There is reason to believe that insects are affected by confinement like the higher animals. It is well known that the Sphingidae rarely breed when thus treated. An entomologist (18/60. Loudon"s "Mag. of Nat. Hist." volume 5 1832 page 153.) in Paris kept twenty-five specimens of Saturnia pyri, but did not succeed in getting a single fertile egg. A number of females of Orthosia munda and of Mamestra suasa reared in confinement were unattractive to the males.
(18/61. "Zoologist" volumes 5-6 1847-48 page 1660.) Mr. Newport kept nearly a hundred individuals of two species of Vanessa, but not one paired; this, however, might have been due to their habit of coupling on the wing. (18/62.
"Transact. Entomolog. Soc." volume 4 1845 page 60.) Mr. Atkinson could never succeed in India in making the Tarroo silk-moth breed in confinement. (18/63.
"Transact. Linn. Soc." volume 7 page 40.) It appears that a number of moths, especially the Sphingidae, when hatched in the autumn out of their proper season, are completely barren; but this latter case is still involved in some obscurity. (18/64. See an interesting paper by Mr. Newman in the "Zoologist"
1857 page 5764; and Dr. Wallace in "Proc. Entomolog. Soc." June 4, 1860 page 119.)]
Independently of the fact of many animals under confinement not coupling, or, if they couple, not producing young, there is evidence of another kind that their s.e.xual functions are disturbed. For many cases have been recorded of the loss by male birds when confined of their characteristic plumage. Thus the common linnet (Linota cannabina) when caged does not acquire the fine crimson colour on its breast, and one of the buntings (Emberiza pa.s.serina) loses the black on its head. A Pyrrhula and an Oriolus have been observed to a.s.sume the quiet plumage of the hen-bird; and the Falco albidus returned to the dress of an earlier age. (18/65. Yarrell "British Birds" volume 1 page 506; Bechstein "Stubenvogel" s. 185; "Philosoph. Transact." 1772 page 271. Bronn "Geschichte der Natur" b. 2 s. 96 has collected a number of cases. For the case of the deer see "Penny Cyclop." volume 8 page 350.) Mr. Thompson, the superintendent of the Knowsley menagerie, informed me that he had often observed a.n.a.logous facts. The horns of a male deer (Cervus canadensis) during the voyage from America were badly developed; but subsequently in Paris perfect horns were produced.
When conception takes place under confinement, the young are often born dead, or die soon, or are ill-formed. This frequently occurs in the Zoological Gardens, and, according to Rengger, with native animals confined in Paraguay.
The mother"s milk often fails. We may also attribute to the disturbance of the s.e.xual functions the frequent occurrence of that monstrous instinct which leads the mother to devour her own offspring,--a mysterious case of perversion, as it at first appears.
Sufficient evidence has now been advanced to prove that animals when first confined are eminently liable to suffer in their reproductive systems. We feel at first naturally inclined to attribute the result to loss of health, or at least to loss of vigour; but this view can hardly be admitted when we reflect how healthy, long-lived, and vigorous many animals are under captivity, such as parrots, and hawks when used for hawking, cheetahs when used for hunting, and elephants. The reproductive organs themselves are not diseased; and the diseases, from which animals in menageries usually perish, are not those which in any way affect their fertility. No domestic animal is more subject to disease than the sheep, yet it is remarkably prolific. The failure of animals to breed under confinement has been sometimes attributed exclusively to a failure in their s.e.xual instincts: this may occasionally come into play, but there is no obvious reason why this instinct should be especially liable to be affected with perfectly tamed animals, except, indeed, indirectly through the reproductive system itself being disturbed. Moreover, numerous cases have been given of various animals which couple freely under confinement, but never conceive; or, if they conceive and produce young, these are fewer in number than is natural to the species. In the vegetable kingdom instinct of course can play no part; and we shall presently see that plants when removed from their natural conditions are affected in nearly the same manner as animals.
Change of climate cannot be the cause of the loss of fertility, for, whilst many animals imported into Europe from extremely different climates breed freely, many others when confined in their native land are completely sterile.
