varieties of maize belong to the same

species; and we have seen that in North America, in proceeding

northward, the varieties cultivated in each zone produce their flowers

and ripen their seed within shorter and shorter periods. So that the

tall, slowly maturing southern varieties do not succeed in New England,

and the New English varieties do not succeed in Canada. I have not met

with any statement that the southern varieties are actually injured or

killed by a degree of cold which the northern varieties withstand with

impunity, though this is probable; but the production of early

flowering and early seeding varieties deserves to be considered as one

form of acclimatisation. Hence it has been found possible, according to

Kalm, to cultivate maize further and further northwards in America. In

Europe, also, as we learn from the evidence given by Alph. De Candolle,

the culture of maize has extended since the end of the last century

thirty leagues north of its former boundary.[773] On the authority of

the great Linnaeus,[774] I may quote an {308} analogous case, namely,

that in Sweden tobacco raised from home-grown seed ripens its seed a

month sooner and is less liable to miscarry than plants raised from

foreign seed.

With the Vine, differently from the maize, the line of practical

culture has retreated a little southward since the middle ages;[775]

but this seems due to commerce, including that of wine, being now freer

or more easy. Nevertheless the fact of the vine not having spread

northward shows that acclimatisation has made no progress during

several centuries. There is, however, a marked difference in the

constitution of the several varieties,--some being hardy, whilst

others, like the muscat of Alexandria, require a very high temperature

to come to perfection. According to Labat,[776] vines taken from France

to the West Indies succeed with extreme difficulty, whilst those

imported from Madeira, or the Canary Islands, thrive admirably.

Gallesio gives a curious account of the naturalisation of the Orange in

Italy. Daring many centuries the sweet orange was propagated

exclusively by grafts, and so often suffered from frosts that it

required protection. After the severe frost of 1709, and more

especially after that of 1763, so many trees were destroyed that

seedlings from the sweet orange were raised, and, to the surprise of

the inhabitants, their fruit was found to be sweet. The trees thus

raised were larger, more productive, and hardier than the former kinds;

and seedlings are now continually raised. Hence Gallesio concludes that

much more was effected for the naturalisation of the orange in Italy by

the accidental production of new kinds during a period of about sixty

years, than had been effected by grafting old varieties during many

ages.[777] I may add that Risso[778] describes some Portuguese

varieties of the orange as extremely sensitive to cold, and as much

tenderer than certain other varieties.

The peach was known to Theophrastus, 322 B.C.[779] According to the

authorities quoted by Dr. F. Rolle,[780] it was tender when first

introduced into Greece, and even in the island of Rhodes only

occasionally bore fruit. If this be correct, the peach, in spreading

during the last two thousand years over the middle parts of Europe,

must have become much hardier. At the present day different varieties

differ much in hardiness: some French varieties will not succeed in

England; and near Paris, the _Pavie de Bonneuil_ does not ripen its

fruit till very late, even when grown on a wall; "it is, therefore,

only fit for a very hot southern climate."[781]

I will briefly give a few other cases. A variety of _Magnolia

grandiflora_, raised by M. Roy, withstands cold several degrees lower

than that which any other variety can resist. With camellias there is

much difference in hardiness. One particular variety of Noisette rose

withstood the severe frost of 1860 "untouched and hale amidst a

universal destruction of other {309} Noisettes." In New York the "Irish

yew is quite hardy, but the common yew is liable to be cut down." I may

add that there are varieties of the sweet potato (_Convolvulus

batatas_) which are suited for warmer, as well as for colder,

climates.[782]

The plants as yet mentioned have been found capable of resisting an unusual

degree of cold or heat, when fully grown. The following cases refer to

plants whilst young. In a large bed of young Araucarias of the same age,

growing close together and equally exposed, it was observed,[783] after the

unusually severe winter of 1860-61, that, "in the midst of the dying,

numerous individuals remained on which the frost had absolutely made no

kind of impression." Dr. Lindley, after alluding to this and other similar

cases, remarks, "Among the lessons which the late formidable winter has

taught us, is that, even in their power of resisting cold, individuals of

the same species of plants are remarkably different." Near Salisbury, there

was a sharp frost on the night of May 24th, 1836, and all the French beans

(_Phaseolus vulgaris_) in a bed were killed except about one in thirty,

which completely escaped.[784] On the same day of the month, but in the

year 1864, there was a severe frost in Kent, and two rows of

scarlet-runners (_P. multiflorus_) in my garden, containing 390 plants of

the same age and equally exposed, were all blackened and killed except

about a dozen plants. In an adjoining row of "Fulmer's dwarf bean" (_P.

vulgaris_), one single plant escaped. A still more severe frost occurred

four days afterwards, and of the dozen plants which had previously escaped

only three survived; these were not taller or more vigorous than the other

young plants, but they escaped completely, with not even the tips of their

leaves browned. It was impossible to behold these three plants, with their

blackened, withered, and dead brethren all round them, and not see at a

glance that they differed widely in constitutional power of resisting

frost.

