CHAP. I.] VARIATION UNDER DOMESTICATION. 



CHAPTER I. 

 VARIATION UNDER DOMESTICATION. 



Causes of variability Effects of habit and the use or disuse of parts Corre- 

 lated 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 election, anciently followed, their effects 

 Methodical and unconscious selection Unknown origin of our domestic 

 productions Circumstances favourable 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 variation ; 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 varieties : 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 

 action, we must bear in mind that in every case, as Professor 

 Weismann has lately insisted, and as I have incidentally shown 

 in my work on 'Variation under Domestication,' there are two 



