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Show 260 CAUSES OF VARIABILITY. CnAP. XXJT. even as late as the 5th of June.16 Such facts as these are well fitted to show, on what obscure aucl slight causes variabihty rests. I may here just allude to the appearance of new and v~luable vari~ties of fruit-trees and of wheat in woods and waste places, whiCh at first s1ght seems a most anomalous circumstance. In France a considerable number of the best pears have been discovered in woods; and. tJ:is bas occurre~ so frequently, that Poiteau asserts that "improved vanehes of our cultivated fruits rarely originate with nurserymen.17 In England, on the other hand, no instance of a good pear having been found wild h_as been recorded; and Mr. Rivers informs me that he knows of only one mstance with apples, namely, the Bess Poole, which was discovered in a wood in Nottinghamshire. This difference between the two countries may be in part accounted for by the more favourable climate of France, but chiefly from the great number of seedlings which spring up there in the woods. I infer that this is the case from a remark made by a French gardener,I8 who regards it as a national calamity that such a number of pear-trees are periodically cut down for firewood, before they have borne fruit. The new varieties which thus spring up in the woods, though they cannot have received any excess of nutriment, will have been exposed to abruptly changed conditions, but whether this is the cause of their production is very doubtful. These varieties, however, are probably all descended 19 from old cultivated kinds growing in adjoining orchards,-a circumstance which will account for theil' variability; and out of a vast number of varying trees there will always be a good chance of the appearance of a valuable kind. In North America, where fruit-trees fTequently spring up in waste places, the Washington pear was found in a hedge, and the Emperor peach in a wood.20 With respect to wheat, some writers have spoken 21 as if it were an ordinary event for new varieties to be found in waste places; the Fenton wheat was certainly discovered growing on a pile of basaltic detritus in a quarry, but in such a situation the plant would probably receive a sufficient amount 16 M. Cardan, in ' Comptes Rendus,' Dec. 1848, quoted in 'Gard. Chronicle, ' 1849, p. 101. Ii M. AlexiP Jordan mentions four excellent pears found in woods in France, and alludes to others (' 1\iem. Acad. de Lyun,' tom. ii. 1852, p. 159). Poiteau's remark is quoted in ' Gardener's Mag.,' vol. iv., 1828, p. 385. See 'Gard. Chronicle,' 1SG2, p. 335, for another case of a new variety of the pear found in a hedge in France. Also for another case, see Louclon's 'Encyclop. of Gardening,' p. 901. 1\'fr. Rivt:rs has given me similar informfl.tion. 18 Duvfl.l, 'Hist. du Poirier,' 1849, p. 2. 19 I infer that this is the fact from Van Mons' statement (' Arbres Fruitiers,' 1835, tom. i. p. 446; that he finds in the woods seedlings resembling all the chief cultivated races of both the pear and apple. Van Mons, however, looked at these wild varieties as aboriginal species. 2o Downing, 'Fruit-trees of North America, ' p. 422; Foley, in 'Transact. Hort. Soc.,' vol. vi. p. 412. 21 'Gnnl. Chronicle,' 1847, p. 244-. CIIAP. XXII. CAUSES OF VARIABILITY. 261 of nutriment. The Chidham wheat was raised from an ear found o'll a hedge; and Hunter's wheat was discovered by the roadside in Scotland, but it is not said that this latter variety gTew where it was found.2'J Whether our domestic productions would ever become so completely habituated to the conditions under which they now live, as to cease varying, we have no sufficient means for judging. But, in fact, our domestic productions are never exposed for a great length of time to uniform conditions, and it is eertain that our most anciently cultivated plants, as well as animals, still go on varying, for all have receutly undergone marked improvement. In some few cases, however, plants have become habituated to new conditions. Thus Metzger, who cultivated in Germany during many years numerous varieties of wheat, brought from different countries,23 states that some kinds were at first extremely variable, but gradually, in one iustance after an interval of twentyfive years, became constant; and it does not appear that this resulted from the selection of the more constant forms. On the Accumulative Action of changed Conditions of Life.We have good grounds for believing that the influence of changed conditions accumulates, so that no effect is produced on a species until it has been exposed during several generations to continued cultivation or domestication. Universal experience shows us that when new flowers are first introduced into our gardens they do not vary; but ultimately all, with the rarest exceptions, vary to a greater or less extent. In a few cases the requisite nuinber of generations, as well as the successive steps in the progress of variation, have been recorded, as in the often-quoted instance of the Dahlia.24 After several years' culture the Zinnia has only lately (1860) begun to vary in any great degree. "In the first seven or "eight years of high cultivation the Swan River daisy ( Brachycome "iberidifolia) kept to its original colour; it then varied into lilac "and purple and other minor shades." 25 Analogous facts have been recorded with the Scotch rose. In discussing the variability of plants several experienced horticulturists have spoken to the 22 Gardener's Chronicle,' 1841, p. 383; 1850,p.700; 1854, p. 650. 23 'Die Getreidearten,' 18·.1:3, s. 66, 116, 117. 2 ~ Sabine, in ' Hort. Transact.,' vol. iii. p. 225 ; Bronn, ' Geschichte der Natur,' b. ii. s. 119. 2s 'Journal of Horticulture,' 1861, p. 112; on Zinnia, 'Gardener's Chronicle,' 1860, p. 852. |