_Corydalis solida,_ according to Godron,[95] 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 sexual 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 Compositæ bear what are called double flowers by the abnormal development of the corolla of their central florets. Doubleness is sometimes connected with prolification,[96] or the continued growth of the axis of the flower. Doubleness is strongly inherited. No one has produced, as Lindley remarks,[97] 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.[98] Long-continued cultivation in rich soil seems to be the commonest exciting cause. A double narcissus and a double _Anthemis nobilis,_ transplanted into very poor soil, has been observed to become single;[99] 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,[100] 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[101] 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, and _Staphylea_), growing under very unfavourable conditions. Professor Lehmann[102] 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 circumstances, 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[103] 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 homological 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.[104] 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.[105] So it is with hybrids; for instance, Prof. Lecoq[106] 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.[107] 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[108] 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, sexual 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 sexual generation.
Several distinguished botanists and good practical judges believe that long- continued propagation by cuttings, runners, tubers, bulbs, etc., independently of any excessive development of these parts, is the cause of many plants failing to produce flowers, or producing only barren flowers,—it is as if they had lost the habit of sexual generation.[109] That many plants when thus propagated are sterile there can be no doubt, but as to whether the long continuance of this form of propagation is the actual cause of their sterility, I will not venture, from the want of sufficient evidence, to express an opinion.
That plants may be propagated for long periods by buds, without the aid of sexual generation, we may safely infer from this being the case with many plants which must have long survived in a state of nature. As I have had occasion before to allude to this subject, I will here give such cases as I have collected. Many alpine plants ascend mountains beyond the height at which they can produce seed.[110] Certain species of Poa and Festuca, when growing on mountain-pastures, propagate themselves, as I hear from Mr. Bentham, almost exclusively by bulblets.
Kalm gives a more curious instance[111] of several American trees, which grow so plentifully in marshes or in thick woods, that they are certainly well adapted for these stations, yet scarcely ever produce seeds; but when accidentally growing on the outside of the marsh or wood, are loaded with seed. The common ivy is found in Northern Sweden and Russia, but flowers and fruits only in the southern provinces. The _Acorus calamus_ extends over a large portion of the globe, but so rarely perfects fruit that this has been seen only by a few botanists; according to Caspary, all its pollen-grains are in a worthless condition.[112] The _Hypericum calycinum,_ which propagates itself so freely in our shrubberies by rhizomes, and is naturalised in Ireland, blossoms profusely, but rarely sets any seed, and this only during certain years; nor did it set any when fertilised in my garden by pollen from plants growing at a distance. The _Lysimachia nummularia,_ which is furnished with long runners, so seldom produces seed-capsules, that Prof. Decaisne,[113] who has especially attended to this plant, has never seen it in fruit. The _Carex rigida_ often fails to perfect its seed in Scotland, Lapland, Greenland, Germany, and New Hampshire in the United States.[114] The periwinkle (_Vinca minor_), which spreads largely by runners, is said scarcely ever to produce fruit in England;[115] but this plant requires insect-aid for its fertilisation, and the proper insects may be absent or rare. The Jussiaea grandiflora has become naturalised in Southern France, and has spread by its rhizomes so extensively as to impede the navigation of the waters, but never produces fertile seed.[116] The horse-radish (_Cochleria armoracia_) spreads pertinaciously and is naturalised in various parts of Europe; though it bears flowers, these rarely produce capsules: Professor Caspary informs me that he has watched this plant since 1851, but has never seen its fruit; 65 per cent of its pollen-grains are bad.
The common _ Ranunculus ficaria_ rarely bears seed in England, France, or Switzerland; but in 1863 I observed seeds on several plants growing near my house.[117] Other cases analogous with the foregoing could be given; for instance, some kinds of mosses and lichens have never been seen to fructify in France.
Some of these endemic and naturalised plants are probably rendered sterile from excessive multiplication by buds, and their consequent incapacity to produce and nourish seed. But the sterility of others more probably depends on the peculiar conditions under which they live, as in the case of the ivy in the northern part of Europe, and of the trees in the swamps of the United States; yet these plants must be in some respects eminently well adapted for the stations which they occupy, for they hold their places against a host of competitors.
