128 A MANUAL OF MENDELISM 



high yielding and least when the parents are low 

 yielding. Thus it can be explained how Professor 

 Nilsson-Ehle obtained higher and higher yielding 

 varieties according as he crossed higher and higher 

 yielding parents. 



The final result would have been the same had all 

 the high yielding factors been recessives. The hybrids 

 would have been lower in yield than their parents, but 

 among their progeny a number of high yielding varieties 

 would have been found which would have required no 

 purification. Nor would the final result have been 

 different had some of the high yielding factors been 

 dominants and some recessives. It is possible, indeed 

 not improbable, that some of the pairs of factors for 

 yield produce intermediates when crossed, but this 

 again need cause no difference in the final result, although 

 it might make the process of purification more compli- 

 cated. The essential in every case is the substitution 

 of a high yielding factor for a lower. 



Similar phenomena have been found in connexion 

 with the efforts of Danish farmers to improve the quality 

 of their cows' milk. The quantity of butter-fat in 

 cows' milk ranges from about 2*5 per cent, at one extreme 

 to about 6 per cent, at the other ; but all statements 

 in connexion with this subject must be regarded as no 

 more than close approximations, for every cow's milk 

 varies from day to day, and even the annual average, 

 which is what is referred to in the following pages, may 

 vary about 10 per cent, in any one year above or below 

 the average of several years. The milk of a cow which 

 contains 4 per cent, of fat on the average of several 

 years may contain as much as 4*4 per cent, in one year 

 and as little as 3*6 per cent, in another. 