This work is not the proper place to show that wild plants {310} of the

same species, naturally growing at different altitudes or under different

latitudes, become to a certain extent acclimatised, as is proved by the

different behaviour of their seedlings when raised in England. In my

'Origin of Species' I have alluded to some cases, and I could add others.

One instance must suffice: Mr. Grigor, of Forres,[785] states that

seedlings of the Scotch fir (_Pinus sylvestris_), raised from seed from the

Continent and from the forests of Scotland, differ much. "The difference is

perceptible in one-year-old, and more so in two-year-old seedlings; but the

effects of the winter on the second year's growth almost uniformly makes

those from the Continent quite brown, and so damaged, that by the month of

March they are quite unsaleable, while the plants from the native Scotch

pine, under the same treatment, and standing alongside, although

considerably shorter, are rather stouter and quite green, so that the beds

of the one can be known from the other when seen from the distance of a

mile." Closely similar facts have been observed with seedling larches.

Hardy varieties would alone be valued or noticed in Europe; whilst

tender varieties, requiring more warmth, would generally be neglected;

but such occasionally arise. Thus Loudon[786] describes a Cornish

variety of the elm which is almost an evergreen, and of which the

shoots are often killed by the autumnal frosts, so that its timber is

of little value. Horticulturists know that some varieties are much more

tender than others: thus all the varieties of the broccoli are more

tender than cabbages; but there is much difference in this respect in

the sub-varieties of the broccoli; the pink and purple kinds are a

little hardier than the white Cape broccoli, "but they are not to be

depended on after the thermometer falls below 24deg Fahr.:" the

Walcheren broccoli is less tender than the Cape, and there are several

varieties which will stand much severer cold than the Walcheren.[787]

Cauliflowers seed more freely in India than cabbages.[788] To give one

instance with flowers: eleven plants raised from a hollyhock, called

the _Queen of the Whites_,[789] were found to be much more tender than

various other seedlings. It may be presumed that all tender varieties

would succeed better under a climate warmer than ours. With

fruit-trees, it is well known that certain varieties, for instance of

the peach, stand forcing in a hot-house better than others; and this

shows {311} either pliability of organisation or some constitutional

difference. The same individual cherry-tree, when forced, has been

observed during successive years gradually to change its period of

vegetation.[790] Few pelargoniums can resist the heat of a stove, but

_Alba multiflora_ will, as a most skilful gardener asserts, "stand

pine-apple top and bottom heat the whole winter, without looking any

more drawn than if it had stood in a common greenhouse; and _Blanche

Fleur_ seems as if it had been made on purpose for growing in winter,

like many bulbs, and to rest all summer."[791] There can hardly be a

doubt that the _Alba multiflora_ pelargonium must have a widely

different constitution from that of most other varieties of this plant;

it would probably withstand even an equatorial climate.

We have seen that according to Labat the vine and wheat require

acclimatisation in order to succeed in the West Indies. Similar facts

have been observed at Madras: "two parcels of mignonette-seed, one

direct from Europe, the other saved at Bangalore (of which the mean

temperature is much below that of Madras) were sown at the same time:

they both vegetated equally favourably, but the former all died off a

few days after they appeared above ground; the latter still survive,

and are vigorous healthy plants." So again, "turnip and carrot seed

saved at Hyderabad are found to answer better at Madras than seed from

Europe or from the Cape of Good Hope."[792] Mr. J. Scott, of the

Calcutta Botanic Gardens, informs me that seeds of the sweet-pea

(_Lathyrus odoratus_) imported from England produce plants, with thick,

rigid stems and small leaves, which rarely blossom and never yield

seed; plants raised from French seed blossom sparingly, but all the

flowers are sterile; on the other hand, plants raised from sweet-peas

grown near Darjeeling in Upper India, but originally derived from

England, can be successfully cultivated on the plains of India; for

they flower and seed profusely, and their stems are lax and scandent.

