198 MENDELISM chap, xvi 



and, according to Professor Wilson, the roan of the 

 Shorthorn is similar, being the heterozygous form 

 produced by mating red with white. The characters 

 of certain breeds of canaries and pigeons again 

 appear to depend upon their heterozygous nature. 

 Such forms cannot, of course, ever be bred true, 

 and where several factors are concerned they may 

 when bred together produce but a small proportion 

 of offspring like themselves. As soon, however, as 

 their constitution has been analysed and expressed 

 in terms of Mendelian factors, pure strains can be 

 built up which when crossed will give nothing but 

 offspring of the desired heterozygous form. 



The points with which the breeder is concerned 

 are often fine ones, not very evident except to the 

 practised eye. Between an ordinary Dutch rabbit 

 and a winner, or between the comb of a Hamburgh 

 that is fit to show and one that is not, the differences 

 are not very apparent to the uninitiated. Whether 

 Mendelism will assist the breeder in the production 

 of these finer points is at present doubtful. It may 

 be that these small differences are heritable, such as 

 those that form the basis of Johannsen's pure lines. 

 In this case the breeder's outlook is hopeful. But it 

 may be that the variations which he seeks to per- 

 petuate are of the nature of fluctuations, dependent 

 upon the earlier life conditions of the individual, and 

 not upon the constitution of the gametes by which 

 it was formed. If such is the case, he will get no 

 help from the science of heredity, for we know of 

 no evidence which might lead us to suppose that 

 variations of this sort can ever become fixed and 

 heritable. 



