

SELECTION 147 



greater degree of variation than there obtains 

 between the individuals of a wild species or 

 " variety." The higher variability of domes- 

 tic productions is to be ascribed to the less 

 uniform conditions of their upbringing, per- 

 haps in part to excess of food. Exposure to 

 new conditions must be continued for genera- 

 tions to set up any great variation; but this, 

 once set up, continues indefinitely. Changed 

 conditions may directly influence the whole 

 organization of the creature or certain parts 

 alone; or they may act indirectly through 

 the reproductive system. With respect to the 

 direct action, the nature of the organization 

 seems to count for more than that of the 

 conditions. The effect on the offspring may 

 be definite: e.g. size may depend upon the 

 amount of food, colour upon quality of food, 

 thickness of skin and hair upon climate, etc. 

 But indefinite variability is a much com- 

 moner result of changed conditions, and has 

 probably played a more important part in 

 the formation of our domestic races. The 

 reproductive system is peculiarly sensitive 

 to very slight external changes. Many plants 

 and animals will not reproduce in domesti- 

 cation, even though individually vigorous ; 

 others, though weak and sickly, breed freely. 

 Hence we need not be surprised at the 