In some of the foregoing cases, as Dr. Hooker has remarked to me, the

greater success may perhaps be attributed to the seeds having been more

fully ripened under a more favourable climate; but this view can hardly

be extended to so many cases, including plants, which, from being

cultivated under a climate hotter than their native one, become fitted

for a still hotter climate. We may therefore safely conclude that

plants can to a certain extent become accustomed to a climate either

hotter or colder than their own; although these latter cases have been

more frequently observed.

We will now consider the means by which acclimatisation may be effected,

namely, through the spontaneous appearance of varieties having a different

constitution, and through the effects of use or habit. In regard to the

first process, there is no evidence that a change in the constitution of

the {312} offspring necessarily stands in any direct relation with the

nature of the climate inhabited by the parents. On the contrary, it is

certain that hardy and tender varieties of the same species appear in the

same country. New varieties thus spontaneously arising become fitted to

slightly different climates in two different ways; firstly, they may have

the power, either as seedlings or when full-grown, of resisting intense

cold, as with the Moscow pear, or of resisting intense heat, as with some

kinds of Pelargonium, or the flowers may withstand severe frost, as with

the Forelle pear. Secondly, plants may become adapted to climates widely

different from their own, from flowering and fruiting either earlier or

later in the season. In both these cases the power of acclimatisation by

man consists simply in the selection and preservation of new varieties. But

without any direct intention on his part of securing a hardier variety,

acclimatisation may be unconsciously effected by merely raising tender

plants from seed, and by occasionally attempting their cultivation further

and further northwards, as in the case of maize, the orange, and the peach.

How much influence ought to be attributed to inherited habit or custom in

the acclimatisation of animals and plants is a much more difficult

question. In many cases natural selection can hardly have failed to have

come into play and complicated the result. It is notorious that mountain

sheep resist severe weather and storms of snow which would destroy lowland

breeds; but then mountain sheep have been thus exposed from time

immemorial, and all delicate individuals will have been destroyed, and the

hardiest preserved. So with the Arrindy silk-moths of China and India; who

can tell how far natural selection may have taken a share in the formation

of the two races, which are now fitted for such widely different climates?

It seems at first probable that the many fruit-trees, which are so well

fitted for the hot summers and cold winters of North America, in contrast

with their poor success under our climate, have become adapted through

habit; but when we reflect on the multitude of seedlings annually raised in

that country, and that none would succeed unless born with a fitting

constitution, it is possible that mere habit may have done nothing towards

their acclimatisation. On the other hand, when we {313} hear that Merino

sheep, bred during no great number of generations at the Cape of Good

Hope--that some European plants raised during only a few generations in the

cooler parts of India, withstand the hotter parts of that country much

better than the sheep or seeds imported directly from England, we must

attribute some influence to habit. We are led to the same conclusion when

we hear from Naudin[793] that the races of melons, squashes, and gourds,

which have long been cultivated in Northern Europe, are comparatively more

precocious, and need much less heat for maturing their fruit, than the

varieties of the same species recently brought from tropical regions. In

the reciprocal conversion of summer and winter wheat, barley, and vetches

into each other, habit produces a marked effect in the course of a very few

generations. The same thing apparently occurs with the varieties of maize,

which, when carried from the Southern to the Northern States of America, or

into Germany, soon become accustomed to their new homes. With vine-plants

taken to the West Indies from Madeira, which are said to succeed better

than plants brought directly from France, we have some degree of

acclimatisation in the individual, independently of the production of new

varieties by seed.

The common experience of agriculturists is of some value, and they often

advise persons to be cautious in trying in one country the productions of

another. The ancient agricultural writers of China recommend the

preservation and cultivation of the varieties peculiar to each country.

During the classical period, Columella wrote, "Vernaculum pecus peregrino

longe praestantius est."[794]

