244 



CAUSES OF VARIABILITY. 



Chap. XXn. 



fact, though not immediately connected with our present 

 Bubject, that the plants and shells of the Arctic regions are 

 eminently variable.^ Moreover, 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 ' Geographic Botanique,' repeatedly shows 

 that the native country of a plant, where in most cases it has 

 been longest cultivated, is that where it has yielded the 

 greatest number of varieties. 



It is doubtful whether a change in the nature of the food 

 is a potent cause of variability. Scarcely any domesticated 

 animal has varied more than the pigeon or the fowl, but 

 their food, especially that of highly-bred pigeons, is generally 

 the same. Kor can our cattle and sheep have been subjected 

 to any great change in this respect. But in all these cases 

 the food probably is much less varied in kind than that which 

 was consumed by the species in its natural state.^ 



Of all the causes which induce variability, excess of food, 

 whether or not changed in nature, is jDrobably the most 

 powerful. This view was held with regard to plants by 

 Andrew Knight, and is now held by Schleiden, more esj)ecially 

 in reference to the inorganic elements of the food.^° In 

 order to give a plant more food it suffices in most cases to 

 grow it separately, and thus prevent other plants robbing its 

 roots. It is surprising, as I have often seen, how vigorously 

 our common wild species flourish when planted by them- 

 selves, though not in hio;hty manured land ; separate growth 

 is, in fact, the first step in cultivation. We see the converse 

 of the belief that excess of food induces variability in the 

 following statement by a great raiser of seeds of all kinds :^^ 



* See Dr. Hooker's Memoir on 

 Arctic Plants in ' Linn. Transact.,' 

 voL xxiii. part ii. Mr. Woodward, 

 and a higher authority cannot be 

 quoted, speaks of the Arctic mollusca 

 (in his ' Rudimentary Treatise,' 1856, 

 p. 355) as remarkably subject to 

 variation. 



^ Bechstein, in his ' Naturge- 

 schuihte der Stubenvcigel,' 1840, s, 

 238, has some good remarks on this 



subject. He states that his canary- 

 birds varied in coloui*, though kept on 

 uniform food. 



" 'The Plant,' by Schleiden, trans- 

 lated by Henfrey, 1848, p. 169. See 

 also Alex. Braun, in ' Bot. Memoirs,' 

 Ray Soc, 1853, p. 313. 



'' Messrs. Hardy and Son, of Mal- 

 don, in ' Gard. Chronicle,' 1856, p. 

 458. Carriere, ' Production et Fixa 

 Vion des Varietes,' 1865, p. 31. 



