CHAPTER XXXVII 
METHODS OF BREEDING 
Like modes of research, methods of breeding are the means by which 
certain results are attained. It is necessary to emphasize this fact, 
because even yet there is much confusion in the minds of breeders as to 
the relation which a particular method of breeding bears to results which 
have been produced by its employment. Not infrequently statements 
are made to the effect that a certain method of breeding is the cause of 
the excellence of one race or strain or the inferiority of another. There 
is a wide difference between the method of producing a given result, and 
the cause of its attainment. For the sake of clarity of thought we shall 
endeavor to emphasize this distinction, so far as is possible in the present 
state of our knowledge, in the discussions which follow. 
Phenotypic ‘Selection.—The oldest method of breeding was simply 
that of mating together the most excellent individuals. In popular 
phraseology this is the method of breeding from the best—its funda- 
mental postulate is expressed in the old statement, like produces like. 
We have called it the method of phenotypic selection in order to empha- 
size the fact that the basis of choice for breeding in this method is the 
sum total of expressed characters of the individual. 
It is not necessary to recount here at any great length the sort: of 
improvement which has been effected in modern breeds of domestic 
animals by the application of this method of breeding. Let it be sufficient 
to state that much of the excellence of modern breeds is an earnest of the 
efficiency of phenotypic selection as a mode of breed amelioration. It 
may, also, be stated justly that all later methods of breeding; out- 
breeding, line-breeding, inbreeding, and genotypic selection; are simply 
refined methods of breeding from the best—they are methods of pheno- 
typic selection plus something else; the something else usually ill-defined, 
but sometimes, as in genotypic selection, more definitely conceived. 
The limitations of the cruder form of phenotypic selection depend 
upon two primary causes, somatic modifiability of characters and geno- 
typic differences among like phenotypic individuals. Since differences 
which are due to modifiability tend in the long run to group themselves 
around a mean in the form of a normal variability curve, it may be stated 
dogmatically that long-continued phenotypic selection should tend to 
obliterate them. But it is not enough for the practical breeder to know 
37 577 
Digitized by Microsoft® 
