Sept. IS, 1919 Variation in Milk of Ayrshire Cows 319 



The chief results of this first part of the investigation may be sum- 

 marized as follows: 



(i) The mean weekly yield and fat percentage of the milk change in a 

 considerable degree and definite manner with increasing age of the cow. 



(2) The weighted mean standard deviation and coefficient of varia- 

 bility for mean weekly yield of cows of any given age are 2.806 gallons 

 and 17.081 per cent respectively. Reasons are given tending to show 

 that these may be taken as very close approximations to true normal 

 values. For cows of all ages lumped together the corresponding values 

 are 3.329 gallons and 20.816 per cent. 



(3) For fat percentage the weighted mean values for cows of any given 

 age are as follows: Mean = 3.738, standard deviation = 0.330, and coeffi- 

 cient of variation =8.827. 



(4) A table is presented (p. 18) showing the relative variability of milk 

 production as compared with other physiological characters. The udder 

 as a secreting organ is compared with the oviduct of the hen; and it is 

 shown that the oviduct considered as a mechanism operates with some- 

 what less variability than does the udder, having regard to the absolute 

 weight of the product in the two cases. 



(5) Evidence is presented which indicates that about one-half of the 

 observed variation in milk production results from the varying genotypic 

 individuality of the animals with respect to this character and that the 

 other half results from varying environmental influences. 



(6) Milk production curves, analytically considered, tend definitely 

 toward positive skewness. This is true in respect to yield and to 

 quality. The weighted mean value of the skewness for mean weekly 

 yield is found to be 4-0.1047, and that for fat percentage -{-0.1338. 



(7) Evidence is presented which indicates that selection can have had 

 little if anything to do with determining the direction or the amount of 

 skewness showTi by milk production curves. 



(8) The curves for milk yield tend on the whole to fall more frequently 

 in unlimited range types, while those for fat percentage tend more to 

 limited range types. The estimation of range ends given by the theoreti- 

 cal curves are, on the whole, good. 



(9) In general the tendency of milk yield curves is toward the 

 leptokurtic condition — that is, they are more peaked than the corre- 

 sponding normal curves would be. Fat percentage curves do not show 

 any definite tendency with respect to kurkosis. 



(10) Certain of the milk yield curves were dissected into two normal 

 curves by Pearson's method. The resulting graduation was not so good 

 as that given by the appropriate unimodal skew frequency curv^e. There 

 is no evidence that variation curves for milk production curves are 

 biomodal. 



(11) The change in mean weekly yield of milk with advancing age is 

 found to be represented by a logarithmic curve, and to be in accordance 



122502° — 19 6 



