VABIATION UNDER DOMESTIGATLON. 



CHAPTBE I. 



VAKIATIOIT USTDEE DOJIESTICATIOIT. 



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

 Parts— Correlated Variation— Inheritance— Character of Domes- 

 tic 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 followed, their Effects — 

 Methodical and Unconscious Selection— Unknown Origin of 

 our Domestic Productions— Circumstances favorable to Man's 

 power of Selection. 



CAUSES OF VAEIABILITY. 



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 gen- 

 erally 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 treatment, 

 we are driven to conclude that this great variability is due 

 to our domestic productions having been raised under con- 

 ditions 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 gener- 

 ations to new conditions to cause any great amount of 

 variation; and that, when the organization has once begun 

 to vary, it generally continues varying for many gener- 

 ations. 'So case is on record of a variable organism ceas- 

 ing to vary under cultivation. Our oldest cultivated 

 plants, such as wheat, still yield new varieties; our oldest 



