22 VARIATION Chap. I. 



GIIAPTER I. 



VAEIATION UNDER DOMESTICATION. 



C!aii8PS of Variability— Effects of Habit— Correlated Variation— Inheritance— Char- 

 acter of Domestic Varieties — Ditliculty of (liHtintruiehinj,' between Varieties and 

 Species — Orii^in of Dome.stic Varieties from oiie or inorc (species — Domestic 

 Pigeons, their Differences and Origin— Principles of Selection, anciently followed, 

 their Effects — Methodical and Uuconscious Selection— Unknown Orisin of our 

 Domestic Productions— Circumstances favorable to Man's Power of Selection. 



Causes of Yariahility. 



WnEN 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 generallj' differ 

 from each other more than do the indi^aduals of any one spe- 

 cies or variety in a state of nature. And if we reflect on the 

 vast diversity of the plants and animals which have been cul- 

 tivated, and which have varied during all ages under the most 

 different climates and treatment, Ave 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 luider nature. There is also, I think, 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 appreciable 

 amount of variation ; and that, when the organization has once 

 l)cgun to vary, it generally continues varying for many gencr- 

 atitms. No case is on record of a variable organism ceasing 

 to vary imder cultivation. Our oldest cultivated plants, siicli 

 as wheat, still yield new varieties : our oldest domesticated 

 animals arc still capable of rapid improvement or modifica- 

 tion. 



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

 subject, the conditions of life ajipcar to act in two ways — di- 