I am aware that the attempt to acclimatise either animals or plants has

been called a vain chimaera. No doubt the attempt in most cases deserves to

be thus called, if made independently of the production of new varieties

endowed with a different constitution. Habit, however much prolonged,

rarely produces any effect on a plant propagated by buds; it apparently

acts only through successive seminal generations. {314} The laurel, bay,

laurestinus, &c., and the Jerusalem artichoke, which are propagated by

cuttings or tubers, are probably now as tender in England as when first

introduced; and this appears to be the case with the potato, which until

recently was seldom multiplied by seed. With plants propagated by seed, and

with animals, there will be little or no acclimatisation unless the hardier

individuals are either intentionally or unconsciously preserved. The

kidney-bean has often been advanced as an instance of a plant which has not

become hardier since its first introduction into Britain. We hear, however,

on excellent authority,[795] that some very fine seed, imported from

abroad, produced plants "which blossomed most profusely, but were nearly

all but abortive, whilst plants grown alongside from English seed podded

abundantly;" and this apparently shows some degree of acclimatisation in

our English plants. We have also seen that seedlings of the kidney-bean

occasionally appear with a marked power of resisting frost; but no one, as

far as I can hear, has ever separated such hardy seedlings, so as to

prevent accidental crossing, and then gathered their seed, and repeated the

process year after year. It may, however, be objected with truth that

natural selection ought to have had a decided effect on the hardiness of

our kidney-beans; for the tenderest individuals must have been killed

during every severe spring, and the hardier preserved. But it should be

borne in mind that the result of increased hardiness would simply be that

gardeners, who are always anxious for as early a crop as possible, would

sow their seed a few days earlier than formerly. Now, as the period of

sowing depends much on the soil and elevation of each district, and varies

with the season; and as new varieties have often been imported from abroad,

can we feel sure that our kidney-beans are not somewhat hardier? I have not

been able, by searching old horticultural works, to answer this question

satisfactorily.

On the whole the facts now given show that, though habit does something

towards acclimatisation, yet that the spontaneous appearance of

constitutionally different individuals is a far more effective agent. As no

single instance has been recorded, either with animals or plants, of

hardier individuals {315} having been long and steadily selected, though

such selection is admitted to be indispensable for the improvement of any

other character, it is not surprising that man has done little in the

acclimatisation of domesticated animals and cultivated plants. We need not,

however, doubt that under nature new races and new species would become

adapted to widely different climates, by spontaneous variation, aided by

habit, and regulated by natural selection.

_Arrests of Development: Rudimentary and Aborted Organs._

These subjects are here introduced because there is reason to believe

that rudimentary organs are in many cases the result of disuse.

Modifications of structure from arrested development, so great or so

serious as to deserve to be called monstrosities, are of common

occurrence, but, as they differ much from any normal structure, they

require here only a passing notice. When a part or organ is arrested

during its embryonic growth, a rudiment is generally left. Thus the

whole head may be represented by a soft nipple-like projection, and the

limbs by mere papillae. These rudiments of limbs are sometimes

inherited, as has been observed in a dog.[796]

Many lesser anomalies in our domesticated animals appear to be due to

arrested development. What the cause of the arrest may be, we seldom

know, except in the case of direct injury to the embryo within the egg

or womb. That the cause does not generally act at a very early

embryonic period we may infer from the affected organ seldom being

wholly aborted,--a rudiment being generally preserved. The external

ears are represented by mere vestiges in a Chinese breed of sheep; and

in another breed, the tail is reduced "to a little button, suffocated,

in a manner, by fat."[797] In tailless dogs and cats a stump is left;

but I do not know whether it includes at an early embryonic age

rudiments of all the caudal vertebrae. In certain breeds of fowls the

comb and wattles are reduced to rudiments; in the Cochin-China breed

scarcely more than rudiments of spurs exist. With polled Suffolk

cattle, "rudiments of horns can often be felt at an early age;"[798]

and with species in a state of nature, the relatively greater

development of rudimentary organs at an early period of life is highly

characteristic of such organs. With hornless breeds of cattle and

sheep; another and singular kind of rudiment has been observed, namely,

minute dangling horns attached to the skin alone, and which are often

shed and grow again. With hornless goats, according to Desmarest,[799]

{316} the bony protuberances which properly support the horns exist as

mere rudiments.

