Chap. XXII. CAUSES OF VARIABILITY. 259 



I have never seen exceeded; yet they could not have been 

 exposed to any great change in their conditions. 



With respect to animals, Azara has remarked with much sur- 

 prise, 12 that, whilst the feral horses on the Pampas are always of 

 one of three colours, and the cattle always of a uniform colour, 

 yet these animals, when bred on the unenclosed estancias, though 

 kept in a state which can hardly be called domesticated, and 

 apparently exposed to almost identically the same conditions as 

 when they are feral, nevertheless display a great diversity of colour. 

 So again in India several species of fresh-water fish are only so 

 far treated artificially, that they are reared in great tanks ; but 

 this small change is sufficient to induce much variability. 13 



Some facts on the effects of grafting, in regard to the variability 

 of trees, deserve attention. Cabanis asserts that when certain 

 pears are grafted on the quince, their seeds yield more varieties 

 than do the seeds of the same variety of pear when grafted on 

 the wild pear. 14 But as the pear and quince are distinct species, 

 though so closely related that the one can be readily grafted 

 and succeeds admirably on the other, the fact of variability being 

 thus caused is not surprising ; we are, however, here enabled 

 to see the cause, namely, the different nature of the stock with 

 its roots and the rest of the tree. Several North American 

 varieties of the plum and peach are well known to reproduce 

 themselves truly by seed ; but Downing asserts, 15 " that when a 

 " graft is taken from one of these trees and placed upon another 

 * ( stock, this grafted tree is found to lose its singular property of 

 "producing the same variety by seed, and becomes like all other 

 " worked trees ;"— that is, its seedlings become highly variable. 

 Another case is worth giving : the Lalande variety of the 

 walnut-tree leafs between April 20th and May 15th, and its 

 seedlings invariably inherit the same habit; whilst several 

 other varieties of the walnut leaf in June. Now, if seedlings are 

 raised from the May-leafing Lalande variety, grafted on another 

 May-leafing variety, though both stock and graft have the same 

 early habit of leafing, yet the seedlings leaf at various times, 



12 ' Quadruples du Paraguay,' 1801, 14 Quoted by Sageret <Pom PW ' 



torn. ii. p. 319. 1830, p. 43. ' ' ' ' 



13 _ M'Clelland on Indian CyprinidsB, » « The Fruits of America ' 1845 



' Asiatic Kesearches,' vol. xix. part ii., p. 5. " ' 

 1839, pp. 266, 268, 313. 



S 2 