Change of food cannot be the chief cause; for ostriches, ducks, and many other animals, which must have undergone a great change in this respect, breed freely. Carnivorous birds when confined are extremely sterile, whilst most carnivorous mammals, except plantigrades, are moderately fertile. Nor can the amount of food be the cause; for a sufficient supply will certainly be given to valuable animals; and there is no reason to suppose that much more food would be given to them than to our choice domestic productions which retain their full fertility. Lastly, we may infer from the case of the elephant, cheetah, various hawks, and of many animals which are allowed to lead an almost free life in their native land, that want of exercise is not the sole cause.
It would appear that any change in the habits of life, whatever these habits may be, if great enough, tends to affect in an inexplicable manner the powers of reproduction. The result depends more on the const.i.tution of the species than on the nature of the change; for certain whole groups are affected more than others; but exceptions always occur, for some species in the most fertile groups refuse to breed, and some in the most sterile groups breed freely.
Those animals which usually breed freely under confinement, rarely breed, as I was a.s.sured, in the Zoological Gardens, within a year or two after their first importation. When an animal which is generally sterile under confinement happens to breed, the young apparently do not inherit this power: for had this been the case, various quadrupeds and birds, which are valuable for exhibition, would have become common. Dr. Broca even affirms (18/66. "Journal de Physiologie" tome 2 page 347.) that many animals in the Jardin des Plantes, after having produced young for three or four successive generations, become sterile; but this may be the result of too close interbreeding. It is a remarkable circ.u.mstance that many mammals and birds have produced hybrids under confinement quite as readily as, or even more readily than, they have procreated their own kind. Of this fact many instances have been given (18/67.
For additional evidence on this subject see F. Cuvier in "Annales du Museum"
tome 12 page 119.); and we are thus reminded of those plants which when cultivated refuse to be fertilised by their own pollen, but can easily be fertilised by that of a distinct species. Finally, we must conclude, limited as the conclusion is, that changed conditions of life have an especial power of acting injuriously on the reproductive system. The whole case is quite peculiar, for these organs, though not diseased, are thus rendered incapable of performing their proper functions, or perform them imperfectly.
[STERILITY OF DOMESTICATED ANIMALS FROM CHANGED CONDITIONS.
With respect to domesticated animals, as their domestication mainly depends on the accident of their breeding freely under captivity, we ought not to expect that their reproductive system would be affected by any moderate degree of change. Those orders of quadrupeds and birds, of which the wild species breed most readily in our menageries, have afforded us the greatest number of domesticated productions. Savages in most parts of the world are fond of taming animals (18/68. Numerous instances could be given. Thus Livingstone ("Travels" page 217) states that the King of the Barotse, an inland tribe which never had any communication with white men, was extremely fond of taming animals, and every young antelope was brought to him. Mr. Galton informs me that the Damaras are likewise fond of keeping pets. The Indians of South America follow the same habit. Capt. Wilkes states that the Polynesians of the Samoan Islands tamed pigeons; and the New Zealanders, as Mr. Mantell informs me, kept various kinds of birds.); and if any of these regularly produced young, and were at the same time useful, they would be at once domesticated.
If, when their masters migrated into other countries, they were in addition found capable of withstanding various climates, they would be still more valuable; and it appears that the animals which breed readily in captivity can generally withstand different climates. Some few domesticated animals, such as the reindeer and camel, offer an exception to this rule. Many of our domesticated animals can bear with undiminished fertility the most unnatural conditions; for instance, rabbits, guinea-pigs, and ferrets breed in miserably confined hutches. Few European dogs of any kind withstand the climate of India without degenerating, but as long as they survive, they retain, as I hear from Dr. Falconer, their fertility; so it is, according to Dr. Daniell, with English dogs taken to Sierra Leone. The fowl, a native of the hot jungles of India, becomes more fertile than its parent-stock in every quarter of the world, until we advance as far north as Greenland and Northern Siberia, where this bird will not breed. Both fowls and pigeons, which I received during the autumn direct from Sierra Leone, were at once ready to couple. (18/69. For a.n.a.logous cases with the fowl see Reaumur "L"Art de faire Eclore" etc. 1749 page 243; and Col. Sykes in "Proc. Zoolog. Soc." 1832 etc. With respect to the fowl not breeding in northern regions see Latham "Hist. of Birds" volume 8 1823 page 169.) I have, also, seen pigeons breeding as freely as the common kinds within a year after their importation from the upper Nile. The guinea- fowl, an aboriginal of the hot and dry deserts of Africa, whilst living under our damp and cool climate, produces a large supply of eggs.
