Chapter VIII (2)

It would be useless, even if it were possible, to describe all the many kinds of silkworms. Several distinct species exist in India and China which produce useful silk, and some of these are capable of freely crossing with the common silk-moth, as has been recently ascertained in France. Captain Hutton[65] states that throughout the world at least six species have been domesticated; and he believes that the silk-moths reared in Europe belong to two or three species. This, however, is not the opinion of several capable judges who have particularly attended to the cultivation of this insect in France; and hardly accords with some facts presently to be given.

The common silk-moth (_Bombyx mori_) was brought to Constantinople in the sixth century, whence it was carried into Italy, and in 1494 into France.[66] Everything has been favourable for the variation of this insect. It is believed to have been domesticated in China as long ago as 2700 B.C. It has been kept under unnatural and diversified conditions of life, and has been transported into many countries. There is reason to believe that the nature of the food given to the caterpillar influences to a certain extent the character of the breed.[67] Disuse has apparently aided in checking the development of the wings. But the most important element in the production of the many now existing, much modified races, no doubt has been the close attention which has long been applied in many countries to every promising variation. The care taken in Europe in the selection of the best cocoons and moths for breeding is notorious,[68] and the production of eggs is followed as a distinct trade in parts of France. I have made inquiries through Dr. Falconer, and am assured that in India the natives are equally careful in the process of selection. In China the production of eggs is confined to certain favourable districts, and the raisers are precluded by law from producing silk, so that their whole attention may be necessarily given up to this one object.[69]

The following details on the differences between the several breeds are taken, when not stated to the contrary, from M. Robinet’s excellent work,[70] which bears every sign of care and large experience. The _eggs_ in the different races vary in colour, in shape (being round, elliptic or oval), and in size. The eggs laid in June in the south of France, and in July in the central provinces, do not hatch until the following spring; and it is in vain, says M. Robinet, to expose them to a temperature gradually raised, in order that the caterpillar may be quickly developed. Yet occasionally, without any known cause, batches of eggs are produced, which immediately begin to undergo the proper changes, and are hatched in from twenty to thirty days. From these and some other analogous facts it may be concluded that the Trevoltini silkworms of Italy, of which the caterpillars are hatched in from fifteen to twenty days, do not necessarily form, as has been maintained, a distinct species. Although the breeds which live in temperate countries produce eggs which cannot be immediately hatched by artificial heat, yet when they are removed to and reared in a hot country they gradually acquire the character of quick development, as in the Trevoltini races.[71]

_Caterpillars._—These vary greatly in size and colour. The skin is generally white, sometimes mottled with black or grey, and occasionally quite black. The colour, however, as M. Robinet asserts, is not constant, even in perfectly pure breeds; except in the race _tigrée,_ so called from being marked with transverse black stripes. As the general colour of the caterpillar is not correlated with that of the silk,[72] this character is disregarded by cultivators, and has not been fixed by selection. Captain Hutton, in the paper before referred to, has argued with much force that the dark tiger-like marks, which so frequently appear during the later moults in the caterpillars of various breeds, are due to reversion; for the caterpillars of several allied wild species of Bombyx are marked and coloured in this manner. He separated some caterpillars with the tiger-like marks, and in the succeeding spring (pp. 149, 298) nearly all the caterpillars reared from them were dark-brindled, and the tints became still darker in the third generation. The moths reared from these caterpillars[73] also became darker, and resembled in colouring the wild _B. huttoni._ On this view of the tiger-like marks being due to reversion, the persistency with which they are transmitted is intelligible.

Several years ago Mrs. Whitby took great pains in breeding silkworms on a large scale, and she informed me that some of her caterpillars had dark eyebrows. This is probably the first step in reversion towards the tiger-like marks, and I was curious to know whether so trifling a character would be inherited. At my request she separated in 1848 twenty of these caterpillars, and having kept the moths separate, bred from them. Of the many caterpillars thus reared, “every one without exception had eyebrows, some darker and more decidedly marked than the others, but _all_ had eyebrows more or less plainly visible.” Black caterpillars occasionally appear amongst those of the common kind, but in so variable a manner, that, according to M. Robinet, the same race will one year exclusively produce white caterpillars, and the next year many black ones; nevertheless, I have been informed by M. A. Bossi of Geneva, that, if these black caterpillars are separately bred from, they reproduce the same colour; but the cocoons and moths reared from them do not present any difference.

