VARIATION OF AYRSHIRE COWvS IN THE QUANTITY 

 AND FAT CONTENT OF THEIR MILK^ 



By Raymond Pearl and John Rice Miner 



The present paper has for its purpose a biometrical analj'sis of the 

 normal individual variation in the milk flow and the fat content of the 

 milk in Ayrshire cattle. 



This work has been undertaken because of a strong conviction on the 

 part of the authors that a fairly comprehensive knowledge of the normal 

 variation of a character which is to be made the basis of genetic study is 

 essential if such study is to be critical. This viewpoint is entirely inde- 

 pendent of any position which one may hold regarding the genetic signifi- 

 cance 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 expression of this variation, a superimposed 

 fluctuating variation of nongerminal origin. The student of genetics 

 ordinarily, and quite rightly, neglects these superimposed fluctuations 

 and confines his attention to the underlying germinal variation. 



This is logically a perfectly justifiable procedure, but an essential to its 

 successful operation is that one shall have such an intimate and thorough 

 knowledge of the normal variability of the character in question that he 

 can make his rejections of the unimportant with substantial correctness 

 and hence safety. 



These considerations become particularly significant when the char- 

 acter dealt with is one especially subject to environmental influences, in 

 consequence of which the fluctuations assume highly significant propor- 

 tions in relation to the underlying germinal differences. Such characters 

 are, for example, fecundity, fertility, and, to a marked degree, milk pro- 

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

 action of a complex of factors, of which those classed broadly as environ- 

 mental certainly play a very important part. To arrive at any sound 

 conclusions regarding the inheritance of these characters it will be essen- 

 tial to form some sort of judgment as to the proportionate parts which 

 genetic and environmental factors play in the production of particular, 

 individual records. It seems perfectly clear that a prerequisite to 



' Papers from the Biological Laboratory of the Maine Agricultural Experiment Station, No. 125. 



This work was begun while the authors were actively connected with the Maine Agricultural Experiment 

 station. It was interrupted by the entry of the United States into the war and has been completed in the 

 Laboratory of Biometry and Vital Statistics of the School of Hygiene and Public Health of Johns Hopkins 

 University. 



Journal of Agricultural Research, Vol. XVII, No. 6 



Washington, D. C. Sept. 15, 1919 



sh Key No. Me. -15 



(285) 