With cultivated plants it is far from rare to find the petals, stamens,

and pistils represented by rudiments, like those observed in natural

species. So it is with the whole seed in many fruits; thus near

Astrakhan there is a grape with mere traces of seeds, "so small and

lying so near the stalk that they are not perceived in eating the

grape."[800] In certain varieties of the gourd, the tendrils, according

to Naudin, are represented by rudiments or by various monstrous

growths. In the broccoli and cauliflower the greater number of the

flowers are incapable of expansion, and include rudimentary organs. In

the Feather hyacinth (_Muscari comosum_) the upper and central flowers

are brightly coloured but rudimentary; under cultivation the tendency

to abortion travels downwards and outwards, and all the flowers become

rudimentary; but the abortive stamens and pistils are not so small in

the lower as in the upper flowers. In the _Viburnum opulus_, on the

other hand, the outer flowers naturally have their organs of

fructification in a rudimentary state, and the corolla is of large

size; under cultivation, the change spreads to the centre, and all the

flowers become affected; thus the well-known Snow-ball bush is

produced. In the Compositae, the so-called doubling of the flowers

consists in the greater development of the corolla of the central

florets, generally accompanied with some degree of sterility; and it

has been observed[801] that the progressive doubling invariably spreads

from the circumference to the centre,--that is, from the ray florets,

which so often include rudimentary organs, to those of the disc. I may

add, as bearing on this subject, that, with Asters, seeds taken from

the florets of the circumference have been found to yield the greatest

number of double flowers.[802] In these several cases we have a natural

tendency in certain parts to become rudimentary, and this under culture

spreads either to, or from, the axis of the plant. It deserves notice,

as showing how the same laws govern the changes which natural species

and artificial varieties undergo, that in a series of species in the

genus Carthamus, one of the Compositae, a tendency in the seeds to the

abortion of the pappus may be traced extending from the circumference

to the centre of the disc: thus, according to A. de Jussieu,[803] the

abortion is only partial in _Carthamus creticus_, but more extended in

_C. lanatus_; for in this species two or three alone of the central

seeds are furnished with a pappus, the surrounding seeds being either

quite naked or furnished with a few hairs; and lastly, in _C.

tinctorius_, even the central seeds are destitute of pappus, and the

abortion is complete.

With animals and plants under domestication, when an organ disappears,

leaving only a rudiment, the loss has generally been sudden, as with

hornless and tailless breeds; and such cases may be ranked as inherited

monstrosities. But in some few cases the loss has been gradual, and

{317} has been partly effected by selection, as with the rudimentary

combs and wattles of certain fowls. We have also seen that the wings of

some domesticated birds have been slightly reduced by disuse, and the

great reduction of the wings in certain silk-moths, with mere rudiments

left, has probably been aided by disuse.

With species in a state of nature, rudimentary organs are so extremely

common that scarcely one can be named which is wholly free from a

blemish of this nature. Such organs are generally variable, as several

naturalists have observed; for, being useless, they are not regulated

by natural selection, and they are more or less liable to reversion.

The same rule certainly holds good with parts which have become

rudimentary under domestication. We do not know through what steps

under nature rudimentary organs have passed in being reduced to their

present condition; but we so incessantly see in species of the same

group the finest gradations between an organ in a rudimentary and

perfect state, that we are led to believe that the passage must have

been extremely gradual. It may be doubted whether a change of structure

so abrupt as the sudden loss of an organ would ever be of service to a

species in a state of nature; for the conditions to which all organisms

are closely adapted usually change very slowly. Even if an organ did

suddenly disappear in some one individual by an arrest of development,

intercrossing with the other individuals of the same species would

cause it to reappear in a more or less perfect manner, so that its

final reduction could only be effected by the slow process of continued

disuse or natural selection. It is much more probable that, from

changed habits of life, organs first become of less and less use, and

ultimately superfluous; or their place may be supplied by some other

organ; and then disuse, acting on the offspring through inheritance at

corresponding periods of life, would go on reducing the organ; but as

most organs could be of no use at an early embryonic period, they would

not be affected by disuse; consequently they would be preserved at this

stage of growth, and would remain as rudiments. In addition to the

effects of disuse, the principle of economy of growth, already alluded

to in this chapter, would lead to the still further reduction of all

superfluous parts. With respect to the final and total suppression or

abortion of any organ, another and distinct principle, which will be

discussed in the chapter on pangenesis, probably takes a share in the

work.

With animals and plants reared by man there is no severe or recurrent

struggle for existence, and the principle of economy will not come into

action. So far, indeed, is this from being the case, that in some

instances organs, which are naturally rudimentary in the

parent-species, become partially redeveloped in the domesticated

descendants. Thus cows, like most other ruminants, properly have four

active and two rudimentary mammae; but in our domesticated animals, the

latter occasionally become considerably developed and yield milk. The

atrophied mammae, which, in male domesticated animals, including man,

have in some rare cases grown to full size and secreted milk, perhaps

offer an analogous case. The hind feet of dogs include rudiments of a

fifth toe, and in certain large breeds these toes, though still

rudimentary, become considerably developed {318} and are furnished with

claws. In the common Hen, the spurs and comb are rudimentary, but in

certain breeds these become, independently of age or disease of the

ovaria, well developed. The stallion has canine teeth, but the mare has

only traces of the alveoli, which, as I am informed by the eminent

veterinary Mr. G. T. Brown, frequently contain minute irregular nodules

of bone. These nodules, however, sometimes become developed into

imperfect teeth, protruding through the gums and coated with enamel;

and occasionally they grow to a third or even a fourth of the length of

the canines in the stallion. With plants I do not know whether the

redevelopment of rudimentary organs occurs more frequently under

culture than under nature. Perhaps the pear-tree may be a case in

point, for when wild it bears thorns, which though useful as a

protection are formed of branches in a rudimentary condition, but, when

the tree is cultivated, the thorns are reconverted into branches.

