60 Maine Agricultural Experiment Station. 1919. 



It is of interest to compare the coefficients of variation for 

 total yield and absolute amount of fat in the mixed milk of a 

 large herd with those for milk yield as discussed in the present 

 paper." It is seen that the former are about 9, whereas the co- 

 efficients for milk yield give values of about 17 to 25, depend- 

 ing upon whether cows of all ages or of a single age are con- 

 sidered. 



In the case of secular variation in the amount of quality 

 of the mixed milk of a large herd, individuality of the animal 

 as a source of variation is entirely eliminated. The observed 

 variation must there be due to the combined action of all the 

 external environmental influences which affect in greater or 

 less degree the milk yield of every cow. 



On the other hand, the constants of variation for milk 

 yield determined in this paper are based upon the diversity or 

 variation exhibited among a large number of different cows in 

 respect of weekly yield. Here one primary factor in the causa- 

 tion of the observed variation must be the individuality of the 

 animal in respect of milking ability. By individuality in this 

 sense is meant the genotype of the individual with regard to 

 the character named. But in the causation of the variation in 

 milk yield as here discussed there must be involved the com- 

 bined influence of the individuality of the animal plus that of 

 all the environmental factors which act in producing variation 

 in the mixed milk of the herd, since each of these causes in- 

 fluence every individual animal while it is making its individ- 

 ual record. 



It is, therefore, possible to make comparison here between 

 observed variations (as measured by the coefficient) due, on 

 the one hand, to environmental influences alone, and, on the 

 other hand, to genotypic differences plus environmental influ- 

 ences. The differences should represent that part of the ob- 

 served variation due to genotypic differences. 



The figures as they stand suggest that roughly about one 

 half of the variation (measured by the coefficients of varia- 

 tion) in milk yield results from the varying genotypic individ- 

 uality of the animals in respect of this character, and the other 

 half results from the varying external circumstances to which 

 cows are subjected during lactation and which have an effect 

 upon the flow of milk. Or, to put the matter in another way, 





