Chap. XXII. CAUSES OP VARIABILITY. 261 



of nutriment. The Chidham wheat was raised from an ear fonnd on a 

 hedge; and Hunter's wheat was discovered by the roadside in Scotland, 

 but it is not said that this latter variety grew where it was found. 22 



Whether our domestic productions would ever become so com- 

 pletely 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 certain that 

 our most anciently cultivated plants, as well as animals, still go 

 on varying, for all have recently undergone marked improve- 

 ment. 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 instance after an interval of 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 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 number 

 of generations, as well as the successive steps in the progress of 

 variation, have been recorded, as in the often-quoted instance of 



^o^^' 24 Aft6r SeVeial yearS ' Culture the Zinnia has only lately 



(1860) begun to vary in any great degree. « Iu the first seven or 



eight -years of high cultivation the Swan Eiver daisy (Brachycome 



iberidifoha) 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 horticulturists have spoken to the 



23 Gardener's Chronicle' 1841 ^ ••• „~~ ~ 



383; 1850, p. 700; 1854 p' 650 ^ 5" * ™\ ^^ ' Geschichte ** 



■ ' Die Getreidearten / P 1843 B 66 fK ^ * f* ^ + - 1+ ,, 



I16 117 ' ' s> bb > 25 Journal of Horticulture,' 1861, p. 



2+ Qnl-,ii-,o ir. <-H7„.f <t ,, 112 > on Zinnia, ' Gardener's Chronicle/ 



babme, m Hort. Transact.,' vol. i860, p. 852 