Finally, though organs which must be classed as rudimentary frequently

occur in our domesticated animals and cultivated plants, these have

generally been formed suddenly, through an arrest of development. They

usually differ in appearance from the rudiments which so frequently

characterise natural species. In the latter, rudimentary organs have been

slowly formed through continued disuse, acting by inheritance at a

corresponding age, aided by the principle of the economy of growth, all

under the control of natural selection. With domesticated animals, on the

other hand, the principle of economy is far from coming into action, and

their organs, although often slightly reduced by disuse, are not thus

almost obliterated with mere rudiments left.

* * * * *

{319}

CHAPTER XXV.

LAWS OF VARIATION, _continued_--CORRELATED VARIABILITY.

EXPLANATION OF TERM--CORRELATION AS CONNECTED WITH

DEVELOPMENT--MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED

SIZE OF PARTS--CORRELATED VARIATION OF HOMOLOGOUS PARTS--FEATHERED FEET

IN BIRDS ASSUMING THE STRUCTURE OF THE WINGS--CORRELATION BETWEEN THE

HEAD AND THE EXTREMITIES--BETWEEN THE SKIN AND DERMAL

APPENDAGES--BETWEEN THE ORGANS OF SIGHT AND HEARING--CORRELATED

MODIFICATIONS IN THE ORGANS OF PLANTS--CORRELATED

MONSTROSITIES--CORRELATION BETWEEN THE SKULL AND EARS--SKULL AND CREST

OF FEATHERS--SKULL AND HORNS--CORRELATION OF GROWTH COMPLICATED BY THE

ACCUMULATED EFFECTS OF NATURAL SELECTION--COLOUR AS CORRELATED WITH

CONSTITUTIONAL PECULIARITIES.

All the parts of the organisation are to a certain extent connected or

correlated together; but the connexion may be so slight that it hardly

exists, as with compound animals or the buds on the same tree. Even in the

higher animals various parts are not at all closely related; for one part

may be wholly suppressed or rendered monstrous without any other part of

the body being affected. But in some cases, when one part varies, certain

other parts always, or nearly always, simultaneously vary; they are then

subject to the law of correlated variation. Formerly I used the somewhat

vague expression of correlation of growth, which may be applied to many

large classes of facts. Thus, all the parts of the body are admirably

coordinated for the peculiar habits of life of each organic being, and they

may be said, as the Duke of Argyll insists in his 'Reign of Law,' to be

correlated for this purpose. Again, in large groups of animals certain

structures always co-exist; for instance, a peculiar form of stomach with

teeth of peculiar form, and such structures may in one sense be said to be

correlated. But these cases have no necessary connexion with the law to be

discussed in the present chapter; for we do not know that {320} the initial

or primary variations of the several parts were in any way related; slight

modifications or individual differences may have been preserved, first in

one and then in another part, until the final and perfectly co-adapted

structure was acquired; but to this subject I shall presently recur. Again,

in many groups of animals the males alone are furnished with weapons, or

are ornamented with gay colours; and these characters manifestly stand in

some sort of correlation with the male reproductive organs, for when the

latter are destroyed these characters disappear. But it was shown in the

twelfth chapter that the very same peculiarity may become attached at any

age to either sex, and afterwards be exclusively transmitted by the same

sex at a corresponding age. In these cases we have inheritance limited by,

or correlated with, both sex and age; but we have no reason for supposing

that the original cause of the variation was necessarily connected with the

reproductive organs, or with the age of the affected being.

In cases of true correlated variation, we are sometimes able to see the

nature of the connexion; but in most cases the bond is hidden from us, and

certainly differs in different cases. We can seldom say which of two

correlated parts first varies, and induces a change in the other; or

whether the two are simultaneously produced by some distinct cause.

Correlated variation is an important subject for us; for when one part is

modified through continued selection, either by man or under nature, other

parts of the organisation will be unavoidably modified. From this

correlation it apparently follows that, with our domesticated animals and

plants, varieties rarely or never differ from each other by some single

character alone.

