Cat? XXII. CAUSES OF VARIABILITY. 243 



On the Nature of the Changes in the Conditions of Life which 

 induce Variability. 



From a remote period to the present day, under climates 

 and circumstances as different as it is possible to conceive, 

 organic beings of all kinds, when domesticated or cultivated, 

 have varied. We see this with the many domestic races of 

 quadrupeds and birds belonging to different orders, with gold- 

 fish and silkworms, with plants of many kinds, raised in 

 various quarters of the world. In the deserts of northern 

 Africa the date-palm has yielded thirty-eight varieties ; in 

 the fertile plains of India it is notorious how many varieties 

 of rice and of a host of other plants exist ; in a single Poly- 

 nesian island, twenty-four varieties of the bread-fruit, the 

 same number of the banana, and twenty-two varieties of the 

 arum, are cultivated by the natives ; the mulberry-tree in 

 India and Europe has yielded many varieties serving as food 

 for the silkworm ; and in China sixty-three varieties of the 

 bamboo are used for various domestic purposes. 5 These facts, 

 and innumerable others which could be added, indicate that 

 a change of almost any kind in the conditions of life suffices 

 to cause variability — different changes acting on different 

 organisms. 



Andrew Knight 6 attributed the variation of both animals 

 and plants to a more abundant supply of nourishment, or to a 

 more favourable climate, than that natural to the species. A 

 more genial climate, however, is far from necessary ; the 

 kidney-bean, which is often injured by our spring frosts, and 

 peaches, which require the protection of a wall, have varied 

 much in England, as has the orange-tree in northern Italy, 

 where it is barely able to exist. 7 Nor can we overlook the 



5 On the date-palm, tee Vogel, of the Pandanns and other trees in 



'Annals an! Mag. of .Nat. Hist.,' the Marianue Island, see 'Hooker's 



1854, p. 460. On Indian varieties, Miscellany,' vol. i. p. 308. On the 



Dr. F. Hamilton, 'Transact. Linn. bamboo in China, see Hue's ' Chinese 



Soc.,' vol. xiv. p. 296. On the varie- Empire,' vol. ii. p. 307. 

 ties cultivated in Tahiti, see Dr. 6 'Treatise on the Culture of the 



Bennett, in Loudon's ' Mag. of N. Apple,' &c, p. 3. 

 Hist.,' vol. v., 1832, p. 484. Also 7 Gallesio, 'Teoria della Ripro- 



Ellis, ' Polynesian Researches,' vol. i. duzione Veg.,' p. 125. 

 pp. 370, 375. On twenty varieties 



P. 2 