The caterpillar in Europe ordinarily moults four times before passing into the cocoon stage; but there are races “à trois mues,” and the Trevoltini race likewise moults only thrice. It might have been thought that so important a physiological difference would not have arisen under domestication; but M. Robinet[74] states that, on the one hand, ordinary caterpillars occasionally spin their cocoons after only three moults, and, on the other hand, “presque toutes les races à trois mues, que nous avons expérimentees, ont fait quatre mues à la seconde ou à la troisième année, ce qui semble prouver qu’il a suffi de les placer dans des conditions favorables pour leur rendre une faculté qu’elles avaient perdue sous des influences moins favorables.”

_Cocoons._—The caterpillar in changing into the cocoon loses about 50 per cent of its weight; but the amount of loss differs in different breeds, and this is of importance to the cultivator. The cocoon in the different races presents characteristic differences; being large or small;—nearly spherical with no constriction, as in the Race de Loriol, or cylindrical, with either a deep or slight constriction in the middle; with the two ends, or with one end alone, more or less pointed. The silk varies in fineness and quality, and in being nearly white, but of two tints, or yellow. Generally the colour of the silk is not strictly inherited: but in the chapter on Selection I shall give a curious account how, in the course of sixty-five generations, the number of yellow cocoons in one breed has been reduced in France from one hundred to thirty-five in the thousand. According to Robinet, the white race, called Sina, by careful selection during the last seventy-five years, “est arrivée à un tel état de pureté, qu’on ne voit pas un seul cocon jaune dans des millions de cocons blancs.”[75] Cocoons are sometimes formed, as is well known, entirely destitute of silk, which yet produce moths; unfortunately Mrs. Whitby was prevented by an accident from ascertaining whether this character would prove hereditary.

_Adult stage._—I can find no account of any constant difference in the moths of the most distinct races. Mrs. Whitby assured me that there was none in the several kinds bred by her; and I have received a similar statement from the eminent naturalist, M. de Quatrefages. Captain Hutton also says[76] that the moths of all kinds vary much in colour, but in nearly the same inconstant manner. Considering how much the cocoons in the several races differ, this fact is of interest, and may probably be accounted for on the same principle as the fluctuating variability of colour in the caterpillar, namely, that there has been no motive for selecting and perpetuating any particular variation.

The males of the wild Bombycidæ “fly swiftly in the day-time and evening, but the females are usually very sluggish and inactive.”[77] In several moths of this family the females have abortive wings, but no instance is known of the males being incapable of flight, for in this case the species could hardly have been perpetuated. In the silk-moth both sexes have imperfect, crumpled wings, and are incapable of flight; but still there is a trace of the characteristic difference in the two sexes; for though, on comparing a number of males and females, I could detect no difference in the development of their wings, yet I was assured by Mrs. Whitby that the males of the moths bred by her used their wings more than the females, and could flutter downwards, though never upwards. She also states that, when the females first emerge from the cocoon, their wings are less expanded than those of the male. The degree of imperfection, however, in the wings varies much in different races and under different circumstances. M. Quatrefages[78] says that he has seen a number of moths with their wings reduced to a third, fourth, or tenth part of their normal dimensions, and even to mere short straight stumps: “il me semble qu’il y a là un véritable arrêt de développement partiel.” On the other hand, he describes the female moths of the André Jean breed as having “leurs ailes larges et étalées. Un seul présente quelques courbures irrégulières et des plis anormaux.” As moths and butterflies of all kinds reared from wild caterpillars under confinement often have crippled wings, the same cause, whatever it may be, has probably acted on silk-moths, but the disuse of their wings during so many generations has, it may be suspected, likewise come into play.

The moths of many breeds fail to glue their eggs to the surface on which they are laid,[79] but this proceeds, according to Capt. Hutton,[80] merely from the glands of the ovipositor being weakened.

As with other long-domesticated animals, the instincts of the silk-moth have suffered. The caterpillars, when placed on a mulberry-tree, often commit the strange mistake of devouring the base of the leaf on which they are feeding, and consequently fall down; but they are capable, according to M. Robinet,[81] of again crawling up the trunk. Even this capacity sometimes fails, for M. Martins[82] placed some caterpillars on a tree, and those which fell were not able to remount and perished of hunger; they were even incapable of passing from leaf to leaf.

Some of the modifications which the silk-moth has undergone stand in correlation with one another. Thus, the eggs of the moths which produce white cocoons and of those which produce yellow cocoons differ slightly in tint. The abdominal feet, also, of the caterpillars which yield white cocoons are always white, whilst those which give yellow cocoons are invariably yellow.[83] We have seen that the caterpillars with dark tiger-like stripes produce moths which are more darkly shaded than other moths. It seems well established[84] that in France the caterpillars of the races which produce white silk, and certain black caterpillars, have resisted, better than other races, the disease which has recently devastated the silk-districts. Lastly, the races differ constitutionally, for some do not succeed so well under a temperate climate as others; and a damp soil does not equally injure all the races.[85]

From these various facts we learn that silk-moths, like the higher animals, vary greatly under long-continued domestication. We learn also the more important fact that variations may occur at various periods of life, and be inherited at a corresponding period. And finally we see that insects are amenable to the great principle of Selection.