One of the simplest cases of correlation is that a modification which

arises during an early stage of growth tends to influence the subsequent

development of the same part, as well as of other and intimately connected

parts. Isidore Geoffroy St. Hilaire states[804] that this may constantly be

observed with monstrosities {321} in the animal kingdom; and

Moquin-Tandon[805] remarks, that, as with plants the axis cannot become

monstrous without in some way affecting the organs subsequently produced

from it, so axial anomalies are almost always accompanied by deviations of

structure in the appended parts. We shall presently see that with

short-muzzled races of the dog certain histological changes in the basal

elements of the bones arrest their development and shorten them, and this

affects the position of the subsequently developed molar teeth. It is

probable that certain modifications in the larvae of insects would affect

the structure of the mature insects. But we must be very careful not to

extend this view too far, for, during the normal course of development,

certain members in the same group of animals are known to pass through an

extraordinary course of change, whilst other and closely allied members

arrive at maturity with little change of structure.

Another simple case of correlation is that with the increased or decreased

dimensions of the whole body, or of any particular part, certain organs are

increased or diminished in number, or are otherwise modified. Thus

pigeon-fanciers have gone on selecting pouters for length of body, and we

have seen that their vertebrae are generally increased in number, and their

ribs in breadth. Tumblers have been selected for their small bodies, and

their ribs and primary wing-feathers are generally lessened in number.

Fantails have been selected for their large, widely-expanded tails, with

numerous tail-feathers, and the caudal vertebrae are increased in size and

number. Carriers have been selected for length of beak, and their tongues

have become longer, but not in strict accordance with the length of beak.

In this latter breed and in others having large feet, the number of the

scutellae on the toes is greater than in the breeds with small feet. Many

similar cases could be given. In Germany it has been observed that the

period of gestation is longer in large-sized than in small-sized breeds of

cattle. With our highly-improved animals of all kinds the period of

maturity has advanced, both with respect to the full growth of the body and

the period of reproduction; and, in correspondence with this, the teeth are

now developed earlier than formerly, so that, {322} to the surprise of

agriculturists, the ancient rules for judging the age of an animal by the

state of its teeth are no longer trustworthy.[806]

_Correlated Variation of Homologous Parts._--Parts which are homologous

tend to vary in the same manner; and this is what might have been expected,

for such parts are identical in form and structure during an early period

of embryonic development, and are exposed in the egg or womb to similar

conditions. The symmetry, in most kinds of animals, of the corresponding or

homologous organs on the right and left sides of the body, is the simplest

case in point; but this symmetry sometimes fails, as with rabbits having

only one ear, or stags with one horn, or with many-horned sheep which

sometimes carry an additional horn on one side of their heads. With flowers

which have regular corollas, the petals generally vary in the same manner,

as we see in the same complicated and elegant pattern, on the flowers of

the Chinese pink; but with irregular flowers, though the petals are of

course homologous, this symmetry often fails, as with the varieties of the

_Antirrhinum_ or snapdragon, or that variety of the kidney-bean (_Phaseolus

multiflorus_) which has a white standard-petal.

In the vertebrata the front and hind limbs are homologous, and they tend to

vary in the same manner, as we see in long and short-legged, or in thick

and thin-legged races of the horse and dog. Isidore Geoffroy[807] has

remarked on the tendency of supernumerary digits in man to appear, not only

on the right and left sides, but on the upper and lower extremities. Meckel

has insisted[808] that, when the muscles of the arm depart in number or

arrangement from their proper type, they almost always imitate those of the

leg; and so conversely the varying muscles of the leg imitate the normal

muscles of the arm.

In several distinct breeds of the pigeon and fowl, the legs and the two

outer toes are heavily feathered, so that in the trumpeter pigeon they

appear like little wings. In the feather-legged bantam the "boots" or

feathers, which grow from the outside of the leg and generally from the two

outer toes, have, {323} according to the excellent authority of Mr.

Hewitt,[809] been seen to exceed the wing-feathers in length, and in one

case were actually nine and a half inches in length! As Mr. Blyth has

remarked to me, these leg-feathers resemble the primary wing-feathers, and

are totally unlike the fine down which naturally grows on the legs of some

birds, such as grouse and owls. Hence it may be suspected that excess of

food has first given redundancy to the plumage, and then that the law of

homologous variation has led to the development of feathers on the legs, in

a position corresponding with those on the wing, namely, on the outside of

the tarsi and toes. I am strengthened in this belief by the following

curious case of correlation, which for a long time seemed to me utterly

inexplicable, namely, that in pigeons of any breed, if the legs are

feathered, the two outer toes are partially connected by skin. These two

outer toes correspond with our third and fourth toes. Now, in the wing of

the pigeon or any other bird, the first and fifth digits are wholly

aborted; the second is rudimentary and carries the so-called

"bastard-wing;" whilst the third and fourth digits are completely united

and enclosed by skin, together forming the extremity of the wing. So that

in feather-footed pigeons, not only does the exterior surface support a row

of long feathers, like wing-feathers, but the very same digits which in the

wing are completely united by skin become partially united by skin in the

feet; and thus by the law of the correlated variation of homologous parts

we can understand the curious connection of feathered legs and membrane

between the two outer toes.

