GENETICS: HARRIS, BLAKESLEE AND KIRKPATRICK 565 
optic plate (or in general), a full compensation is secured by rotating 
the mirror nearest the telescope, provided it is the thinner of the pair. 
In such a case the system of mirrors will not be parallel, however, when 
the fringes are infinite in size (circles) and the zero position of the 
micrometer must be independently found, as one of the constants of 
the apparc'tus. 
iSee these Proceedings, 3, June, 1917, 412, 432, 436. 
2 From a Report to the Carnegie Institution, of Washington, D. C. 
INTER-PERIODIC CORRELATION IN THE EGG PRODUCTION OF 
THE DOMESTIC FOWL 
By J. Arthur Harris, A. F. Blakeslee, and Wm. F. Kirkpatrick 
STATION FOR EXPERIMENTAL EVOLUTION, COLD SPRING HARBOR. N. Y.. AND CONNECTICUT 
AGRICULTURAL COLLEGE, STORRS. CONN. 
Communicated by C. B. Davenport, July 13, 1917 
In no animal organism except man has the investigation of the inter- 
relationship of the various morphological and physiological charac- 
teristics of the individual by means of the modern methods of statistical 
analysis applied to large masses of quantitatively recorded observations 
been carried out on a more extensive scale than in the domestic fowl, 
to which Pearl and Surface, and others associated with them, have 
devoted their attention for a number of years. 
Notwithstanding the many problems dealt with in this series of 
investigations, our knowledge of the interrelationships of characters 
of economic importance in this organism is still far from complete. 
Data from breeds different from those used by Pearl and Surface are 
particularly needed for purposes of comparison. 
In an earlier paper^ we discussed the correlation between the concen- 
tration of yellow pigment in the somatic tissues and egg production 
in the White Leghorn fowl. In the present investigation we have 
considered the correlations between the egg production of various 
periods. 
The intensity of the correlation between the number of eggs laid in 
the several individual months and the total egg production of the year 
as a whole is shown by the solid dots in figure L A curve in excellent 
agreement was found for the previous year. 
Economically these coefficients are of the greatest importance since 
they make possible the selection of groups of birds of high annual egg 
production from the trap nest records of individual months. Biologi- 
cally they are in some degree spurious because of the fact that the cor- 
