84 THE ORIGIN OF SPECIES 



coughs or colds, rheumatism, or influmination of various 

 organs. 



With respect to what I have called the indirect action 

 of changed conditions; namely, through the reproductive 

 system 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 ob- 

 served 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 ani- 

 mals there are which will not breed, though kept in aa 

 almost free state in their native country! This is gen- 

 erally, but erroneously, attributed to vitiated instincts. 

 Many cultivated plants display the utmost vigor, and 

 yet rarely or never seed! Ln 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 laws are which determine the 

 reproduction of animals under confinement, I may men- 

 tion 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 