Finally, the high degree of sterility which often accompanies the doubling of flowers, or an excessive development of fruit, seldom supervenes at once. An incipient tendency is observed, and continued selection completes the result. The view which seems the most probable, and which connects together all the foregoing facts and brings them within our present subject, is, that changed and unnatural conditions of life first give a tendency to sterility; and in consequence of this, the organs of reproduction being no longer able fully to perform their proper functions, a supply of organised matter, not required for the development of the seed, flows either into these organs and renders them foliaceous, or into the fruit, stems, tubers, etc., increasing their size and succulency. But it is probable that there exists, independently of any incipient sterility, an antagonism between the two forms of reproduction, namely, by seed and buds, when either is carried to an extreme degree. That incipient sterility plays an important part in the doubling of flowers, and in the other cases just specified, I infer chiefly from the following facts. When fertility is lost from a wholly different cause, namely, from hybridism, there is a strong tendency, as Gärtner[118] affirms, for flowers to become double, and this tendency is inherited. Moreover, it is notorious that with hybrids the male organs become sterile before the female organs, and with double flowers the stamens first become foliaceous. This latter fact is well shown by the male flowers of dioecious plants, which, according to Gallesio[119] first become double. Again, Gärtner[120] often insists that the flowers of even utterly sterile hybrids, which do not produce any seed, generally yield perfect capsules or fruit,—a fact which has likewise been repeatedly observed by Naudin with the Cucurbitaceæ; so that the production of fruit by plants rendered sterile through any cause is intelligible. Kölreuter has also expressed his unbounded astonishment at the size and development of the tubers in certain hybrids; and all experimentalists[121] have remarked on the strong tendency in hybrids to increase by roots, runners, and suckers. Seeing that hybrid plants, which from their nature are more or less sterile, thus tend to produce double flowers; that they have the parts including the seed, that is the fruit, perfectly developed, even when containing no seed; that they sometimes yield gigantic roots; that they almost invariably tend to increase largely by suckers and other such means;—seeing this, and knowing, from the many facts given in the earlier parts of this chapter, that almost all organic beings when exposed to unnatural conditions tend to become more or less sterile, it seems much the most probable view that with cultivated plants sterility is the exciting cause, and double flowers, rich seedless fruit, and in some cases largely-developed organs of vegetation, etc., are the indirect results—these results having been in most cases largely increased through continued selection by man.
REFERENCES [1] For England, _see_ below. For Germany, _see_ Metzger, (‘Consid. sur les Céréales,’ 1843, p. 200) gives numerous references on this subject. For Southern France, _see_ Godron, ‘Florula for the second statement, Ibid., 1850, p. 702. On this same subject vol. ii. p. 200. Also Marshall ‘Minutes of Agriculture,’ November, 1775.
For Mr. Abbey’s remarks on grafting, etc., Ibid., July 18, 1865, p.
44.
[8] Mr. Spencer has fully and ably discussed this whole subject in his ‘Principles of Biology,’ 1864, vol. ii. ch. x. In the first edition of my ‘Origin of Species,’ 1859, p. 267, I spoke of the good effects from slight changes in the conditions of life and from cross-breeding, and of the evil effects from great changes in the conditions and from crossing widely distinct forms, as a series of facts “connected together by some common but unknown bond, which is essentially related to the principle of life.”
[10] Since the appearance of the first edition of this work, Mr.
Sclater has published (‘Proc. Zoolog. Soc.,’ 1868, p. 623) a list of the species of mammals which have bred in the gardens from 1848 to 1867 inclusive. Of the Artiodactyla 85 species have been kept, and of these 1 species in 1·9 have bred at least once during the 20 years; of 28 Marsupialia, 1 in 2·5 have bred; of 74 Carnivora, 1 in 3·0 have bred; of 52 Rodentia, 1 in 4·7 have bred; and of Quadrumana 75 species have been kept, and 1 in 6·2 have bred.
249, 265, 327.
ii. p. 113.
[17] On the Breeding of the Larger Felidæ, ‘Proc. Zoolog. Soc.,’ 1861, p. 140.
[20] Rengger ‘Säugethiere,’ etc., s. 276. On the parentage of the sent to Mr. H. Denny of Leeds the lice which I collected from the wild aperea in La Plata, and he informs me that they belong to a genus distinct from those found on the guinea-pig. This is important evidence that the aperea is not the parent of the guinea-pig; and is worth giving, as some authors erroneously suppose that the guinea-pig since being domesticated has become sterile when crossed with the aperea.
