10 VAKIATION UNDEK DOMESTICATION. [Chap. I. 



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, 

 according 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 

 action of changed conditions, namely, through the 

 reproductive system of being affected, we may infer 

 that variability 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 animals when reared under new or unnatural 

 conditions. Many facts clearly show how eminently 

 susceptible 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 

 confinement, 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 

 country ! 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 published 

 on this curious subject; but to show how singular the 



