Chap. XXII. CAUSES OF VARIABILITY. 249 



twenty-five 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. — 

 AVe 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 genera- 

 tions 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 number 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.^* 

 After several years' culture the Zinnia has only lately (1860j 

 begun to vary in any great degree. *' In the first seven or 

 " eight years of high cultivation, the Swan Eiver daisy 

 " {Bracliycome iheridifolia) kept to its original colour ; it then 

 " varied into lilac and purple and other minor shades."-^ 

 Analogous facts have been recorded with the Scotch rose. In 

 discussing the variability of plants several experienced hor- 

 ticulturists have spoken to the same general effect. Mr. 

 Salter '^^ remarks, " Every one knows that the chief difficulty 

 *' is in breaking through the original form and colour of the 

 " species, and every one will be on the look-out for any 

 " natural sport, either from seed or branch ; that being once 

 " obtained, however trifling the change may be, the result 

 " depends upon himself." M. de Jonghe, who has had so much 

 success in raising new varieties of pears and strawberries,^^ 

 remarks with respect to the former, " There is another prin- 

 " ciple, namely, that the more a type has entered into a state 

 " of variation, the greater is its tendency to continue doing 

 " so ; and the more it has varied from the original type, the 



2* Sabine, in' Hort. Transact,,' vol. ^^ 'The Chrysanthemum, its His- 



iii. p. 225; Brona, ' Geschichte der tory, &c.,' 1865, p. 3. 

 Natur,' b. ii. s. 119. ^^ 'Gardener's Chron.,* 1855, ]), 



" 'Journal of Horticulture,' 1861, 5-i ; 'Journal of Horticulture,' Ma/ 



p. 112; on Zinnia, 'Gardener's 9, 1865, p. 363. 

 Chi-onicle,* 1860, p. 852. 



