134 Maine Agricultural Experiment Station. 1920. 



: slight but consistent decrease in the butter-fat percentage which 

 Jier milk will contain. 



The problems of milk secretion taken from their economic 

 and scientific aspects may be said to be twofold, the first phase 

 of the subject dealing with the problems connected with the 

 production of the quantity of milk, the second phase consider- 

 ing the quality or amount of the constituents per unit volume 

 of the milk. This second phase to the minds of most people 

 has come to mean for milk production the amount of butter-fat 

 per unit volume of the milk or the percentage of this butter-fat. 



In studies in milk secretion V of this series of papers the 

 subject of the variations and correlations of milk secretion with 

 age was examined analytically. In this paper it is proposed to 

 deal with the normal fluctuations and associations of the butter- 

 fat percentage for the milk of the same cows used in the pre- 

 ceding study. 



The theorem chosen is a small part of that greater problem 

 which lias come to be known under the title of developmental 

 mechanics. If a group of like animals are measured for any 

 character and the measurements brought together in a curve rep- 

 resenting the individuals in the group, the position of any indi- 

 vidual in the curve and the shape of the curve itself are the 

 functions of the two basic variables given such prominence by 

 the work of Galton, environment and heredity. This environ- 

 ment may play a larger or a smaller part in its influence on the 

 character. In most inheritance studies of what might be called 

 qualitative characters, commonly classified as the chemist does in 

 analyzing a chemical compound for its constituent elements as 

 barium present or barium absent, the environmental differences 

 cause little variation in the somatic appearance of the character. 

 In other words put in quantitative terms coefficient of variability 

 of the character is low or as the physicist says the character is 

 constant. The place of the individual in a curve then is due 

 largely if not entirely to heredity. 



In the so called quantitative characters the conditions are 

 reversed to a certain extent. The superimposed variability of 

 the conditions under which the organism exists play their part 

 along with heredity in determining the place in the variation 

 .curve that the individual will take. Clearly in a study of the 



