84 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ I, II

variables is not quite linear, their degree of correlation is quite high, i.e., a high
S. P. will go with a long lactation period.

         Again though no similar statistical constants for the relationship existing
between the lactation length and total milk yielded in it are known to the author
to have been determined as between S. P. and lactation length, yet it is common
knowledge that the total amount of milk produced during any lactation, i.e., from
parturition to the end of lactation or until recurrence of parturition, is greatly
influenced by the length of time over which the lactation extends. From the
correlation surface drawn for these two variables for our data the statistical cons-
tants given in Table XII were obtained:—

                           TABLE XII.

         Correlation of milk yield with lactation length.

         Although the value of r as given above does not represent the true physiolo-
gical relationship between the two variables, for the length of lactation is generally
a controlled factor in most herds, yet its very high positive value leaves no doubt
in its significance. The value of n²—r² is again interesting ; for it together with
the value of r shows that though with longer lactation periods one gets more milk
yet the rate of increase is not expressed by a linear equation but is represented in
some other form.

         The above discussion about the relations between the S. P., lactation length
and milk yield therefore goes to explain the cause of the variation of lactation
milk yield with S. P., for it clearly brings out that since S. P. and lactation length
and lactation length and lactation milk yield are all positively correlated, a longer
S. P. in a cow results in a longer lactation and therefore more milk yield as
against shorter one in which the length of lactation and consequently the milk
yield are both reduced.

         Before this discussion about the causes of variation of milk yield with S. P.
is left off, it must of course be mentioned that this is not the only way in which