Nevertheless, our domesticated animals under new conditions occasionally show signs of lessened fertility. Roulin a.s.serts that in the hot valleys of the equatorial Cordillera sheep are not fully fecund (18/70. "Mem. par divers Savans" "Acad. des Sciences" tome 6 1835 page 347.); and according to Lord Somerville (18/71. "Youatt on Sheep" page 181.) the merino-sheep which he imported from Spain were not at first perfectly fertile, it is said (18/72. J.
Mills "Treatise on Cattle" 1776 page 72.) that mares brought up on dry food in the stable, and turned out to gra.s.s, do not at first breed. The peahen, as we have seen, is said not to lay so many eggs in England as in India. It was long before the canary-bird was fully fertile, and even now first-rate breeding birds are not common. (18/73. Bechstein "Stubenvogel" s. 242.) In the hot and dry province of Delhi, as I hear from Dr. Falconer, the eggs of the turkey, though placed under a hen, are extremely liable to fail. According to Roulin, geese taken to the lofty plateau of Bogota, at first laid seldom, and then only a few eggs; of these scarcely a fourth were hatched, and half the young birds died; in the second generation they were more fertile; and when Roulin wrote they were becoming as fertile as our geese in Europe. With respect to the valley of Quito, Mr. Orton says (18/74. "The Andes and the Amazon" 1870 page 107.) "the only geese in the valley are a few imported from Europe, and these refuse to propagate." In the Philippine Archipelago the goose, it is a.s.serted, will not breed or even lay eggs. (18/75. Crawford "Descriptive Dict.
of the Indian Islands" 1856 page 145.) A more curious case is that of the fowl, which, according to Roulin, when first introduced would not breed at Cusco in Bolivia, but subsequently became quite fertile; and the English Game fowl, lately introduced, had not as yet arrived at its full fertility, for to raise two or three chickens from a nest of eggs was thought fortunate. In Europe close confinement has a marked effect on the fertility of the fowl: it has been found in France that with fowls allowed considerable freedom only twenty per cent of the eggs failed; when allowed less freedom forty per cent failed; and in close confinement sixty out of the hundred were not hatched.
(18/76. "Bull. de la Soc. d"Acclimat." tome 9 1862 pages 380, 384.) So we see that unnatural and changed conditions of life produce some effect on the fertility of our most thoroughly domesticated animals, in the same manner, though in a far less degree, as with captive wild animals.
It is by no means rare to find certain males and females which will not breed together, though both are known to be perfectly fertile with other males and females. We have no reason to suppose that this is caused by these animals having been subjected to any change in their habits of life; therefore such cases are hardly related to our present subject. The cause apparently lies in an innate s.e.xual incompatibility of the pair which are matched. Several instances have been communicated to me by Mr. W.C. Spooner (well known for his essay on Cross-breeding), by Mr. Eyton of Eyton, by Mr. Wicksted and other breeders, and especially by Mr. Waring of Chelsfield, in relation to horses, cattle, pigs, foxhounds, other dogs, and pigeons. (18/77. For pigeons see Dr.
Chapuis "Le Pigeon Voyageur Belge" 1865 page 66.) In these cases, females, which either previously or subsequently were proved to be fertile, failed to breed with certain males, with whom it was particularly desired to match them.
