VARIATION UNDER DOMESTICATION 7 



body or constitution, causing coughs or colds, rheumatism, or in- 

 flammation of various organs. 



With respect to what I have called the indirect action of changed 

 conditions, namely, through the reproductive system being af- 

 fected, 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 con- 

 ditions. 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 con- 

 finement, even when the male and female unite. How many ani- 

 mals 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 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 de- 

 termine 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 carnivorous 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 confinement; 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 having their reproductive 

 system so seriously affected by unperceived causes as to fail to act, 

 we need not be surprised at this system, when it does act under 

 confinement, acting irregularly, and producing offspring somewhat 

 unlike their parents. I may add that as some organisms breed 

 freely under the most unnatural conditions — for instance, rabbits 

 and ferrents kept in hutches — showing that their reproductive or- 

 gans are not easily affected ; so will some animals and plants with- 