REFERENCES

[1] ‘Poultry Chronicle,’ 1854, vol. ii. p. 91 and vol. i. p. 330.

[2] Dr. Turral, ‘Bull. Soc. d’Acclimat.,’ tom. vii., 1860, p. 541.

[3] Willughby’s ‘Ornithology,’ by Ray, p. 381. This breed is also figured by Albin in 1734 in his ‘Nat. Hist. of Birds,’ vol. ii. p. 86.

[4] F. Cuvier, in ‘Annales du Muséum,’ tom. ix. p. 128, says that moulting and incubation alone stops these ducks laying. Mr. B. P. Brent makes a similar remark in the ‘Poultry Chronicle,’ 1855, vol. iii. p. 512.

[5] Rev. E. S. Dixon, ‘Ornamental and Domestic Poultry’ (1848), p. 117. Mr. B. P. Brent, in ‘Poultry Chronicle,’ vol. iii., 1855, p. 512.

[6] Crawfurd on the ‘Relation of Domesticated Animals to Civilisation,’ read before the Brit. Assoc. at Oxford, 1860.

[7] Dureau de La Malle, in ‘Annales des Sciences Nat.,’ tom. xvii. p. 164; and tom. xxi. p. 55. Rev. E. S. Dixon, ‘Ornamental Poultry,’ p. 118. Tame ducks were not known in Aristotle’s time, as remarked by Volz, in his ‘Beiträge zur Kulturgeschichte,’ 1852, s. 78.

[8] I quote this account from ‘Die Enten-und Schwanenzucht,’ Ulm 1828, s. 143. _See_ Audubon’s ‘Ornithological Biography,’ vol. iii. p. 168, on the taming of ducks on the Mississippi. For the same fact in England, _see_ Mr. Waterton in Loudon’s ‘Mag. of Nat. Hist.,’ vol. viii. 1835, p. 542; and Mr. St. John, ‘Wild Sports and Nat. Hist. of the Highlands,’ 1846, p. 129.

[9] Mr. E. Hewitt, in ‘Journal of Horticulture,’ 1862, p. 773; and 1863, p. 39.

[10] I have met with several statements on the fertility of the several breeds when crossed. Mr. Yarrell assured me that Call and common ducks are perfectly fertile together. I crossed Hook-billed and common ducks, and a Penguin and Labrador, and the crossed Ducks were quite fertile, though they were not bred _ inter se,_ so that the experiment was not fully tried. Some half-bred Penguins and Labradors were again crossed with Penguins, and subsequently bred by me _inter se,_ and they were extremely fertile.

[11] ‘Poultry Chronicle,’ 1855, vol. iii. p. 512.

[12] ‘Journal of the Indian Archipelago,’ vol. v. p. 334.

[13] ‘The Zoologist,’ vols. vii, viii. (1849-1850), p. 2353.

[14] ‘Poultry Chronicle,’ 1855, vol. iii. p. 512.

[15] ‘Poultry Chronicle,’ vol. iii. 1855, p. 312. With respect to Rouens _see_ ditto vol. i. 1854, p. 167.

[16] Col. Hawker’s ‘Instructions to young Sportsmen,’ quoted by Mr. Dixon in his ‘Ornamental Poultry,’ p. 125.

[17] ‘Cottage Gardener,’ April 9th, 1861.

[18] These hybrids have been described by M. Selys-Longchamps in the ‘Bulletins (tom. xii. No 10) Acad. Roy. de Bruxelles.’

[19] ‘Proc. Zoolog. Soc.,’ 1861, p. 261.

[20] ‘Ceylon,’ by Sir J. E. Tennent, 1859, vol. i. p. 485; also J. Crawfurd on the ‘Relation of Domest. Animals to Civilisation,’ read before Brit. Assoc. 1860. _See also_ ‘Ornamental Poultry,’ by Rev. E. S. Dixon, 1848, p. 132. The goose figured on the Egyptian monuments seems to have been the Red goose of Egypt.

[21] Macgillivray’s ‘British Birds,’ vol. iv. p. 593.

[22] Mr. A. Strickland (‘Annals and Mag. of Nat. Hist.,’ 3rd series, vol. iii. 1859, p. 122) reared some young wild geese, and found them in habits and in all characters identical with the domestic goose.