A change in the const.i.tution of the female may sometimes have occurred before she was put to the second male; but in other cases this explanation is hardly tenable, for a female, known not to be barren, has been unsuccessfully paired seven or eight times with the same male likewise known to be perfectly fertile. With cart-mares, which sometimes will not breed with stallions of pure blood, but subsequently have bred with cart-stallions, Mr. Spooner is inclined to attribute the failure to the lesser s.e.xual power of the racehorse.
But I have heard from the greatest breeder of racehorses at the present day, through Mr. Waring, that "it frequently occurs with a mare to be put several times during one or two seasons to a particular stallion of acknowledged power, and yet prove barren; the mare afterwards breeding at once with some other horse." These facts are worth recording, as they show, like so many previous facts, on what slight const.i.tutional differences the fertility of an animal often depends.]
STERILITY OF PLANTS FROM CHANGED CONDITIONS OF LIFE, AND FROM OTHER CAUSES.
In the vegetable kingdom cases of sterility frequently occur, a.n.a.logous with those previously given in the animal kingdom. But the subject is obscured by several circ.u.mstances, presently to be discussed, namely, the contabescence of the anthers, as Gartner has named a certain affection--monstrosities-- doubleness of the flower--much-enlarged fruit--and long-continued or excessive propagation by buds.
[It is notorious that many plants in our gardens and hot-houses, though preserved in the most perfect health, rarely or never produce seed. I do not allude to plants which run to leaves, from being kept too damp, or too warm, or too much manured; for these do not flower, and the case may be wholly different. Nor do I allude to fruit not ripening from want of heat or rotting from too much moisture. But many exotic plants, with their ovules and pollen appearing perfectly sound, will not set any seed. The sterility in many cases, as I know from my own observation, is simply due to the absence of the proper insects for carrying the pollen to the stigma. But after excluding the several cases just specified, there are many plants in which the reproductive system has been seriously affected by the altered conditions of life to which they have been subjected.
It would be tedious to enter on many details. Linnaeus long ago observed (18/78. "Swedish Acts" volume 1 1739 page 3. Pallas makes the same remark in his "Travels" English translation volume 1 page 292.) that Alpine plants, although naturally loaded with seed, produce either few or none when cultivated in gardens. But exceptions often occur: the Draba sylvestris, one of our most thoroughly Alpine plants, multiplies itself by seed in Mr. H.C.
Watson"s garden, near London; and Kerner, who has particularly attended to the cultivation of Alpine plants, found that various kinds, when cultivated, spontaneously sowed themselves. (18/79. A. Kerner "Die Cultur der Alpenpflanzen" 1864 s. 139; Watson "Cybele Britannica" volume 1 page 131; Mr.
D. Cameron also has written on the culture of Alpine plants in "Gard.
Chronicle" 1848 pages 253, 268, and mentions a few which seed.) Many plants which naturally grow in peat-earth are entirely sterile in our gardens. I have noticed the same fact with several liliaceous plants, which nevertheless grew vigorously.
Too much manure renders some kinds utterly sterile, as I have myself observed.
The tendency to sterility from this cause runs in families; thus, according to Gartner (18/80. "Beitrage zur Kenntniss der Befruchtung" 1844 s. 333.), it is hardly possible to give too much manure to most Gramineae, Cruciferae, and Leguminosae, whilst succulent and bulbous-rooted plants are easily affected.
Extreme poverty of soil is less apt to induce sterility; but dwarfed plants of Trifolium minus and repens, growing on a lawn often mown and never manured, were found by me not to produce any seed. The temperature of the soil, and the season at which plants are watered, often have a marked effect on their fertility, as was observed by Kolreuter in the case of Mirabilis. (18/81.
"Nova Acta Petrop." 1793 page 391.) Mr. Scott, in the Botanic Gardens of Edinburgh, observed that Oncidium divaricatum would not set seed when grown in a basket in which it throve, but was capable of fertilisation in a pot where it was a little damper. Pelargonium fulgidum, for many years after its introduction, seeded freely; it then became sterile; now it is fertile (18/82.
