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. — 

 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 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. 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 Eiver daisy 

 " {Br achy come 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 hor- 

 ticulturists have spoken to the same general effect. Mr. 

 Salter 26 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, 27 

 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 



24 Sabine, in 'Hort. Transact.,' vol. 26 'The Chrysanthemum, its Bis- 

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



Natur,' b. ii. s. 119. 2 " 7 < Gardener's Chron.,' 1855, p, 



25 ' Journal of Horticulture,' 1861, 54; 'Journal of Horticulture,' May 

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



Chronicle,' 1860, p. 852. 



