454 GENETICS IN RELATION TO AGRICULTURE 



to be very simple and easy of solution. This deficiency will undoubtedly 

 be remedied as the results of definitely planned experiments become 

 known; for the present it is necessary to make the most of the meagre 

 data now at hand. 



Selection as a Cause of Variation. It is unnecessary to reopen here 

 the question of the causal connection between variation and selection, 

 for the arguments which have been presented in various places for the 

 belief that modifying factors, rather than quantitative changes in a given 

 Mendelian factor, are responsible for changes in a given character by 

 selection apply just as well to domestic animals as to any other living 

 beings. It is, however, true that a large number of the characters of 

 farm animals are of a type such that they may be shifted with compara- 

 tive ease in a -given direction by selection. The characteristic white- 

 face pattern of Hereford cattle behaves as a unit in heredity, but unques- 

 tionably there is sufficient evidence to show that by continued selection 

 the extent of the white area may be increased or decreased in exactly 

 the same fashion that Castle by selection was able to increase or decrease 

 the amount of the black pigmented area in hooded rats. No experiment, 

 however, has yet been carried out, or is likely to be undertaken, in 

 which over 30,000 Hereford cattle have been bred and raised for the 

 purpose of determining definitely what are the limits of selection for 

 this character. Similarly in such breeds of cattle as the Ayrshire and 

 Holstein-Friesian the proportion of pigmented area to non-pigmented 

 may be shifted at the will of the breeder from a solid pigmented condition to 

 one almost entirely white. With more complex characters such as speed, 

 milk production, and others of utilitarian value the evidence is even more 

 convincing that selection does gradually shift the mean of the race. 

 Whatever may be the true interpretation, there can be no question that 

 the breeder may guide the variation of nearly every character in a definite 

 direction by proper methods of selection. 



Variation by Modifiability. The success of the art of the caretaker 

 and feeder in animal breeding depends upon the existence of a high degree 

 of modifiability in domestic animals. Extreme cases such as the effect of 

 starvation contrasted with the effect of liberal feeding are easily recog- 

 nized, but in more o.bscure cases it is difficult to state how much of a given 

 effect is due to inheritance and how much to modifiability. Thus the 

 high standard of present racing records does not depend solely upon more 

 careful attention to the selection of breeding stock, for improvement in 

 methods of feeding, care, and training has been associated with this 

 greater discrimination in the selection of breeding stock. The same fact 

 is true in even greater degree with respect to the marvellous records which 

 have been made by dairy cows during recent years, for elaborate methods 

 of development and feeding have been devised to stimulate production to 



