No. 552] INHERITANCE OF FECUNDITY 



711 



method per se is precisely the burden of a very great 

 proportion of the teaching of breeding (in whatever form 

 that teaching is done) at the present time. 



It seems to me that it has never been demonstrated, up 

 to the present time, that continued selection can do any- 

 thing more than : 



1. Isolate pure biotypes from a mixed population, 

 which contains individuals of different heredity constitu- 

 tion in respect to the character or characters considered. 



2. Bring about and perpetuate as a part of a logical 

 system of breeding for a particular end, certain combina- 

 tions of hereditary factors which would never (or very 

 rarely) have occurred and would have been lost in the 

 absence of such systematic selection ; which combinations 

 give rise to somatic types which may be quite different 

 from the original types. In this way a real evolutionary 

 change (i. e., the formation of a race of qualitatively 

 different hereditary constitution from anything existing 

 before) may be brought about. This can unquestionably 

 be done for fecundity in the domestic fowl. But here 

 " selection " is simply one part of a system of breeding, 

 which to be successful must be based on a definite knowl- 

 edge of gametic as well as somatic conditions. It is 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 absolutely 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 hereditary 

 factor or determinant (t. e., its power to cause a quanti- 

 tatively definite degree of somatic development of a char- 

 acter) can be changed by selection on a somatic basis, 

 however long continued. To determine, by critical ex- 

 periments which shall exclude beyond doubt or question 

 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 ol 

 the fundamental problems of genetics. 



