LAW OF VARIATION. 41 



Habit has a decided influence towards inducing vari- 

 ation. As the blacksmith's right arm becomes more 

 muscular from the habit of exercise induced by his 

 vocation, so we find in domestic animals that use, or 

 the demand created by habit, is met by a development 

 or change in the organization adapted to the require- 

 ment. For instance, with cows in a state of nature or 

 where required only to suckle their young, the supply 

 of milk is barely fitted to the requirement. If more is 

 desired, and if the milk be drawn completely and regu- 

 larly, the yield is increased and continued longer. By 

 keeping up the demand there is induced in the next 

 generation a greater development of the secreting 

 organs, and more milk is given. By continuing the 

 practice, by furnishing the needful conditions of suita- 

 ble food, &c., and by selecting in each generation 

 those animals showing the greatest tendency towards 

 milk, a breed specially adapted for the dairy may be 

 established. It is just by this mode that the Ayrshires 

 have, in the past eighty or a hundred years, been 

 brought to be what they are, a breed giving more good 

 milk upon a given quantity of food than any other. 



It is because the English breeders of modern Short- 

 horns altogether prefer beef-making to milk-giving prop- 

 stock-getter ; and his progeny is more likely to inherit full health and 

 vigor. 



