PROBLEMS OF PRACTICAL BREEDER 85 



ANIMAL BREEDING 



If we study the history of the development of economic 

 characters in animals, we must reach the conclusion 

 that most of them have been developed slowly, by a 

 long process of selection. Large mutations are occa- 

 sionally observed ; the Ancon sheep was one ; the bull- 

 dog was probably another. But these large mutations 

 are usually mere curiosities, and are preserved as such. 

 The domestic animals must have been selected for 

 certain valuable characters at the outset, and these 

 characters have been slowly improved, either by the 

 accumulation of small variations or by the gradual 

 building up of certain factor combinations. Under 

 these circumstances we must expect simple relations 

 only rarely. The heredity of economic factors will 

 usually be a complex matter. For example, certain 

 breeds of sheep, such as the Merino, have been selected 

 for fineness of wool. Others, such as the Lincoln, have 

 been selected, among other points, for a long and coarser 

 type of wool. Actually, Lincoln wool may be four or 

 five times as long, and about twice as thick as 

 Merino. If these two breeds are crossed, as they 

 frequently are, the hybrid is about intermediate, and 

 in the subsequent generations there is no obvious segre- 

 gation into Merino and Lincoln types. The second 

 generation is probably more variable than the first, but 

 in any case we have a highly complex relationship. 

 Our theory is not yet perfect enough to deal with such 

 matters. In fact, as regards the ordinary methods of 

 the animal breeder, theory can scarcely go beyond 

 giving general reasons for what was already known 

 empirically. Briefly, the method of the animal breeder 

 is to select individuals showing as nearly as possible 

 the qualities required, to attempt to balance defects 

 in one parent with excellencies in the other, and to 

 mate together individuals which are neither very closely 

 related nor entirely unrelated. 



The degree of relationship which is compatible with 



