Chap. XXII. CAUSES OF VARIABILITY. 245 



" 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." 

 According also to Carriere, who has had great experience with 

 flower-garden seeds, " On remarque en general les plantes de 

 *' vigeur moyenne sont celles qui conservent le mieux leurs 

 " caracteres." 



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

 exercise, as Bechstein remarked, has perhaps played, inde- 

 pendently of the direct effects of the disuse of any parti- 

 cular 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, they will be in excess ; and 

 as growth, nutrition, and reproduction are intimately allied 

 processes, this superfluity might disturb the due and proper 

 action of the reproductive organs, and consequently afiect the 

 character of the future offspring. But it may be argued that 

 neither an excess of food nor a superfluity in the organised 

 fluids of the body necessarily induces variability. The goose 

 and the turkey have been well fed for many generations, yet 

 have varied very little. Our fruit-trees and culinary plants, 

 which are so variable, have been cultivated from an ancient 

 period, and, though they probably still receive more nutri- 

 ment than in their natural state, yet they must have received 

 during many generations nearly the same amount ; and it 

 might be thought that they would have become habituated to 

 the excess. Nevertheless, on the whole, Knight's view, that 

 excess of food is one of the most potent causes of variability, 

 appears, as far as I can judge, probable. 



Whether or not our various cultivated plants have received 

 nutriment in excess, all have been exposed to changes of 

 various kinds. Fruit-trees are grafted on diSerent stocks, 

 and grown in various soils. The seeds of culinary and agri- 

 cultural plants are carried from place to place; and during 

 the last century the rotation of our crops and the manures 

 used have been greatly changed. 



Slight changes of treatment often suflSce to induce varia- 