"Cottage Gardener" 1856 pages 44, 109.) if kept in a dry stove during the winter. Other varieties of pelargonium are sterile and others fertile without our being able to a.s.sign any cause. Very slight changes in the position of a plant, whether planted on a bank or at its base, sometimes make all the difference in its producing seed. Temperature apparently has a much more powerful influence on the fertility of plants than on that of animals.
Nevertheless it is wonderful what changes some few plants will withstand with undiminished fertility: thus the Zephyranthes candida, a native of the moderately warm banks of the Plata, sows itself in the hot dry country near Lima, and in Yorkshire resists the severest frosts, and I have seen seeds gathered from pods which had been covered with snow during three weeks.
(18/83. Dr. Herbert "Amaryllidaceae" page 176.) Berberis wallichii, from the hot Khasia range in India, is uninjured by our sharpest frosts, and ripens its fruit under our cool summers. Nevertheless, I presume we must attribute to change of climate the sterility of many foreign plants; thus, the Persian and Chinese lilacs (Syringa persica and chinensis), though perfectly hardy here, never produce a seed; the common lilac (S. vulgaris) seeds with us moderately well, but in parts of Germany the capsules never contain seed. (18/84. Gartner "Beitrage zur Kenntniss" etc. s. 560, 564.) Some few of the cases, given in the last chapter, of self-impotent plants, might have been here introduced, as their state seems due to the conditions to which they have been subjected.
The liability of plants to be affected in their fertility by slightly changed conditions is the more remarkable, as the pollen when once in process of formation is not easily injured; a plant may be transplanted, or a branch with flower-buds be cut off and placed in water, and the pollen will be matured.
Pollen, also, when once mature, may be kept for weeks or even months. (18/85.
"Gardener"s Chronicle" 1844 page 215; 1850 page 470. Faivre gives a good resume on this subject in his "La Variabilite des Especes" 1868 page 155.) The female organs are more sensitive, for Gartner (18/86. "Beitrage zur Kenntniss"
etc. s. 252, 338.) found that dicotyledonous plants, when carefully removed so that they did not in the least flag, could seldom be fertilised; this occurred even with potted plants if the roots had grown out of the hole at the bottom.
In some few cases, however, as with Digitalis, transplantation did not prevent fertilisation; and according to the testimony of Mawz, Bra.s.sica rapa, when pulled up by its roots and placed in water, ripened its seed. Flower-stems of several monocotyledonous plants when cut off and placed in water likewise produce seed. But in these cases I presume that the flowers had been already fertilised, for Herbert (18/87. "Journal of Hort. Soc." volume 2 1847 page 83.) found with the Crocus that the plants might be removed or mutilated after the act of fertilisation, and would still perfect their seeds; but that, if transplanted before being fertilised, the application of pollen was powerless.
Plants which have been long cultivated can generally endure with undiminished fertility various and great changes; but not in most cases so great a change of climate as domesticated animals. It is remarkable that many plants under these circ.u.mstances are so much affected that the proportion and the nature of their chemical ingredients are modified, yet their fertility is unimpaired.
Thus, as Dr. Falconer informs me, there is a great difference in the character of the fibre in hemp, in the quant.i.ty of oil in the seed of the Linum, in the proportion of narcotin to morphine in the poppy, in gluten to starch in wheat, when these plants are cultivated on the plains and on the mountains of India; nevertheless, they all remain fully fertile.
CONTABESCENCE.
Gartner has designated by this term a peculiar condition of the anthers in certain plants, in which they are shrivelled, or become brown and tough, and contain no good pollen. When in this state they exactly resemble the anthers of the most sterile hybrids. Gartner (18/88. "Beitrage zur Kenntniss" etc. s.
