xiv ECONOMICAL 163 



ing on a continuously varying homogeneous material and 

 producing a permanent effect. This is because the in- 

 terval between the average weight of the different pure 

 lines is small compared with the environmental fluctua- 

 tions. None the less it is there, and the secret of separat- 

 ing and fixing any of these pure lines is again to breed from 

 the individual separately. As soon as the pure line is 

 separated further selection becomes superfluous. 



Since the publication of Darwin's famous work upon 

 the effects of cross and self fertilisation, it has been 

 generally accepted that the effect of a cross is commonly, 

 though not always, to introduce fresh vigour into the 

 offspring, though why this should be so we are quite at a 

 loss to explain. Continued close inbreeding, on the con- 

 trary, eventually leads to deterioration, though, as in 

 many self-fertilised plants, a considerable number of gen- 

 erations may elapse before it shows itself in any marked 

 degree. The fine quality of many of the seedsman's 

 choice varieties of vegetables probably depends upon the 

 fact that they had resulted from a cross but a few 

 generations back, and it is possible that they often 

 oust the older kinds not because they started as some- 

 thing intrinsically better, but because the latter had 

 gradually deteriorated through continuous self-fertilisa- 

 tion. Most breeders are fully alive to the beneficial re- 

 sults of a cross so far as vigour is concerned, but they 

 often hesitate to embark upon it owing to what was held 



