The Variation of Animals and Plants under Domestication Volume II Part 18

Absorption often plays an important part in the repair 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. (24/7. Prof. Marey's discussion on the power of co- adaptation in all parts of the organisation is excellent. 'La Machine Animale'

1873 chapter 9. See also Paget 'Lectures' etc. page 257.) A dislocated bone will form for itself a new socket. Displaced tendons and varicose veins excavate new channels in the bones against which they press. 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 almost all parts of the organisation, even whilst retaining the same form, undergo constant renewal; so that a part which is not renewed would be liable to absorption.

Some cases, usually cla.s.sed under the so-called nisus formativus, at first appear to come under a distinct head; for not only are old structures reproduced, but new structures 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 quant.i.ty 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 hydrocephalus the open and dangerous s.p.a.ces in the skull are filled up with new bones, which interlock by perfect serrated sutures. (24/8. These cases are given by Blumenbach in his 'Essay on Generation' pages 52, 54.) But most physiologists, especially on the Continent, have now given up the belief in plastic lymph or blastema, and Virchow (24/9. 'Cellular Pathology' translation by Dr. Chance 1860 pages 27, 441.) 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 a "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." (24/10. Paget 'Lectures on Pathology' volume 1 1853 page 357.)

No doubt the power of reparation, though not always perfect, is an admirable provision, ready for various emergencies, even for such as occur only at long intervals of time. (24/11. Paget ibid page 150.) 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 again all the parts into harmony with one another.]

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, etc.; and that disuse, on the other hand, weakens them. It has been experimentally proved by Ranke (24/12. 'Die Blutvertheilung, etc. der Organe' 1871 as quoted by Jaeger 'In Sachen Darwin's' 1874 page 48. See also H. Spencer 'The Principles of Biology'

volume 2 1866 chapters 3-5.) that the flow of blood is greatly increased towards any part which is performing work, and sinks again when the part is at rest. Consequently, if the work is frequent, the vessels increase in size and the part is better nourished. Paget (24/13. 'Lectures on Pathology' 1853 volume 1 page 71.) also accounts for the long, thick, dark-coloured hairs which occasionally grow, even in young children, near old-standing inflamed surfaces or fractured bones by an increased flow of blood to the part. When Hunter inserted the spur of a c.o.c.k into the comb, which is well supplied with blood-vessels, it grew in one case spirally 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. According to the interesting observations of M. Sedillot (24/14. 'Comptes Rendus' September 26, 1864 page 539.), when a portion of one of the bones of the leg of an animal is removed, the a.s.sociated 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 that of the tibia. Now, it is at first difficult to believe that increased weight acting on a straight bone could, by alternately increasing and diminishing the pressure, cause the blood to flow more freely in the vessels which permeate the periosteum and thus supply more nutriment to the bone.

Nevertheless the observations adduced by Mr. Spencer (24/15. H. Spencer 'The Principles of Biology' volume 2 page 243.), on the strengthening of the bowed bones of rickety children, along their concave sides, leads to the belief that this is possible.

The rocking of the stem of a tree increases in a marked manner the growth of the woody tissue in the parts which are strained. Prof. Sachs believes, from reasons which he a.s.signs, that this is due to the pressure of the bark being relaxed in such parts, and not as Knight and H. Spencer maintain, to an increased flow of sap caused by the movement of the trunk. (24/16. Ibid volume 2 page 269. Sachs 'Text-book of Botany' 1875 page 734.) But hard woody tissue may be developed without the aid of any movement, as we see with ivy closely attached to an old wall. In all such cases, it is very difficult to distinguish between the effects of long-continued selection and those which follow from the increased action of the part, or directly from some other cause. Mr. H. Spencer (24/17. Ibid volume 2 page 273.) acknowledges this difficulty, and gives as an instance the thorns on trees and the sh.e.l.ls of nuts. Here we have extremely hard woody tissue without the possibility of any movement, 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 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 (24/18. Paget 'Lectures on Pathology' volume 2 page 209.), 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. (24/19. Muller 'Phys.' English translation pages 54, 791. Prof. Reed has given ('Physiological and Anat. Researches' page 10) a curious account of the atrophy of the limbs of rabbits after the destruction of the nerve.) The Proteus is furnished with branchiae as well as with lungs: and Schreibers (24/20. Quoted by Lecoq in 'Geograph. Bot.' tome 1 1854 page 182.) 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 only in the individual but in the race, their power of flight; for we do not see a young fowl, 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 furculum, the length of the wings as measured from tip to tip of the radii, 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 it 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 furculum, 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 (24/21. 'Das Abandern der Vogel' 1833 s. 74.) a.s.serts 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 coc.o.o.ns with their wings distorted, incapable of flight, often greatly reduced in size, or even, according to Quatref.a.ges, 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 coc.o.o.n; or it may 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 be attributed to long-continued disuse.]

