242 BREEDING 



others. This is the kind of thing which would have 

 to be found out. 



But if the characters bred for are inherited in a 

 Mendelian manner, there is no difficulty as to how 

 to proceed in practice, and, moreover, no question 

 but that the only guide which will conduct the breeder 

 to the end which he wishes to achieve is a knowledge 

 of the character-factors concerned, in the germ-cells 

 of the animals or plants under consideration. For 

 instance, it is clear that in the case of the blue colour 

 in Andalusian fowls, or the red-roan in cattle, the 

 attempt, based on the confinement of one's attention 

 to outward bodily characteristics alone, to raise a pure 

 breed possessing these colours, by only mating animals 

 which exhibited them, is doomed to failure. True, the 

 Mendelian cannot tell the breeder how to raise a 

 strain which will breed true to these colours ; but 

 he can tell him how to raise 100 per cent, in each 

 generation where the breeder, acting on the principles 

 which he had learnt from his ancestors, only suc- 

 ceeded in raising 50 per cent. 



With regard, now, to this principle of conducting 

 breeding on the basis of a knowledge of the contents 

 of the germ-cells. It may be asked, " How are we 

 to determine the nature of these contents ? " The 

 answer in most cases is " By breeding from the 

 individuals themselves ; to determine what are the 

 characters in respect of which they are homozygous 

 and what those in respect of which they are hetero- 

 zygous." Indeed, the difference between modern and 

 old-fashioned principles of breeding may be sum- 