117 et seq.; Kolreuter "Zweite Fortsetzung" s. 10, 121; "Dritte Fortsetzung"
s. 57. Herbert "Amaryllidaceae" page 355. Wiegmann "Ueber die b.a.s.t.a.r.derzeugung" s. 27.), in his discussion on this subject, has shown that plants of many orders are occasionally thus affected; but the Caryophyllaceae and Liliaceae suffer most, and to these orders, I think, the Ericaceae may be added. Contabescence varies in degree, but on the same plant all the flowers are generally affected to nearly the same extent. The anthers are affected at a very early period in the flower-bud, and remain in the same state (with one recorded exception) during the life of the plant. The affection cannot be cured by any change of treatment, and is propagated by layers, cuttings, etc., and perhaps even by seed. In contabescent plants the female organs are seldom affected, or merely become precocious in their development. The cause of this affection is doubtful, and is different in different cases. Until I read Gartner"s discussion I attributed it, as apparently did Herbert, to the unnatural treatment of the plants; but its permanence under changed conditions, and the female organs not being affected, seem incompatible with this view. The fact of several endemic plants becoming contabescent in our gardens seems, at first sight, equally incompatible with this view; but Kolreuter believes that this is the result of their transplantation. The contabescent plants of Dianthus and Verbasc.u.m, found wild by Wiegmann, grew on a dry and sterile bank. The fact that exotic plants are eminently liable to this affection also seems to show that it is in some manner caused by their unnatural treatment. In some instances, as with Silene, Gartner"s view seems the most probable, namely, that it is caused by an inherent tendency in the species to become dioecious. I can add another cause, namely, the illegitimate unions of heterostyled plants, for I have observed seedlings of three species of Primula and of Lythrum salicaria, which had been raised from plants illegitimately fertilised by their own-form pollen, with some or all their anthers in a contabescent state. There is perhaps an additional cause, namely, self-fertilisation; for many plants of Dianthus and Lobelia, which had been raised from self-fertilised seeds, had their anthers in this state; but these instances are not conclusive, as both genera are liable from other causes to this affection.
Cases of an opposite nature likewise occur, namely, plants with the female organs struck with sterility, whilst the male organs remain perfect. Dianthus j.a.ponicus, a Pa.s.siflora, and Nicotiana, have been described by Gartner (18/89.
"b.a.s.t.a.r.derzengung" s. 356.) as being in this unusual condition.
MONSTROSITIES AS A CAUSE OF STERILITY.
Great deviations of structure, even when the reproductive organs themselves are not seriously affected, sometimes cause plants to become sterile. But in other cases plants may become monstrous to an extreme degree and yet retain their full fertility. Gallesio, who certainly had great experience (18/90.
"Teoria della Riproduzione" 1816 page 84; "Traite du Citrus" 1811 page 67.), often attributes sterility to this cause; but it may be suspected that in some of his cases sterility was the cause, and not the result, of the monstrous growths. The curious St. Valery apple, although it bears fruit, rarely produces seed. The wonderfully anomalous flowers of Begonia frigida, formerly described, though they appear fit for fructification, are sterile. (18/91. Mr.
C.W. Crocker in "Gardener"s Chronicle" 1861 page 1092.) Species of Primula in which the calyx is brightly coloured are said (18/92. Verlot "Des Varietes"
1865 page 80.) to be often sterile, though I have known them to be fertile. On the other hand, Verlot gives several cases of proliferous flowers which can be propagated by seed. This was the case with a poppy, which had become monopetalous by the union of its petals. (18/93. Verlot ibid page 88.) Another extraordinary poppy, with the stamens replaced by numerous small supplementary capsules, likewise reproduces itself by seed. This has also occurred with a plant of Saxifraga geum, in which a series of advent.i.tious carpels, bearing ovules on their margins, had been developed between the stamens and the normal carpels (18/94. Prof. Allman, Brit. a.s.soc., quoted in the "Phytologist" volume 2 page 483. Prof. Harvey, on the authority of Mr. Andrews, who discovered the plant, informed me that this monstrosity could be propagated by seed. With respect to the poppy see Prof. Goeppert as quoted in "Journal of Horticulture"
July 1, 1863 page 171.) Lastly, with respect to peloric flowers, which depart wonderfully from the natural structure,--those of Linaria vulgaris seem generally to be more or less sterile, whilst those before described of Antirrhinum majus, when artificially fertilised with their own pollen, are perfectly fertile, though sterile when left to themselves, for bees are unable to crawl into the narrow tubular flower. The peloric flowers of Corydalis solida, according to G.o.dron (18/95. "Comptes Rendus" December 19, 1864 page 1039.), are sometimes barren and sometimes fertile; whilst those of Gloxinia are well known to yield plenty of seed. In our greenhouse Pelargoniums, the central flower of the truss is often peloric, and Mr. Masters informs me that he tried in vain during several years to get seed from these flowers. I likewise made many vain attempts, but sometimes succeeded in fertilising them with pollen from a normal flower of another variety; and conversely I several times fertilised ordinary flowers with peloric pollen. Only once I succeeded in raising a plant from a peloric flower fertilised by pollen from a peloric flower borne by another variety; but the plant, it may be added, presented nothing particular in its structure. Hence we may conclude that no general rule can be laid down; but any great deviation from the normal structure, even when the reproductive organs themselves are not seriously affected, certainly often leads to s.e.xual impotence.
