

256 



CAUSES OF VARIABILITY. 



Cpap. XXII. 



We see this with the many domestic races of quadrupeds and 

 birds belonging to different orders, with gold-fish and silk- worms, 

 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 Polynesian 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 alone, and innumerable others could be added, in- 

 dicate that a change of almost any kind in the conditions of life 

 suffices to cause variability — different changes acting on diffe- 

 rent 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 fact, though not 

 immediately connected with our present subject, that the plants 

 and shells of the arctic regions are eminently variable. 8 More- 

 over, it does not appear that a change of climate, whether more 

 or less genial, is one of the most potent causes of variability; 

 for in regard to plants Alph. De Candolle, in his ' Geographie 



5 On the date-palm, see Vogel, 

 * Annals and Mag. of Nat. Hist./ 

 1854, p. 460. On Indian varieties, 

 Dr. F. Hamilton, ' Transact. Linn. Soc./ 

 vol. xiv. p. 296. On the varieties cul- 

 tivated in Tahiti, see Dr. Bennett, in 

 London's 'Mag. of N. Hist./ vol. v., 

 1832, p. 484. Also Ellis, 'Polynesian 



* Chinese Empire/ vol. ii. p. 307. 



6 'Treatise on the Culture of the 

 Apple/ &c, p. 3. 



7 Gallesio, ' Teoria della Kipro- 



duzione Veg./ p. 125. 



8 See Dr. Hooker's Memoir on Arctic 

 Plants in * Linn. Transact./ vol. xxiii. 



part ii. 



Woodward 



* X - v JL — ' — 



Kesearches,' vol. i. pp. 375, 370. On authority cannot be quoted, speaks of 



twenty varieties of the Panclanus and the Arctic mollusca (in his 'Kudimentury 



other trees in the Marianne Island, see Treatise/ 1856, p. 355) as remarkably 



'Hooker's Miscellany/ vol. i. p. 308. subject to variation. 

 On the bamboo in China, see Hue's 



( 