Andrew Knight[810] has remarked that the face or head and the limbs vary

together in general proportions. Compare, for instance, the head and limbs

of a dray and race-horse, or of a greyhound and mastiff. What a monster a

greyhound would appear with the head of a mastiff! The _modern_ bulldog,

however, has fine limbs, but this is a recently-selected character. From

the measurements given in the sixth chapter, we clearly see that in all the

breeds of the pigeon the length of the beak and the size of the feet are

correlated. The view which, as before explained, seems the most probable

is, that disuse in all cases tends {324} to diminish the feet, the beak

becoming at the same time through correlation shorter; but that in those

few breeds in which length of beak has been a selected point, the feet,

notwithstanding disuse, have through correlation increased in size.

With the increased length of the beak in pigeons, not only the tongue

increases in length, but likewise the orifice of the nostrils. But the

increased length of the orifice of the nostrils perhaps stands in closer

correlation with the development of the corrugated skin or wattle at the

base of the beak; for when there is much wattle round the eyes, the eyelids

are greatly increased or even doubled in length.

There is apparently some correlation even in colour between the head and

the extremities. Thus with horses a large white star or blaze on the

forehead is generally accompanied by white feet.[811] With white rabbits

and cattle, dark marks often co-exist on the tips of the ears and on the

feet. In black and tan dogs of different breeds, tan-coloured spots over

the eyes and tan-coloured feet almost invariably go together. These latter

cases of connected colouring may be due either to reversion or to analogous

variation,--subjects to which we shall hereafter return,--but this does not

necessarily determine the question of their original correlation. If those

naturalists are correct who maintain that the jaw-bones are homologous with

the limb-bones, then we can understand why the head and limbs tend to vary

together in shape and even in colour; but several highly competent judges

dispute the correctness of this view.

The lopping forwards and downwards of the immense ears of fancy rabbits is

in part due to the disuse of the muscles, and in part to the weight and

length of the ears, which have been increased by selection during many

generations. Now, with the increased size and changed direction of the

ears, not only has the bony auditory meatus become changed in outline,

direction, and greatly in size, but the whole skull has been slightly

modified. This could be clearly seen in "half-lops"--that is, in rabbits

with one ear alone lopping forward--for the opposite sides of their skulls

were not strictly symmetrical. This seems to me a curious instance of

correlation, between hard {325} bones and organs so soft and flexible, as

well as so unimportant under a physiological point of view, as the external

ears. The result no doubt is largely due to mere mechanical action, that

is, to the weight of the ears, on the same principle that the skull of a

human infant is easily modified by pressure.

The skin and the appendages of hair, feathers, hoofs, horns, and teeth, are

homologous over the whole body. Every one knows that the colour of the skin

and that of the hair usually vary together; so that Virgil advises the

shepherd to look whether the mouth and tongue of the ram are black, lest

the lambs should not be purely white. With poultry and certain ducks we

have seen that the colour of the plumage stands in some connexion with the

colour of the shell of the egg,--that is, with the mucous membrane which

secretes the shell. The colour of the skin and hair, and the odour emitted

by the glands of the skin, are said[812] to be connected, even in the same

race of men. Generally the hair varies in the same way all over the body in

length, fineness, and curliness. The same rule holds good with feathers, as

we see with the laced and frizzled breeds both of fowls and pigeons. In the

common cock the feathers on the neck and loins are always of a particular

shape, called hackles: now in the Polish breed, both sexes are

characterised by a tuft of feathers on the head; but through correlation

these feathers in the male always assume the form of hackles. The wing and

tail-feathers, though arising from parts not homologous, vary in length

together; so that long or short winged pigeons generally have long or short

tails. The case of the Jacobin-pigeon is more curious, for the wing and

tail feathers are remarkably long; and this apparently has arisen in

correlation with the elongated and reversed feathers on the back of the

neck, which form the hood.