822 THE VARIABILITY OP COWs' MILK, i., 



part of the period of lactation, and increase again towards the end 

 of the period. The yield of milk, and the percentage of sugar, 

 on the other hand, decrease steadily during the whole period of 

 lactation. (Crowther and Ruston, 1911; Eckels and Shaw, 1913). 



The factors which determine the composition of the milk secreted 

 by a cow thus fall into two groups: those which are due to con- 

 ditions independent of the animal itself, and those which are fixed 

 by the animal. The chief extrinsic factors are : the food, the sea- 

 sons, and the methods of milking. As long as the variations of 

 these factors are not such as to disturb the health of the cow, they 

 have very little effect on the average output or quality of the milk. 

 The main intrinsic factors are: the idiosyncrasy of the cow, its 

 inherited characters or breed, and the stage of its lactation. These 

 are the factors which determine the kind and amount of milk that 

 a cow secretes, and, of these factors, the first two are predominant, 

 except at the beginning and end of the period of lactation. 



A cow, then, is not to be regarded as a milk-producing machine, 

 whose output is determined by the nature of the raw material sup- 

 plied, and by the conditions of working. It is a physiological 

 mechanism whose product, though not uniform, is independent of 

 considerable variations of the external conditions. The variations 

 of the product are determined by the mechanism itself. This great 

 fact is now fully recognised by the dairy-farmer, who realises that 

 the output of his herds is to be improved by paying attention, not 

 so much to the feed, as to the breed of the cows. 



Since the composition and amount of the milk produced by a 

 cow cannot be varied at all, by adjusting the external conditions 

 and diet, it is important to know over what range of values the 

 spontaneous variations of composition and properties may be 

 expected. Such information is only to be obtained by examining 

 specimens of milk from a large number of single cows. The 

 l>aucity of complete analyses of the milk of individual cows has 

 already been referred to. The bulk of the analyses of this 

 kind have estimated only the volume of milk yielded by the 

 cow and the i3ercentage of fat; the investigators have been con- 