From the foregoing facts there can be no doubt that with our anciently domesticated animals, certain bones have increased or decreased in size and weight owing to increased or decreased use; but they have not been modified, as shown in the earlier chapters, in shape or structure. With animals living a free life and occasionally exposed to severe compet.i.tion the reduction would tend to be greater, as it would be an advantage to them to have the development of every superfluous part saved. With highly-fed domesticated animals, on the other hand, there seems to be no economy of growth, nor any tendency to the elimination of superfluous details. But to this subject I shall recur.

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. (24/22. Nathusius 'Die Racen des Schweines'

1860 s. 53, 57; 'Vorstudien...Schweineschadel' 1864 s. 103, 130, 133. Prof.

Lucae supports and extends the conclusions of Von Nathusius: 'Der Schadel des Maskenschweines' 1870.) These modifications of structure, which are all strictly inherited, characterise several improved breeds, so that they cannot have been derived from any single domestic stock. 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" (24/23. 'Journal of Agriculture of Highland Soc.' July 1860 page 321.); 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 obvious; and perhaps the liver may be affected by the nutritious and artificial food on which they largely subsist. Again, Dr. Wilckens a.s.serts (24/24. 'Landwirth. Wochenblatt'

No. 10.) that various parts of the body certainly differ in Alpine and lowland breeds of several domesticated animals, owing to their different habits of life; for instance, the neck and fore-legs in length, and the hoofs in shape.

[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. With respect to glands, Sir J. Paget observes that "when one kidney is destroyed the other often becomes much larger, and does double work."

(24/25. 'Lectures on Surgical Pathology' 1853 volume 1 page 27.) If we compare the size of the udders and their power of secretion in cows which have been long domesticated, and in certain breeds of the goat in which the udders nearly touch the ground, with 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 (24/26. Andersson 'Travels in South Africa' page 318. For a.n.a.logous cases in South America see Aug. St.-Hilaire 'Voyage dans la Province de Goyaz' tome 1 page 71.), "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 that short-sight is inherited; and we have seen in the twelfth chapter from the statistical researches of M. Giraud-Teulon, that the habit of viewing near objects gives a tendency to short-sight. Veterinarians are unanimous that horses are affected with spavins, splints, ringbones, etc., from being shod and from travelling on hard roads, and they are almost equally unanimous that a tendency to these malformations is transmitted. Formerly horses were not shod in North Carolina, and it has been a.s.serted that they did not then suffer from these diseases of the legs and feet. (24/27. Brickell 'Nat. Hist. of North Carolina' 1739 page 53.)]

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 formerly 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 pa.s.sing 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 (24/28. Livingstone quoted by Youatt on 'Sheep'

page 142. Hodgson in 'Journal of Asiatic Soc. of Bengal' volume 16 1847 page 1006 etc. etc. On the other hand Dr. Wilckens argues strongly against the belief that the drooping of the ears is the result of disuse: 'Jahrbuch der deutschen Viehzucht' 1866.) to disuse, for animals protected by man are not compelled habitually to use their ears. Col. Hamilton Smith (24/29.

'Naturalist's Library' Dogs volume 2 1840 page 104.) 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." G.o.dron also has remarked "that the pigs of the ancient Egyptians had not their ears enlarged and pendent." (24/30. 'De l'Espece' tome 1 1859 page 367.) But it is remarkable that the drooping of the ear is not accompanied by any decrease in size; on the contrary, animals so different as fancy rabbits, certain Indian breeds of the goat, our petted spaniels, blood- hounds, and other dogs, have enormously elongated ears, so that it would appear as if their weight had caused them to droop, aided perhaps by disuse.

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.

The epidermis on our hands is easily thickened, as every one knows, by hard work. In a district of Ceylon the sheep have "h.o.r.n.y 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. (24/31. 'Ceylon' by Sir J.E. Tennent 1859 volume 2 page 531.)

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 cornfields 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. (24/32. For the foregoing statements see Hunter 'Essays and Observations' 1861 volume 2 page 329; Dr.

