382 PRINCIPLES OF STOCK-BREEDING. 



The principle of correlation may likewise be traced 

 in animals that are used for work, or for the produc- 

 tion of milk or of wool. 1 In these cases, however, 

 the relations of particular parts to the general use- 

 fulness of the animal for its special purpose, aside 

 from the indications of constitutional vigor that are 

 the same in all animals, have not been as fully de- 

 termined as they have in the meat-producing breeds, 

 so that there are many details of the organization 

 that need more extended observation and study with 

 reference to the applications of this law of the or- 

 ganization. 



The kind and amount of labor that can best be 

 performed by an animal will largely depend upon the 

 proportions of its body and limbs. 



Temperament and constitutional power are of 

 great importance in all forms of labor, as they deter- 

 mine the efficiency of the power applied; but they 

 cannot act to the best advantage unless the organs of 

 locomotion are adapted by a proper proportion of 

 their parts to the work they are required to do. 



The bones of the legs form a series of levers that 

 are moved by appropriate muscles^ which are in turn 

 brought into activity through the influence of the 

 nervous system. If the proportions and relative posi- 

 tion of these levers make them act at the greatest 

 disadvantage in the performance of a given task, the 

 muscles that constitute the motive power, and the 



1 Virgil and Columella recognize the principle of correlation when 

 they advise that a ram with a " black or spotted" tongue be rejected, 

 as his lambs are liable to be spotted with black (Virgil, " Gcorgics," 

 book iii., p. 80; Columella's "Husbandry," book vii.,chap. iii., p. 306). 



