150 MENDELISM CHAP. 



may give a different result because they happen to 

 be fluctuations of different pure lines. But within 

 the pure line any seed, large or small, produces the 

 average result for that line. Thus a seed of line 

 C which weighs 20 grains will give practically the 

 same result as one that weighs 10 grains. 



On this view we can understand why selection 

 of the largest seed raises the average weight in the 

 next generation. We are picking out more of C 

 'and less of A and B, and as this process is repeated 

 the proportion of C gradually increases and we get 

 the appearance of selection acting on a continuously 

 varying homogeneous material and producing a 

 permanent effect. This is because the interval 

 between the average weight of the different pure 

 lines is small compared with the environmental 

 fluctuations. None the less it is there, and the 

 secret of separating 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 contrary, eventually leads 

 to deterioration, though, as in many self- fertilised 

 plants, a considerable number of generations 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 