Edmondston, as quoted in Macgillivray 'British Birds' volume 5 page 550: Menetries as quoted in Bronn 'Geschichte der Natur' b. 2 s. 110.)

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 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; for instance, 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. (24/33. These statements on the intestines are taken from Isidore Geoffroy Saint-Hilaire 'Hist. Nat. Gen.' tome 3 pages 427, 441.)

CHANGED AND INHERITED HABITS OF LIFE.

This subject, as far as the mental powers of animals are concerned, so blends into instinct, that I will here only remind the reader of such cases as the tameness of our domesticated 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 mere habit, and how much to the selection of individuals which have varied in the desired manner, irrespectively of the special circ.u.mstances under which they have been kept, can seldom be told.

We have already seen that animals may be habituated to a changed diet; but some additional instances may 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. (24/34. Gilbert White 'Nat. Hist.

Selborne' 1825 volume 2 page 121.) Our sporting dogs will not touch the bones of game birds, whilst most 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. (24/35.

Burdach 'Traite de Phys.' tome 2 page 267 as quoted by Dr. P. Lucas 'L'Hered.

Nat.' tome 1 page 388.) One of my relations bred some young pigs from 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. (24/36. This and several other cases are given by Colin 'Physiologie Comp. des Animaux Dom.' 1854 tome 1 page 426.) 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. (24/37. M. Michely de Cayenne in 'Bull.

Soc. d'Acclimat.' tome 8 1861 page 563.)

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 with respect to the season has been recorded. (24/38. Quatref.a.ges 'Unite de l'Espece Humaine' 1861 page 79.) 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 of certain breeds of the pigeon, have been modified and are inherited. Cattle, horses, and pigs have learnt to browse under water in the St. John's River, East Florida, where the Vallisneria has been largely naturalised. The cows were observed by Prof.

Wyman to keep their heads immersed for "a period varying from fifteen to thirty-five seconds." (24/39. 'The American Naturalist' April 1874 page 237.) The voice differs much in certain kinds of fowls and pigeons. Some varieties are clamorous and others silent, as the Call and common duck, or the Spitz and pointer dog. Every one knows how the breeds of the dog differ from one another 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, 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 (24/40.

'Flora' 1835 b. 2 page 504.) raised from seed of Verbasc.u.m phoeniceum, which is usually a biennial, both annual and perennial varieties. Some deciduous bushes become evergreen in hot countries. (24/41. Alph. de Candolle 'Geograph.

Bot.' tome 2 page 1078.) Rice requires much water, but there is one variety in India which can be grown without irrigation. (24/42. Royle 'Ill.u.s.trations of the Botany of the Himalaya' page 19.) Certain varieties of the oat and of our other cereals are best fitted for certain soils. (24/43. 'Gardener's Chronicle' 1850 pages 204, 219.) Endless similar facts could be given in the animal and vegetable kingdoms. They are noticed here because they ill.u.s.trate a.n.a.logous differences in closely allied natural species, and because such changed habits of life, whether due to habit, or to the direct action of external conditions, or to so-called spontaneous variability, 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 a.s.serted (24/44.

Rev. R. Everest 'Journal As. Soc. of Bengal' volume 3 page 19.), that no one has there succeeded in keeping the Newfoundland long alive; but then it may be argued, and 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 (24/45. Youatt on 'Sheep' 1838 page 491.), 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 far better adapted for India than those imported from England. (24/46. Royle 'Prod. Resources of India' page 153.) It is almost certain that all the breeds of the fowl are descended from one species; but the Spanish breed, which there is good reason to believe originated near the Mediterranean (24/47. Tegetmeier 'Poultry Book' 1866 page 102.), 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 ident.i.ty in the caterpillar, coc.o.o.n, and mature states (24/48. Dr. R. Paterson in a paper communicated to Bot. Soc. of Canada quoted in the 'Reader' 1863 November 13.); yet they differ much in const.i.tution: the Indian form "will flourish only in warm lat.i.tudes," 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 const.i.tutions 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 that by 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 (24/49. See remarks by Editor in 'Gardener's Chronicle' 1848 page 5.), not one succeeds. Though the American varieties can withstand a severer winter than ours, the summer here is not hot enough. Fruit-trees have also originated in Europe with different const.i.tutions, but they are not much noticed, because nurserymen here do not supply wide areas. 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 deg and even 14 deg Fahr., which killed the flowers, whether fully expanded or in bud, of all other kinds of pears. (24/50. 'Gardener's Chronicle' 1860 page 938.

