Jan. 20, 1919 Variations and Mode of Secretion of Milk Solids 89 



this increase in age. For the partial correlation between percentage of 

 butter fat and age, holding the milk production constant, is 0.0105 ± 

 0.0181, or there is no significant correlation. 



3. The quantity of milk produced for the year is entirely independent 

 of the percentage content of the solids-not-fat, or put in another way, the 

 factor or factors causing high or low milk production are separate and 

 distinct from those causing a high percentage of these constituents in the 

 milk. 



4. Age is a prominent factor in bringing about the reduction of the 

 percentage of solids-not-fd,t. This reduction is not due to differences in 

 the amount of milk, as the milk held constant by the partial-correlation 

 method gives practically the same correlation as when the milk produc- 

 tion is not considered. 



5. Decrease in the amount of milk raises the percentage of butter fat 

 in a greater degree than it affects the percentage of the solids-not-fat. 



6. Increased age has a marked effect in reducing the percentage of 

 solids-not-fat. It does not so reduce the percentage of butter fat. 



CORRELATION BETWEEN THE BUTTER FAT AND THE SOLIDS-NOT- 

 FAT IN COW'S MILK 



This question of the correlation between the butter fat and the 

 solids-not-fat has considerable importance for the problems of the mech- 

 anism of milk secretion in the mammary gland. Does this gland secrete 

 a high content of butter fat when it secretes a high content of the other 

 solids — ^lactose, protein, and ash? Should such an association exist, it 

 becomes evident that the factors leading to a high fat content also 

 lead to a high solids-not-fat content. Taken in connection with either 

 theory to account for the presence of the organic constituents of milk, 

 such a correlation indicates a regulatory mechanism which balances these 

 constituents together in similar proportions in any given individual cow. 



Apart from the bearing on the problems of the milk secretion, such a 

 correlation has wider significance. It predicates that the factors in in- 

 heritance for this high content of one constituent also transmits the pro- 

 duction of high content of the other solids. The problem thus becomes 

 important for the student of inheritance of quantity and quality in cow's 

 milk. 



To answer these questions for Holstein-Friesian cows, it is necessary 

 to arrange the data for the yearly records of butter fat in a table of double 

 entry or correlation table. In the arrangement of this table loo-pound 

 inten^^als were chosen as the basis of division of both the solids-not-fat 

 and butter-fat production of the year tests. In the first two observa- 

 tions of the butter fat it is necessary to group these between 280 and 300 

 pounds and calculate accordingly. The results are given in Table IX. 



Observation easily shows that these two variates, solids-not-fat and 

 butter fat of cow's milk, are highly correlated. The correlation coeffi- 



