264 RAYMOND PEARL 



as somatic conditions. It is very, very far removed from a blind 

 'breeding of the best to the best to get the best.' The latter 

 plan alone may, as in the case of fecundity, fail absolutel}^ to 

 bring about any progressive change whatever. 



It has never yet been demonstrated, so far as I know, that the 

 absolute somatic value of a particular hereditarj^ factor or deter- 

 minant (i.e., its power to cause a quantitatively definite degree of 

 somatic development of a character) can be changed by selection 

 on a somatic basis, however long continued. To determine, by 

 critical experiments which shall exclude beyond doubt or ques- 

 tion such effects of selection as those noted under 1 and 2 above, 

 whether the absolute somatic value of factors may be changed 

 by selection, or in any other way, is one of the fundamental 

 problems of genetics. 



Prepotency 



One of "the least understood phenomena in genetics is that 

 which the practical breeder calls 'prepotency.' When the scien- 

 tific student of genetics deals with the matter at all he is rather 

 apt either to throw it over entirely as a 'breeder's superstition,' 

 or to take it as something 'given' to help him out of a difficulty 

 in the interpretation of results which fail to conform to expecta- 

 tion. Some time a more searching investigation of this phenome- 

 non must be made than is implied in either of these lines of pro- 

 cedure. 



In a former paper (27, p. 324), it was suggested that the evi- 

 dence indicated, for certain productive characters at least, that 

 hereditary high performance tended to behave as a Mendelian 

 dominant to hereditary low performance. The following state- 

 ment was then made: 



If this suggestion is true it gives at once, I think, a possible clue to the 

 explanation of a part at least of the Ivnown facts regarding what is called 

 prepotency in the practical breeding of domestic animals for. performance. 

 It is customary in practice to regard an animal as prepotent in breeding 

 for performance when the progeny of that individual uniformily tend to 

 resemble it closely in respect to tlie character bred for, regardless of the 

 other parent in each mating. Let it now only be considered that the 

 great sire, say, of speed or of milk production belongs to a line having a 