[23] _See also_ Hunter’s ‘Essays,’ edited by Owen, vol. ii. p. 322.

[24] Yarrell’s ‘British Birds,’ vol. iii. p. 142.

[25] L. Lloyd, ‘Scandinavian Adventures,’ 1854, vol. ii. p. 413, says that the wild goose lays from five to eight eggs, which is a much fewer number than that laid by our domestic goose.

[26] The Rev. L. Jenyns (Blomefield) seems first to have made this observation in his ‘British Animals.’ _See also_ Yarrell, and Dixon in his ‘Ornamental Poultry’ (p. 139), and ‘Gardener’s Chronicle,’ 1857, p. 45.

[27] Mr. Bartlet exhibited the head and neck of a bird thus characterised before the Zoological Soc., Feb. 1860.

[28] W. Thompson, ‘Natural Hist. of Ireland,’ 1851, vol. iii. p. 31. The Rev. E. S. Dixon gave me some information on the varying colour of the beak and legs.

[29] Mr. A. Strickland, in ‘Annals and Mag. of Nat. Hist.,’ 3rd series, vol. iii., 1859, p. 122.

[30] ‘Poultry Chronicle,’ vol. i., 1854, p. 498; vol. iii. p. 210.

[31] ‘The Cottage Gardener.’ Sept. 4th, 1860, p. 348.

[32] ‘L’Hist. de la Nature des Oiseaux,’ par P. Belon, 1555, p. 156. With respect to the livers of white geese being preferred by the Romans _see_ Isid. Geoffroy St.-Hilaire ‘Hist. Nat. Gén.,’ tom. iii. p. 58.

[33] Mr. Sclater on the black-shouldered peacock of Latham, ‘Proc. Zoolog. Soc.,’ April 24th, 1860. Mr. Swinhoe at one time believed, (‘Ibis,’ July, 1868) that this kind of peafowl was found wild in Cochin China, but he has since informed me that he feels very doubtful on this head.

[34] ‘Proc. Zoolog. Soc.,’ April 14th, 1835.

[35] ‘The Field,’ May 6th, 1871. I am much indebted to Mr. Canning for information with respect to his birds.

[36] ‘Proc. Zoolog. Soc.,’ April 8th, 1856, p. 61. Prof. Baird believes (as quoted in Tegetmeier’s ‘Poultry Book,’ 1866, p. 269) that our turkeys are descended from a West Indian species now extinct. But besides the improbability of a bird having long ago become extinct in these large and luxuriant islands, it appears (as we shall presently see) that the turkey degenerates in India, and this fact indicates that it was not aboriginally an inhabitant of the lowlands of the tropics.

[37] Audubon’s ‘Ornithological Biography,’ vol. i., 1831, pp. 4-13; and ‘Naturalist’s Library,’ vol. xiv., Birds, p. 138.

[38] F. Michaux, ‘Travels in N. America,’ 1802, Eng. translat., p. 217.

[39] ‘Ornamental Poetry,’ by the Rev. E. S. Dixon, 1848, p. 34.

[40] Bechstein, ‘Naturgesch. Deutschlands,’ B. iii., 1793, s. 309.

[41] Mr. Bartlett in ‘Land and Water,’ Oct. 31st, 1868, p. 233; and Mr. Tegetmeier in the ‘Field,’ July 17th, 1869, p. 46.

[42] ‘Gardener’s Chronicle,’ 1852, p. 699.

[43] E. Blyth, in ‘Annals and Mag. of Nat. Hist.,’ 1847, vol. xx. p. 391.

[44] Roulin makes this remark in ‘Mém. de divers Savans, l’Acad. des Sciences,’ tom. vi., 1835, p. 349. Mr. Hill, of Spanish Town, in a letter to me, describes five varieties of the Guinea fowl in Jamaica. I have seen singular pale-coloured varieties imported from Barbadoes and Demerara.

[45] For St. Domingo, _see_ M. A. Salle, in ‘Proc. Zoolog. Soc.’ 1857, p. 236. Mr. Hill remarks to me, in his letter, on the colour of the legs of the feral birds in Jamaica.

[46] Mr. B. P. Brent, ‘The Canary, British Finches,’ etc., pp. 21, 30.

[47] ‘Cottage Gardener,’ Dec. 11th, 1855, p. 184: an account is here given of all the varieties. For many measurements of the wild birds, _see_ Mr. E. Vernon Harcourt, ibid., Dec. 25th, 1855, p. 223.