DOUBLE FLOWERS.
When the stamens are converted into petals, the plant becomes on the male side sterile; when both stamens and pistils are thus changed, the plant becomes completely barren. Symmetrical flowers having numerous stamens and petals are the most liable to become double, as perhaps follows from all multiple organs being the most subject to variability. But flowers furnished with only a few stamens, and others which are asymmetrical in structure, sometimes become double, as we see with the double gorse or Ulex, and Antirrhinum. The Compositae bear what are called double flowers by the abnormal development of the corolla of their central florets. Doubleness is sometimes connected with prolification (18/96. "Gardener"s Chronicle" 1866 page 681.), or the continued growth of the axis of the flower. Doubleness is strongly inherited. No one has produced, as Lindley remarks (18/97. "Theory of Horticulture" page 333.), double flowers by promoting the perfect health of the plant. On the contrary, unnatural conditions of life favour their production. There is some reason to believe that seeds kept during many years, and seeds believed to be imperfectly fertilised, yield double flowers more freely than fresh and perfectly fertilised seed. (18/98. Mr. Fairweather "Transact. Hort. Soc."
volume 3 page 406: Bosse quoted by Bronn "Geschichte der Natur" b. 2 s. 77. On the effects of the removal of the anthers see Mr. Leitner in Silliman"s "North American Journ. of Science" volume 23 page 47; and Verlot "Des Varietes" 1865 page 84.) Long-continued cultivation in rich soil seems to be the commonest exciting cause. A double narcissus and a double Anthemis n.o.bilis, transplanted into very poor soil, has been observed to become single (18/99. Lindley"s "Theory of Horticulture" page 3?3.); and I have seen a completely double white primrose rendered permanently single by being divided and transplanted whilst in full flower. It has been observed by Professor E. Morren that doubleness of the flowers and variegation of the leaves are antagonistic states; but so many exceptions to the rule have lately been recorded (18/100. "Gardener"s Chronicle" 1865 page 626; 1866 pages 290, 730; and Verlot "Des Varietes" page 75.), that, though general, it cannot be looked at as invariable. Variegation seems generally to result from a feeble or atrophied condition of the plant, and a large proportion of the seedlings raised from parents, if both are variegated, usually perish at an early age; hence we may perhaps infer that doubleness, which is the antagonistic state, commonly arises from a plethoric condition. On the other hand, extremely poor soil sometimes, though rarely, appears to cause doubleness: I formerly described (18/101. "Gardener"s Chronicle" 1843 page 628. In this article I suggested the theory above given on the doubleness of flowers. This view is adopted by Carriere "Production et Fix. des Varietes" 1865 page 67.) some completely double, bud-like, flowers produced in large numbers by stunted wild plants of Gentiana amarella growing on a poor chalky bank. I have also noticed a distinct tendency to doubleness in the flowers of a Ranunculus, Horse-chestnut, and Bladder-nut (Ranunculus repens, Aesculus pavia, and Staphylea), growing under very unfavourable conditions. Professor Lehmann (18/102. Quoted by Gartner "b.a.s.t.a.r.derzeugung" s.
567.) found several wild plants growing near a hot spring with double flowers.
