300 Practical Application [ch. 



fowls or excessive fertility in pigs should not be entertained 

 with great confidence. 



To avoid raising false expectations it should also be 

 said that many of the small fancy points which distinguish 

 individuals of the same breed from each other are rather of 

 the nature of fluctuations than definite transmissible at- 

 tributes. In the regulation of these finer details it is 

 improbable that heredity plays any very prominent or at 

 least assignable part. A Dutch rabbit, for instance, having 

 the transverse division between the coloured and the white 

 parts of the trunk exactly disposed as the fancier desires is 

 scarcely if at all more likely to have offspring correctly 

 marked than an average specimen of the same strain. In 

 such cases all the fancier can do is to use strains otherwise 

 good. A knowledge of genetic physiology will only help 

 him here in so far as it may warn him not to pay ex- 

 travagant prices for animals whose qualities are not genetic 

 or transmissible. 



The domesticated animals and plants of European 

 countries have already been 'brought to such perfection 

 that in. them the scope for new combination is reduced. 

 Nevertheless the line of progress which Professor Biffen 

 has inaugurated, by combining the resistance to rust-disease 

 of one variety of wheat, with productiveness and other 

 qualities derived from another variety, is one which should 

 be capable of indefinite extension. It may be that resistance 

 to disease is incompatible with some of the valuable qualities 

 of animals and plants, but the attempt to produce these 

 combinations should be made on the largest possible scale. 

 The search for disease-resisting strains of animals and plants 

 is hardly begun. 



In its application to the improvement of the domesticated 

 animals and plants of tropical regions the aid of Mendelian 

 method may be expected to be more immediate and direct. 

 Apart from the actual creation of new types by re-combina- 

 tion much can undoubtedly be accomplished, as Mr W. L. 

 Balls has indicated in the case of the Egyptian Cotton crop, 

 by purification of the cultivated sorts. According to the 

 traditional practice the ** variety " used consists in reality of 

 an immense number of distinct strains which under ordinary 



