24 VARIATION Chap. I. 



changes of structure, ■whether extremely slight or strongly 

 marked, which appear among many individuals living together, 

 may be considered as the indefinite elTects of the conditions of 

 life on each individual organism, in nearly the' same manner as 

 a chill allxicts diflerent men in an indefinite manner, according 

 to their state of body or constitution, causing coughs or colds, 

 rheumatism, or inflammations of various organs. 



With respect to what I have called the indirect action of 

 changed conditions, namely, through the reproductive system 

 being afTected, Ave 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 Kolreutcr and others have remarked, between the variabil- 

 ity which follows from the crossing of distinct species, and 

 that which may be observed with all plants and animals 

 Avhen reared under new or unnatural conclitions. Many facts 

 clearly show how eminently susceptible the reproductive sys- 

 tem 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, 

 oven Avhen the male and female unite. How many animals 

 there are Avhich Avill not breed, though kept in an almost free 

 state in their native country ! This is generally, but errone- 

 ously, 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 determine whether or not a plant will 

 produce seeds. I cannot here give the details which I liave 

 collected and elscAvhere published on this curious subject ; but, 

 to show how singular the laws are Avhich determine the repro- 

 duction of animals under confinement, I may mention that car- 

 nivorous animals, even from the tropics, breed in this country 

 pretty freely under confinement, with the exception of the 

 jilantigrades or bear family, Avhich seldom produce young; 

 whereas carnivorous birds, with the rarest exceptions, hardly 

 ever lay fertile eggs. Many exotic plants have jwllen utterly 

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

 When, on the one hand, we sec domesticated animals and 

 jilants, though often weak and sickly, yet breechng freely un- 

 der confinement ; and when, on the other hand, we see indi- 

 viduals, though taken young from a state of nature, ]>erfectly 

 tamed, long-lived, and healthy (of which I could give numerous 