[21] Although the existence of the _Leporides,_ as described by Dr.
Broca (‘Journal de Phys.,’ tom. ii. p. 370), has been positively denied, yet Dr. Pigeaux (‘Annals and Mag. of Nat. Hist.,’ vol. xx., 1867, p. 75) affirms that the hare and rabbit have produced hybrids.
[22] ‘Quadrupeds of North America,’ by Audubon and Bachman, 1846, p.
268.
[29] A list of the species of birds which have bred in the Zoological Gardens from 1848 to 1867 inclusive has been published by Mr. Sclater in ‘Proc. Zoolog. Soc.,’ 1869, p. 626, since the first edition of this work appeared. Of Columbæ 51 species have been kept, and of Anseres 80 species, and in both these families 1 species in 2·6 have bred at least once in the 20 years. Of Gallinæ 83 species have been kept and 1 in 27 have bred; of 57 Grallæ 1 in 9 have bred; of 110 Prehensores 1 in 22 have bred; of 178 Passeres 1 in 25·4 have bred; of 94 Accipitres 1 in 47 have bred; of 25 Picariæ and of 35 Herodiones not one species in either group has bred.
[31] According to Sir A. Burnes (‘Cabool,’ etc., p. 51), eight species are used for hawking in Sinde.
1851-2, p. 3207.
453. For the siskin breeding, vol. iii.-iv., 1845-46, p. 1075.
Bechstein ‘Stubenvögel,’ s. 139, speaks of bullfinches making nests, but rarely producing young.
parrots breeding in France are here recorded. _ See also_ ‘Report Brit. Assoc. Zoolog.,’ 1843.
[45] Dr. Hancock remarks (‘Charlesworth’s Mag. of Nat. Hist.’ vol.
ii., 1838, p. 492), “it is singular that, amongst the numerous useful birds that are indigenous to Guiana, none are found to propagate among the Indians; yet the common fowl is reared in abundance throughout the country.”
[48] Mowbray on Poultry, 7th edit., p. 133.
[49] Temminck, ‘Hist. Nat. Gén. des Pigeons,’ etc., 1813, tom. iii.
pp. 288, 382; ‘Annals and Mag. of Nat. Hist.,’ vol. xii., 1843, p.
453. Other species of partridge have occasionally bred; as the red-legged (_P. rubra_), when kept in a large court in France (_see_ ‘Journal de Physique,’ tom. xxv. p. 294), and in the Zoological Gardens in 1856.
[51] Temminck, ‘Hist. Nat. Gén. des Pigeons,’ etc., tom. ii. pp. 456, [52] Bates, ‘The Naturalist on the Amazons,’ vol. i. p. 193; vol. ii.
p. 112.
[53] Temminck, ‘Hist. Nat. Gén.,’ etc., tom. ii. p. 125. For _Tetrao urogallus, see_ L. Lloyd, ‘Field Sports of North of Europe,’ vol. i.
pp. 287, 314; and ‘Bull. de la Soc. d’Acclimat.,’ tom. vii., 1860, p.
600. For _T. scoticus,_ Thompson, ‘Nat. Hist. of Ireland,’ vol. ii.
iii. p. 199.
[54] Marcel de Serres, ‘Annales des Sc. Nat.,’ 2nd series, Zoolog., tom. xiii. p. 175.
[55] Dr. Hancock, in ‘Charlesworth’s Mag. of Nat. Hist.,’ vol. ii., Menagerie,’ by D. Gray, 1846, pl. xiv.; E. Blyth, ‘Report Asiatic Soc.
of Bengal,’ May 1855.
[64] _See_ an interesting paper by Mr. Newman in the ‘Zoologist,’ 1857, p. 5764; and Dr. Wallace, in ‘Proc. Entomolog. Soc.,’ June 4th, 1860, p. 119.
(‘Geschichte der Natur,’ Band ii. s. 96) has collected a number of cases. For the case of the deer, _see_ ‘Penny Cyclop.,’ vol. viii. p.
350.
[67] For additional evidence on this subject, _see_ F. Cuvier in [68] Numerous instances could be given. Thus Livingstone (‘Travels,’ p. 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.