With respect to the cause of doubleness, which arises, as we see, under widely different circ.u.mstances, I shall presently attempt to show that the most probable view is that unnatural conditions first give a tendency to sterility, and that then, on the principle of compensation, as the reproductive organs do not perform their proper functions, they either become developed into petals, or additional petals are formed. This view has lately been supported by Mr.
Laxton (18/103. "Gardener"s Chronicle" 1866 page 901.) who advances the case of some common peas, which, after long-continued heavy rain, flowered a second time, and produced double flowers.
SEEDLESS FRUIT.
Many of our most valuable fruits, although consisting in a h.o.m.ological sense of widely different organs, are either quite sterile, or produce extremely few seeds. This is notoriously the case with our best pears, grapes, and figs, with the pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms, and some members of the orange-tribe. Poorer varieties of these same fruits either habitually or occasionally yield seed. (18/104. Lindley "Theory of Horticulture" pages 175-179; G.o.dron "De l"Espece" tome 2 page 106; Pickering "Races of Man;" Gallesio "Teoria della Riproduzione" l816 pages 101-110. Meyen "Reise um Erde" Th. 2 s. 214 states that at Manilla one variety of the banana is full of seeds: and Chamisso (Hooker"s "Bot. Misc." volume 1 page 310) describes a variety of the bread-fruit in the Mariana Islands with small fruit, containing seeds which are frequently perfect. Burnes in his "Travels in Bokhara" remarks on the pomegranate seeding in Mazenderan, as a remarkable peculiarity.) Most horticulturists look at the great size and anomalous development of the fruit as the cause, and sterility as the result; but the opposite view, as we shall presently see, is more probable.
STERILITY FROM THE EXCESSIVE DEVELOPMENT OF THE ORGANS OF GROWTH OR VEGETATION.
Plants which from any cause grow too luxuriantly, and produce leaves, stems, runners, suckers, tubers, bulbs, etc., in excess, sometimes do not flower, or if they flower do not yield seed. To make European vegetables under the hot climate of India yield seed, it is necessary to check their growth; and, when one-third grown, they are taken up, and their stems and tap-roots are cut or mutilated. (18/105. Ingledew in "Transact. of Agricult. and Hort. Soc. of India" volume 2.) So it is with hybrids; for instance, Prof. Lecoq (18/106.
"De la Fecondation" 1862 page 308.) had three plants of Mirabilis, which, though they grew luxuriantly and flowered, were quite sterile; but after beating one with a stick until a few branches alone were left, these at once yielded good seed. The sugar-cane, which grows vigorously and produces a large supply of succulent stems, never, according to various observers, bears seed in the West Indies, Malaga, India, Cochin China, Mauritius, or the Malay Archipelago. (18/107. Hooker "Bot. Misc." volume 1 page 99; Gallesio "Teoria della Riproduzione" page 110. Dr. J. de Cordemoy in "Transact. of the R. Soc.
of Mauritius" new series volume 6 1873 pages 60-67, gives a large number of cases of plants which never seed, including several species indigenous in Mauritius.) Plants which produce a large number of tubers are apt to be sterile, as occurs, to a certain extent, with the common potato; and Mr.
Fortune informs me that the sweet potato (Convolvulus batatas) in China never, as far as he has seen, yields seed. Dr. Royle remarks (18/108. "Transact.
Linn. Soc." volume 17 page 563.) that in India the Agave vivipara, when grown in rich soil, invariably produces bulbs, but no seeds; whilst a poor soil and dry climate lead to an opposite result. In China, according to Mr. Fortune, an extraordinary number of little bulbs are developed in the axils of the leaves of the yam, and this plant does not bear seed. Whether in these cases, as in those of double flowers and seedless fruit, s.e.xual sterility from changed conditions of life is the primary cause which leads to the excessive development of the organs of vegetation, is doubtful; though some evidence might be advanced in favour of this view. It is perhaps a more probable view that plants which propagate themselves largely by one method, namely by buds, have not sufficient vital power or organised matter for the other method of s.e.xual generation.