DIGESTION AND ASSIMILATION 95 



is possible with each succeeding generation. It would seem 

 correct to say that modification in the length of staple in 

 wool is much more quickly made than modification in the 

 fineness of the same. Beginning with sheep producing 

 coarse wool, it would probably require more than a score 

 of generations of careful breeding and selection to produce 

 wool as fine in fibre as that of high class Merino wool, 

 whereas in less than half a dozen generations the form of 

 distinctively wool bearing sheep can be transformed into 

 high-class mutton form. 



That modification in the essential elements of milk is 

 a work that is slow and tedious, is sustained by many exper- 

 iments conducted to throw light on this question. It is so 

 slow and so tedious that it seems as though centuries would 

 be required to effect marked change through food alone. Of 

 course careful and rigid selections would facilitate 

 the process. Contrary to the almost universally prevailing 

 belief, the food fed does not influence materially the per 

 cent of the fat in milk. A cow which furnishes milk low in 

 the per cent of butter fat will continue to furnish milk 

 essentially of the same character, though fed food ever so 

 rich in carbohydrates and fat. The quantity of the milk 

 furnished will of course be increased, and along with such 

 increase will come increase in the amount of butter fat pro- 

 duced, but only, or at least mainly, because there has been 

 increase in the quantity of the milk produced. To increase 

 the per cent of butter fat in milk would require the most 

 persistent selection through long years of careful breeding. 