Remarks by Editor and quotation from Decaisne.) This power in the flower of resisting cold and afterwards producing fruit does not invariably depend, as we know on good authority (24/51. J. de Jonghe of Brussels in 'Gardener's Chronicle' 1857 page 612.), on general const.i.tutional vigour. In proceeding northward, the number of varieties which are found capable of resisting 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. (24/52. Ch. Martius 'Voyage Bot. Cotes Sept. de la Norvege' page 26.) 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. (24/53. 'Journal de l'Acad. Hort. de Gand' quoted in 'Gardener's Chronicle' 1859 page 7.) We thus see that our fruit-trees, like distinct species of the same genus, certainly differ from each other in their const.i.tutional 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" (24/54.

'Gardener's Chronicle' 1851 page 396.); but the failure in this case is at first only in the quant.i.ty, though ultimately in the quality, of the grain produced. The Rev. M.J. 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. (24/55. Ibid 1862 page 235.) 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."

(24/56. On the authority of Labat quoted in 'Gardener's Chronicle' 1862 page 235.) 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. (24/57. MM. Edwards and Colin 'Annal. des Sc. Nat.' 2nd series Bot. tome 5 page 22.)

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 can 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. (24/58. 'Geograph. Bot.' page 337.) On the authority of Linnaeus (24/59. 'Swedish Acts' English translation 1739-40 volume 1. Kalm in his 'Travels' volume 2 page 166 gives an a.n.a.logous case with cotton-plants raised in New Jersey from Carolina seed.), I may quote an a.n.a.logous 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 (24/60. De Candolle 'Geograph. Bot.' page 339.); but this seems due to commerce being now easier, so that it is better to import wine from the south than to make it in northern districts. 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 const.i.tution 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 (24/61. 'Gardener's Chronicle' 1862 page 235.), 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.

During 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 old 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. (24/62. Gallesio 'Teoria della Riproduzione Veg.' 1816 page 125; and 'Traite du Citrus' 1811 page 359.) I may add that Risso (24/63. 'Essai sur l'Hist. des Orangers' 1813 page 20 etc.) 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. (24/64. Alph. de Candolle 'Geograph. Bot.' page 882.) According to the authorities quoted by Dr. F.

Rolle (24/65. 'Ch. Darwin's Lehre von der Entstehung' etc. 1862 s. 87.), 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 in the season, even when grown on a wall; "it is, therefore, only fit for a very hot southern climate." (24/66. Decaisne quoted in 'Gardener's Chronicle' 1865 page 271.)

I will briefly give a few other cases. A variety of Magnolia grandiflora, raised by M. Roy, withstands a temperature several degrees lower than that which any other variety can resist. With camellias there is much difference in hardiness. One particular variety of the Noisette rose withstood the severe frost of 1860 "untouched and hale amidst a universal destruction of other 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. (24/67. For the magnolia see Loudon's 'Gardener's Mag.' volume 13 1837 page 21. For camellias and roses see 'Gardener's Chronicle' 1860 page 384. For the yew 'Journal of Hort.' March 3, 1863 p 174. For sweet potatoes see Col. von Siebold in 'Gardener's Chronicle' 1855 page 822.)]

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 (24/68. The Editor 'Gardener's Chronicle' 1861 page 239.), 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 24, 1836, and all the French beans (Phaseolus vulgaris) in a bed were killed except about one in thirty, which completely escaped. (24/69. Loudon's 'Gardener's Mag.' volume 12 1836 page 378.) 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 around, and not see at a glance that they differed widely in const.i.tutional power of resisting frost.

This work is not the proper place to show that wild plants of the same species, naturally growing at different alt.i.tudes or under different lat.i.tudes, become to a certain extent acclimatised, as is proved by the different behaviour of their seedlings when raised in another country. In my 'Origin of Species' I have alluded to some cases, and I could add many others.

One instance must suffice: Mr. Grigor, of Forres (24/70. 'Gardener's Chronicle' 1865 page 699. Mr. G. Maw gives ('Gardener's Chronicle' 1870 page 895) a number of striking cases; he brought home from southern Spain and northern Africa several plants, which he cultivated in England alongside specimens from northern districts; and he found a great difference not only in their hardiness during the winter, but in the behaviour of some of them during the summer.), 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 make 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.