[48] Bechstein, ‘Naturgesch. der Stubenvögel,’ 1840, s. 243; _see_ s. 252 on the inherited song of Canary-birds. With respect to their baldness _see also_ W. Kidd’s ‘Treatise on Song-Birds.’

[49] W. Kidd’s ‘Treatise on Song-Birds,’ p. 18.

[50] The ‘Indian Field,’ 1858, p. 255.

[51] Yarrell’s ‘British Fishes,’ vol. i. p. 319.

[52] Mr. Blyth in the ‘Indian Field,’ 1858, p. 255.

[53] W. F. Mayers, ‘Chinese Notes and Queries,’ Aug. 1868, p. 123.

[54] ‘Proc. Zoolog. Soc.’ May 25, 1842.)

[55] Yarrell’s ‘British Fishes,’ vol. i. p. 319.

[56] ‘Dict. Class. d’Hist. Nat.,’ tom. v. p. 276.

[57] ‘Observations in Nat. Hist.,’ 1846, p. 211. Dr. Gray has described, in ‘Annals and Mag. of Nat. Hist.,’ 1860, p. 151 a nearly similar variety but destitute of a dorsal fin.

[58] ‘De l’Espèce,’ 1859, p. 459. With respect to the bees of Burgundy _see_ M. Gerard, art. ‘Espèce,’ in ‘Dict. Univers. d’Hist. Nat.’

[59] _See_ a discussion on this subject, in answer to a question of mine, in ‘Journal of Horticulture,’ 1862, pp. 225-242; also Mr. Bevan Fox, in ditto, 1862, p. 284.

[60] This excellent observer may be implicitly trusted; _see_ ‘Journal of Horticulture,’ July 14th, 1863, p. 39.

[61] ‘Journal of Horticulture,’ Sept. 9th, 1862, p. 463; _see also_ Herr Kleine on same subject (Nov. 11th, p. 643, who sums up, that, though there is some variability in colour, no constant or perceptible differences can be detected in the bees of Germany.

[62] Mr. Woodbury has published several such accounts in ‘Journal of Horticulture,’ 1861 and 1862.

[63] ‘Annals and Mag. of Nat. Hist.,’ 3rd series, vol. xi. p. 339.

[64] ‘The Cottage Gardener,’ May 1860, p. 110; and ditto in ‘Journal of Hort.,’ 1862, p. 242.

[65] ‘Transact. Entomolog. Soc.’ 3rd series, vol. iii. pp. 143-173 and pp. 295-331.

[66] Godron, ‘De l’Espèce,’ 1859, tom. i. p. 460. The antiquity of the silkworm in China is given on the authority of Stanislas Julien.

[67] _See_ the remarks of Prof. Westwood, Gen. Hearsey and others at the meeting of the Entomolog. Soc. of London, July, 1861.

[68] _See_ for instance M. A. de Quatrefages’ ‘Études sur les Maladies actuelles du Ver à Soie,’ 1859, p. 101.

[69] My authorities for the statements will be given in the chapter on Selection.

[70] ‘Manuel de l’Éducateur de Vers à Soie,’ 1848.

[71] Robinet, ibid., pp. 12, 318. I may add that the eggs of N. American silkworms taken to the Sandwich Islands produced moths at very irregular periods; and the moths thus raised yielded eggs which were even worse in this respect. Some were hatched in ten days, and others not until after the lapse of many months. No doubt a regular early character would ultimately have been acquired. _See_ review in ‘Athenæum,’ 1844, p. 329, of J. Jarves’ ‘Scenes in the Sandwich Islands.’

[72] ‘The Art of rearing Silk-worms,’ translated from Count Dandolo, 1825, p. 23.

[73] ‘Transact. Ent. Soc.,’ ut supra, pp. 153, 308.

[74] Robinet, ibid., p. 317.

[75] Robinet, ibid., pp. 306-317.

[76] ‘Transact. Ent. Soc.,’ ut supra, p. 317.

[77] Stephen’s Illustrations, ‘Haustellata,’ vol. ii. p. 35. _ See also_ Capt. Hutton, ‘Transact. Ent. Soc.,’ ibid., p. 152.

[78] ‘Études sur les Maladies du Ver à Soie,’ 1859, pp. 304, 209.

[79] Quatrefages, ‘Études,’ etc., p. 214.

[80] ‘Transact. Ent. Soc.,’ ut supra, p. 151.

[81] ‘Manuel de l’Educateur,’ etc., p. 26.

[82] Godron, ‘De l’Espèce,’ p. 462.

[83] Quatrefages, ‘Études,’ etc., pp. 12, 209, 214.

[84] Robinet, ‘Manuel,’ etc., p. 303.

[85] Robinet, ibid., p. 15.