BULLETIN 279 



THE VARIATION OF AYRSHIRE COWS IN THE 



QUANTITY AND FAT CONTENT OF 



THEIR MILK 1 



Raymond Pearl and John Rice Miner 



The present work has for its purpose a biometrical analy- 

 sis of the normal individual variation in the milk flow and the 

 fat content of the milk in Ayrshire cattle. 



The investigation was undertaken because of a strong con- 

 viction that a fairly comprehensive knowledge of the normal 

 variation of a character which is to be made the basis of in- 

 heritance studies is essential if such study is to be critical. 

 This viewpoint is entirely independent of any position which 

 one may hold regarding the significance of different kinds of 

 variation. As a matter of biological fact one never deals 

 actually with one sort of variation absolutely free from the 

 influence or effect of all others. For, even though we may 

 be studying a discontinuous variation of strictly germinal 

 origin and control, there will be, in the actual somatic expres- 

 sion of this variation, a super-imposed fluctuating variation of 

 non-germinal origin. 



These considerations become particularly significant when 

 the character dealt with is one especially subject to environ- 

 mental influences, in consequence of which the fluctuations 

 assume highly significant proportions in relation to the under- 

 lying germinal differences. Such characters are, for example, 

 fecundity, fertility, and, to a marked degree, milk production 

 in cattle. Any milk or fat record represents the result of the 

 action of a complex of factors, of which those classed broadly 

 as environmental certainly play a very important part. To> 

 arrive at any sound conclusions regarding the inheritance of 

 these characters it will be essential to form some sort of judg- 

 ment as to the proportionate parts which genetic and environ- 



"This paper is a resume of an extensive technical paper with the 

 same title published in the Journal of Agricultural Research. 



