8 VARIATION UNDER BOMEBTIOATION. 



ancestor. Even strongly-marked differences occasionally 

 apuear in the young of the same litter, and in seedlings 

 from the same seed-capsule. At long intervals of time, 

 out of millions of individuals reared in the same country 

 and fed on nearly the same food, deviations of structure so 

 strongly pronounced as to deserve to be called monstros- 

 ities arise; but monstrosities cannot be separated by any 

 distinct line from slighter variations. All such changes of 

 structure, whether extremely slight or strongly marked, 

 which appear among many individuals living together, 

 may be considered as the indefinite effects of the condi- 

 tions of life on each individual organism, in nearly the 

 same manner as the chill effects different men in an indefi- 

 nite manner, according to their state of body or constitu- 

 tion, 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 

 irossing of distinct species, and that which may be observed 

 with plants and animals when reared under new or unnat- 

 ural 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 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 determine the reproduction of animals 



