Butter-Fat Percentage in Jersey Cattle. 135 



hereditary nature of such a quantitative character a knowledge 

 of its variation is essential to any adequate study of the subject. 

 Before the milk production or butter-fat percentage of a heifer 

 and an age cow are compared we must know what has come 

 between for this may be and often is a part of heredity itself. 



What these investigations, the first on milk production and 

 the present one on butter-fat percentage, have attempted to do 

 is to analyze the individual variations of the individuals in one 

 curve in terms of their component parts. 



Material and Methods. - 



The material and methods used are the same as those of 

 the previous paper save that certain of the cows kept in the 

 early history of the herd were not tested for butter-fat. The 

 number remaining after these were discarded were 1713 with 

 complete eight months butter-fat percentage records. Through- 

 out this study all of the records are for the first eight months of 

 the lactations that extend at least through the ninth month. For 

 the benefit of those who are unfamiliar with the previous paper, 

 that part of the introduction significant to the data and its man- 

 ner of collection is quoted. 



"The data are exceptional in the following ways : ( 1 ) The 

 records extend back to the year 1897 when the herd was organ- 

 ized; (2) the animals are practically all straight island stock; 

 (3) they have been under the oversight and direction of one 

 manager since 1901 ; (4) exact records are kept of the milk 

 production, butter-fat per cent and butter-fat; (5) many of the 

 individual animals have records for several different lactations. 



The elimination of variation caused by varying the five 

 factors above in the records of cows to be used for exact analy- 

 sis of the laws of milk and butter-fat production is important, 

 as it has been often shown that such differences can influence 

 the herd's production. It is obvious that these records are free 

 from such variables. They constitute a homogeneous group of 

 data representing the island Jersey under constant conditions of 

 management and climate. 



The data used for study are all from normal healthy cows. 

 Two diseases have been present in the herd, tuberculosis and 

 abortion. The tubercular animals were all eliminated early in, 



