274 MILK, BUTTER, AND CHEESE [chap. 



Though this average composition is very well main- 

 tained when a large number of analyses are considered, 

 the milk from a given cow may vary very widely from 

 the average, and these variations are governed by the 

 factors set out below. It will, however, be found that 

 the chief variable is the amount of fat; though the 

 solids also vary, the differences are never so great as 

 those of the fat. 



1. Individuality. — In the milk of the cows compos- 

 ing any given herd, although they may be all animals 

 of the same age belonging to the same breed and all 

 treated alike as regards food and housing, there 

 will be found considerable variations in composi- 

 tion. In an ordinary mixed herd of Shorthorns, 

 cattle not specially selected, it will generally be found 

 that some of the cows habitually yield milk containing 

 less than 3 per cent, of fat, while in others the percent- 

 age is well over 4. These differences are known to be 

 hereditary, so that not only can the average composition 

 of the mixed milk of the herd be considerably raised by 

 weeding out the animals yielding poor milk, but by 

 steadily breeding only from those cows which yield 

 milk possessing a high percentage of fat, a very consider- 

 able and permanent improvement may be effected in 

 the milk. 



2. Breed. — It has already been mentioned that the 

 Jerseys and other Channel Island cattle yield milk in 

 which the fat globules are above the average size. It is 

 also found that these races produce milk containing a 

 higher proportion of fat. The following table shows the 

 average analyses of the milk of a number of races of 

 cows which are usually kept in milk in the British Isles. 

 The effect of individuality must also be superimposed 

 upon these average figures, so that single, cases occur in 