[69] For analogous cases with the fowl, _see_ Réaumur, ‘L’Art de faire Eclore,’ etc., 1749, p. 243; and Col. Sykes, in ‘Proc. Zoolog. Soc.,’ 1832, etc. With respect to the fowl not breeding in northern regions, p. 347.
[71] Youatt on Sheep, p. 181.
145.
[76] ‘Bull. de la Soc. d’Acclimat.,’ tom. ix., 1862, pp. 380, 384.
in his ‘Travels’ (Eng. translat.), vol. i. p. 292.
‘Cybele Britannica,’ vol. i. p. 131; Mr. D. Cameron, also, has written on the culture of Alpine plants in ‘Gard. Chronicle,’ 1848, pp. 253, 268, and mentions a few which seed.
a good résumé on this subject in his ‘La Variabilité des Espèces,’ 1868, p. 155.
1811, p. 67.
[94] Prof. Allman, Brit. Assoc., quoted in the ‘Phytologist,’ vol. ii.
p. 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 [98] Mr. Fairweather, in ‘Transact. Hort. Soc.,’ vol. iii. p. 406: Bosse, quoted by Bronn, ‘Geschichte der Natur,’ B. ii. s. 77. On the effects of the removal of the anthers, _see_ Mr. Leitner, in Silliman’s ‘North American Journ. of Science,’ vol. xxiii. p. 47; and suggested the theory above given on the doubleness of flowers. This view is adopted by Carrière, ‘Production et Fix. des Variétés,’ 1865, p. 67.
l’Espèce,’ tom. ii. p. 106; Pickering, ‘Races of Man;’ Gallesio, Erde,’ Th. ii. s. 214) states that at Manilla one variety of the banana is full of seeds: and Chamisso (Hooker’s ‘Bot. Misc.,’ vol. i.
p. 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.
[105] Ingledew, in ‘Transact. of Agricult. and Hort. Soc. of India,’ vol. ii.
Riproduzione,’ p. 110. Dr. J. de Cordemoy, in ‘Transact. of the R.
Soc. of Mauritius’ (new series), vol. vi. 1873, pp. 60-67, gives a large number of cases of plants which never seed, including several species indigenous in Mauritius.
[109] Godron, ‘De l’Espèce,’ tom. ii. p. 106; Herbert on Crocus, in ‘Journal of Hort. Soc.,’ vol. i., 1846, p. 254: Dr. Wight, from what he has seen in India, believes in this view; ‘Madras Journal of Lit.
[110] Wahlenberg specifies eight species in this state on the Lapland Alps: _see_ Appendix to Linnæus’ ‘Tour in Lapland,’ translated by Sir J. E. Smith, vol. ii. pp. 274-280.
[111] ‘Travels in North America,’ Eng. translat., vol. iii. p. 175.
[112] With respect to the ivy and Acorus, _see_ Dr. Broomfield in the ‘Phytologist,’ vol. iii. p. 376. Also Lindley and Vaucher on the Acorus, and _see_ Caspary as below.
[113] ‘Annal. des Sc. Nat.,’ 3rd series, Zool., tom. iv. p. 280. Prof.
Decaisne refers also to analogous cases with mosses and lichens near Paris.
[114] Mr. Tuckermann, in Silliman’s ‘American Journal of Science,’ vol. xlv. p. 1.
[117] On the non-production of seeds in England, _see_ Mr. Crocker, in tom. iv. p. 466; Dr. D. Clos, in ‘Annal. des Sc. Nat.,’ 3rd series, Bot., tom. xvii. 1852, p. 129: this latter author refers to other analogous cases. _See_ more especially on this plant and on other allied cases Prof. Caspary, “Die Nuphar,” ‘Abhand. Naturw. Gesellsch.
73, 87, 119) also shows that when two species, one single and the other double, are crossed, the hybrids are apt to be extremely double.
CHAPTER XIX. SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON HYBRIDISM.
