8 VARIATION UNDER DOMESTICATION. 



slight or strongly marked, which appear among many indi- 

 viduals living together, may be considered 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 inflam- 

 mation of various organs. 



With respect to what I have called the indirect action of 

 changed conditions, namely, through the reproductive sys- 

 tem 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 sur- 

 rounding 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 vigor, 

 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 deter- 

 mine 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 laws are which determine the reproduction of animals 

 under confinement, I may mention that carnivorous animals, 

 even from the tropics, breed in this country pretty freely 

 under confinement, with the exception of the plantigrades or 

 bear family, which seldom produce young ; whereas carniv- 

 orous birds, with the rarest;exceptions, hardly ever lay fertile 

 eggs. Many exotic plants have pollen utterly worthless, in 

 the same condition as in the most sterile hybrids. When, 

 on the one hand, we see domesticated animals and plants, 

 though often weak and sickly, breeding freely under confine- 

 ment ; and when, on the other hand, we see individuals, though 

 taken young from a state of nature perfectly tamed, long-lived 

 and healthy (of which I could give numerous instances), yet 



