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 larvæ of perfect insects, can regenerate limbs
and antennæ up to the time of their last moult;" and so can the larvæ
of true insects, but not the mature insect. Salamanders correspond in
development with the tadpoles or larvæ 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 branchiæ as
well as with lungs: and Schreibers[735] found that when the animal was
compelled to live in deep water the branchiæ 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 branchiæ 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 scapulæ 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 scapulæ
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 scapulæ 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 Bombycidæ, 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 _Café diable_, but, after having been reared on the Ailanthus, they would not touch the _Café 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 18° and even 14° 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 Linnæus,[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 24° 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 præstantius est."[794]

I am aware that the attempt to acclimatise either animals or plants has been called a vain chimæra. 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 papillæ. 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 vertebræ. 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 Compositæ, 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 Compositæ, 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 mammæ; but in our domesticated animals, the
latter occasionally become considerably developed and yield milk. The
atrophied mammæ, 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}