10 VARIATION UNDER DOMESTICATION. [Chap. 1. 



as the indefinite effects of the conditions of life on each 

 individual organism, in nearly the same manner as the 

 chill affects different men in an indefinite manner, ac- 

 cording to their state of body or constitution, causing 

 coughs or colds, rheumatism, or inflammation of various 

 organs. 



With respect to what I have called the indirect ac- 

 tion of changed conditions, namely, through the repro- 

 ductive system being affected, we may infer that vari- 

 ability is thus induced, partly from the fact of this 

 system being extremely sensitive to any change in the 

 conditions, and partly from the similarity, as Kolreuter 

 and others have remarked, between the variability 

 which follows from the crossing of distinct species, 

 and that which may be observed with plants and ani- 

 mals when reared under new or unnatural condi- 

 tions. Many facts clearly show^ how eminently sus- 

 ceptible the reproductive system is to very slight 

 changes in the surrounding conditions. Nothing is 

 more easy than to tame an animal, and few things more 

 difficult than to get it to breed freely under confine- 

 ment, even when the male and female unite. How 

 many animals there are which will not breed, though 

 kept in an almost free state in their native coun- 

 try! This is generally, but erroneously, attributed to 

 vitiated instincts. Many cultivated plants display the 

 utmost vigour, and yet rarely or never seed! In some 

 few cases it has been discovered that a very trifling 

 change, such as a little more or less water at some 

 particular period of growth, will determine whether or 

 not a plant will produce seeds. I cannot here give the 

 details which I have collected and elsewhere pubhshed 

 on this curious subject; but to show how singular the 



