286 Maine Agricultural Experiment Station. 1919. 



A table is presented showing the relative variability of 

 milk production as compared with other physiological charac- 

 ters. The udder as a secreting organ is compared with the ovi- 

 duct of the hen ; and it is shown that the oviduct considered as 

 a mechanism operates with somewhat less variability than does 

 the udder, having regard to the absolute weight of the product 

 in the two cases. 



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 en- 

 vironmental influences. 



Milk production curves, analytically considered, tend defi- 

 nitely 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 -(-0.1047, and that for fat 

 percentage -{-0.1338. 



Evidence is presented which indicates that selection can 

 have had little if anything to do with determining the direction 

 or the amount of skewness shown by milk production curves. 



The curves for milk yield tend on the whole to fall more 

 frequently in unlimited range types, while those for fat per- 

 centage tend more to limited range types. The estimation of 

 range ends given by the theoretical curves are, on the whole, 

 good. 



In general the tendency of milk yield curves is toward the 

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

 corresponding normal curves would be. Fat percentage curves 

 do not show any definite tendency with respect to kurkosis. 



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 curve. There is no evidence that variation curves 

 for milk production curves are biomodal. 



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 with a law which may be stated in this way: 

 The absolute amount of milk produced per unit of time in- 

 creases with the age of the cow until the maximum is reached, 

 but the rate of increase diminishes with advancing age until 



