Variation of animals and plants under domestication, v.2

Page 72

1~6 GOOD FROM CROSSING. CJIAP. XVII. notice that, when largo size is one of tho dos:iJ:ed characters, as -_vith poutors,34 tho evil effects of close interbreeding are much sooner percmved than when small birds, such as short-faced tumblers, are valued. The extreme delicacy of the high fancy breeds, such as these tumblers and improved English carriers, is remarkable ; they arc liable to m.any cliseaflos, and often die in the egg or during the first moult ; and thmr eggs have generally to be hatched under foflter-mothers. Although .these higl.tly­prizod birds have invariably been subjected to much close mtcrbreoding, yet thc:iJ· extreme delicacy of constitution cannot perhaps be thus fully explained. Mr. Yarrcll informed me that Sir J. Scbright continued closely intcrbreccling some owl-pigeons, lmtil from their extreme sterility he as nearly as possible lost the whole family. Mr. Brcnt35 tried to raise a breed of trumpeters, by crossing a common pigeon, and recrossing the daughter, granddaughter, great-granddaughter, and great-great-granddaughter, with the same male trumpeter, until he obtained a bird with H of trumpeter's blood ; but then tho experiment failed, for "breeding so close stopped re­production." Tho experienced Neumeistor 36 also asserts that the offspring from dovecotes and various other breeds are " generally VCTY fertile and hardy birds:" so again, MM. Boitard and Corbie,37 after forty-five yoars' experience, recommcud persons to cross their breedfl for amusement; for, if they fail to make interesting birds, they will succeed under an economical point of view, "as it is found that mongrels are more fertile than pigeons of pluc race." I will refer only to one other animal, namely, the Hive-bee, because a clistinguished entomologist has advanced this as a cuse of inevitable close interbreecling. As the hive is tenanted by a single female, it might have been thought that her male and female offspring would always have bred together, more especially as bees of cliflerent hives are hostile to each other; a strange worker being almost always attacked when trying to enter another hive. But Mr. Tegetmeier has shown 38 that this instinct docs not apply to drones, which are permitted to enter any hive; so that there is no a pl'io1·i improbability of a queen receiving a foreign drone. The fact of the union invariably and necessarily taking place on the wing, during the quean's nuptial flight, seems to be a special provision agaillst continued interbrcecling. However this may be, experience has shown, since the introduction of the yellow-banded Ligurian race into Germany and England, that bees freely cross: 1\fr. Woodbury, who introduced Ligurian bees into Devonshire, found during a single season that tbJ.·ee stocks, at distances of from one to two miles from his hives, \vere crossed by his drones. In one case the Ligurian drones must have flown over the city of Exeter, and over several intermediate hives. On another occasion several common black queens were crossed by Ligurian drones at a distance of from one to tlll:ee and a half miles.39 34 ' A Treatise on Fancy Pigeons,' by J. M. Eaton, p. 5G. 35 ' The Pigeon Book,' p. 4G. 36 ' Das Ganze dcr Taubcnzucht,' 1837, s. 18. 37 ' Los Pigeons,' 1824, p. 35. 38 'Proc. Entomolog. Soc.,' Aug. Gtb, 1860, p. l2G. 33 • J ournal or Horticulture,' 1861, pp. 3!), 77, 158; and 1804, p. 20G. CnAP. XVII. EVIL FROYI INTERBREEDING. 127 Plants. When a single plant of a new species is introduced into any country, if propagated by seed, many individuals will soon be raised, so that if the proper insects be present there will be crossing. With newly­introduced trees or other plants not propagated by seed we are not here concerned. With old-established plants it is an almost universal practice occasionally to make exchanges of ~:>eecl, by which means individuals which have been exposed to different conditions of lifc,-and this, as we have seen, diminishes the evil from close interbreeding,-will occasionally be introduced into each district. Experiments have not been tried on the effects of fertilising flowers with their own pollen during seveml generations. Bui we shall presently see that certain plants, either normally or abnormally, are more or less sterile, even in the first generation, when fertilised by their own pollen. Although nothing is directly known on the evil effects of long-continued close inter­brooding with plants, the converse proposition that great good is derived from crossing is well established. With respect to the crossing of individuals belonging to the same sub­variety, Gartner, whose accuracy and experience exceeded that of all other hybridisers, states 40 that he has many times observed good effects from this step, especially with exotic genera, of which the fertility is somewhat impaired, such as Passiflora, Lobelia, and Fuchsia. Herbert al~;;o says,~' "I am inclined to think that I have derived advantage from impreg­" nating the flower from which I wished to obtain seed with pollen from "another individual of the same variety, or at least from another flower, "rather than with its own." Again, Professor Lecoq asserts that he has ascertained that crossed offspring are more vigorous and robust than their parents.42 General statements of this kind, however, can seldom be fully trusted; consequently I have begun a series of experiments, which, if they continue to give the same results as hitherto, will for ever settle the question of the good effects of crossing two distinct plants of the same variety, and of the evil effects of self-fertilisation. A clear light will thus also be thrown on the fact that flowers are invariably constructed so as to permit, or favour, or necessitate the union of two individuals. We shall clearly understand why moncecious and dicecious,-why dimorphic and trimorphic plants exist, and many other such cases. The plan which I have followed in my expe­riments is to grow plantF: in the same pot, or in pots of the same size, or close together in the open ground; to carefully exclude insects; and then to fertilise some of the flowers with pollen from the same flower, and others on the same plant with pollen from a distinct but adjoining plant. In many, but not all, of these experiments, the crossed plants yielded much more seed than the self-fertilised plants; and I have never seen the 4° ' Beitrage zur Kenntniss der Befruchtung,' 18-H, s. 366. 41 'Amnryllidaccre,' p. 371. ·1 ~ 'Do h Fecondation,' 2nd edit., l 862, p. 79.