Chap. XXII. CAUSES OF VARIABILITY. 257 



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. 

 Nor 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. 9 



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

 whether or not changed in nature, is probably the most powerful. 

 This view was held with regard to plants by Andrew Knight, and 

 is now held by Schleiden, more especially in reference to the 

 inorganic elements of the food. 10 In order to give a plant more 

 food it suffices in most cases to grow it separately, and thus pre- 

 vent other plants robbing its roots. It is surprising, as I have 

 often seen, how vigorously our common wild plants nourish when 

 planted by themselves, though not in highly manured land. 

 Growing plants separately is, in fact, the first step in culti- 

 vation. 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. 11 "It is a rule invariably with us, when 

 " we desire to keep a true stock of any one kind of seed, to 

 " grow it on poor land without dung ; but when we grow for 

 " quantity, we act contrary, and sometimes have dearly to 

 " repent of it." 



In the case of animals the want of a proper amount of exercise, 

 as Bechstein has remarked, has perhaps played, independently 

 of the direct effects of the disuse of any particular organ, an 

 important part in causing variability. We can see in a vague 

 manner that, when the organised and nutrient fluids of the body 

 are not used during growth, or by the wear and tear of the tissues, 



™L «™ J «, V ' T hM S ° me also Alex - Braun, m ' Bot. Memoirs,* 

 good remarks on this subject. He states R av Sn( , is*q „ qiq 



that his canary-birds varied in colour n m ' w ' f' ," q f ivr i , 



though kept on uniform food. ' , ^T™ ^ ^ J£ ' 



» • The Plant/ by Schleiden, trans- ^ Chr0mcle ' 1856 ' * 458 " 



VOL. II. 



