Chap. I.] VARIATION UNDER DOMESTICATION. 



CHAPTEK L 



Variation under Domestication. 



Causes of Variability — Effects of Habit and the use or disuse cf Parts — 

 Correlated Variation — Inheritance — Character of Domestic Varieties 

 — Difficulty of distinguishing between Varieties and Species — Origin 

 of Domestic Varieties from one or more Species — Domestic Pigeons, 

 their Differences and Origin — Principles of Selection, anciently fol- 

 lowed, their Effects — Methodical and Unconscious Selection — Un- 

 known Origin of our Domestic Productions — Circumstances favour- 

 able to Man's power of Selection. 



Causes of Variability. 



When we compare the individuals of the same variety or sub- 

 variety of our older cultivated plants and animals, one of the first 

 points which strikes us is, that they generally differ more from 

 each other than do the individuals of any one species or variety in 

 a state of nature. And if we reflect on the vast diversity of the 

 plants and animals which have been cultivated, and which have 

 varied during all ages under the most different climates and treat- 

 ment, we are driven to conclude that this great variability is due 

 to our domestic productions having been raised under conditions of 

 life not so uniform as, and somewhat different from, those to which 

 the parent-species had been exposed under nature. There is, also, 

 some probability in the view propounded by Andrew Knight, that 

 this variability may be partly connected with excess of food. It 

 seems clear that organic beings must be exposed during several 

 generations to new conditions to cause any great amount of varia- 

 tion ; and that, when the organisation has once begun to vary, it 

 generally continues varying for many generations. No case is on 

 record of a variable organism ceasing to vary under cultivation. 

 Our oldest cultivated plants, such as wheat, still yield new varie- 

 ties: our oldest domesticated animals are still capable of rapid 

 improvement or modification. 



As far as I am able to judge, after long attending to the subject; 

 the conditions of life appear to act in two ways, — directly on the 

 whole organisation or on certain parts alone, and indirectly by 

 affecting the reproductive system. With respect to the direct 



