Wilson — Inheritance of Quantity and Quality in Cows' Mifk. 471 



forenoon. Tims, he is present while the cows are milked in the evening and 

 in the morning. He weighs the milk of each cow and takes a sample evening 

 and morning. The weiglits are recorded and the samples sent on to an analyst, 

 who tests them for the amount of fat they contain. Thus, means for deter- 

 mining every cow's yield and the quality of her milk are ascertained either 

 every fourteen or every twenty-one days ; and when the means are all 

 collected, the total can be worked out for the year or for such time as the 

 work is continued. 



It cannot be maintained, of course, that this method is absolutely accurate. 

 More accurate results would have been obtained if the observations had been 

 taken more frequently — most of all if they had been taken daily — but they are 

 sufficiently accurate for the purpose of the Ayrshire cattle-breeders ; and 

 when it is considered that, given a sufficient number of cases, the errors below 

 the line are cancelled by those above, the results are sufficient for our 

 purpose also. 



By grouping the cows according to the quality of their milk and 

 eliminating all that have been observed for less than thirty weeks and which, 

 in consequence, might not show the true approximate average, we find that 

 the bulk of Ayrshire cows' milk contains from 3*3 to 4 per cent, of fat, but 

 that there are many cows with milk both above and below these limits : 

 some running over 5 and others below 3 per cent. It is from this fact, 

 which is expressed graphically in diagram 1 (p. 472), together with the fact 

 that the quantity of Ayrshire cows' milk varies in a similar manner, that 

 we can infer that the yield and quality of cows' milk are separately 

 inherited. 



If we group together all the low-yielding cows, and find their milk 

 invariably high in quality, we may infer that low yield and high quality are 

 of the nature of concomitant variations. If we group the high-yielding 

 cows together, and find their milk invariably of low quality, we may infer 

 that high yield and low quality run together. But if we take these groups 

 and any other groups we can form, and find that the quality varies the same 

 way in them all — that is that there are low qualities, high qualities, and 

 medium qualities in every one of them — then we are justified in inferring 

 that the quantity and quality of the milk are independent of each other. And 

 this is what we do find. 



But, to get a fair view of the case, we must eliminate some other cows in 

 addition to those that were eliminated to form our first diagram. 



In the Ayrshire cattle country there are two main systems of dairying : 

 one in which milk is produced throughout the year, and the other in which 

 it is produced from early spring till late autumn chiefly for cheese-making. 



3u2 