ON THE GOOD DERIVED ON THE EFFECTS OF CROSSING—THE INFLUENCE OF DOMESTICATION ON FERTILITY—CLOSE INTERBREEDING—GOOD AND EVIL RESULTS FROM CHANGED CONDITIONS OF LIFE—VARIETIES WHEN CROSSED NOT INVARIABLY FERTILE—ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND VARIETIES—CONCLUSIONS WITH RESPECT TO HYBRIDISM—LIGHT THROWN ON HYBRIDISM BY THE ILLEGITIMATE PROGENY OF HETEROSTYLED PLANTS—STERILITY OF CROSSED SPECIES DUE TO DIFFERENCES CONFINED TO THE REPRODUCTIVE SYSTEM—NOT ACCUMULATED THROUGH NATURAL SELECTION—REASONS WHY DOMESTIC VARIETIES ARE NOT MUTUALLY STERILE—TOO MUCH STRESS HAS BEEN LAID ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND CROSSED It was shown in the fifteenth chapter that when individuals of the same variety, or even of a distinct variety, are allowed freely to intercross, uniformity of character is ultimately acquired. Some few characters, however, are incapable of fusion, but these are unimportant, as they are often of a semi-monstrous nature, and have suddenly appeared. Hence, to preserve our domesticated breeds true, or to improve them by methodical selection, it is obviously necessary that they should be kept separate. Nevertheless, a whole body of individuals may be slowly modified, through unconscious selection, as we shall see in a future chapter, without separating them into distinct lots.
Domestic races have often been intentionally modified by one or two crosses, made with some allied race, and occasionally even by repeated crosses with very distinct races; but in almost all such cases, long-continued and careful selection has been absolutely necessary, owing to the excessive variability of the crossed offspring, due to the principle of reversion. In a few instances, however, mongrels have retained a uniform character from their first production.
When two varieties are allowed to cross freely, and one is much more numerous than the other, the former will ultimately absorb the latter.
Should both varieties exist in nearly equal numbers, it is probable that a considerable period would elapse before the acquirement of a uniform character; and the character ultimately acquired would largely depend on prepotency of transmission and on the conditions of life; for the nature of these conditions would generally favour one variety more than another, so that a kind of natural selection would come into play.
Unless the crossed offspring were slaughtered by man without the least discrimination, some degree of unmethodical selection would likewise come into action. From these several considerations we may infer, that when two or more closely allied species first came into the possession of the same tribe, their crossing will not have influenced, in so great a degree as has often been supposed, the character of the offspring in future times; although in some cases it probably has had a considerable effect.
Domestication, as a general rule, increases the prolificness of animals and plants. It eliminates the tendency to sterility which is common to species when first taken from a state of nature and crossed. On this latter head we have no direct evidence; but as our races of dogs, cattle, pigs etc., are almost certainly descended from aboriginally distinct stocks, and as these races are now fully fertile together, or at least incomparably more fertile than most species when crossed, we may with entire confidence accept this conclusion.
Abundant evidence has been given that crossing adds to the size, vigour, and fertility of the offspring. This holds good when there has been no previous close interbreeding. It applies to the individuals of the same variety but belonging to different families, to distinct varieties, sub-species, and even to species. In the latter case, though size is gained, fertility is lost; but the increased size, vigour, and hardiness of many hybrids cannot be accounted for solely on the principle of compensation from the inaction of the reproductive system.
Certain plants whilst growing under their natural conditions, others when cultivated, and others of hybrid origin, are completely self-impotent, though perfectly healthy; and such plants can be stimulated to fertility only by being crossed with other individuals of the same or of a distinct species.
On the other hand, long-continued close interbreeding between the nearest relations diminishes the constitutional vigour, size, and fertility of the offspring; and occasionally leads to malformations, but not necessarily to general deterioration of form or structure. This failure of fertility shows that the evil results of interbreeding are independent of the augmentation of morbid tendencies common to both parents, though this augmentation no doubt is often highly injurious.
Our belief that evil follows from close interbreeding rests to a certain extent on the experience of practical breeders, especially of those who have reared many animals of quickly propagating kinds; but it likewise rests on several carefully recorded experiments. With some animals close interbreeding may be carried on for a long period with impunity by the selection of the most vigorous and healthy individuals; but sooner or later evil follows. The evil, however, comes on so slowly and gradually that it easily escapes observation, but can be recognised by the almost instantaneous manner in which size, constitutional vigour, and fertility are regained when animals that have long been interbred are crossed with a distinct family.
These two great classes of facts, namely, the good derived from