506 ?ENETICS IN RELATION TO AGRICULTURE 
mal to the wild fowl, and some fowls do not show this winter cycle at all. 
The period of March, April and May on the other hand is the natural 
reproductive cycle of the fowl. As might be expected it is the period 
of maximum egg production. It is terminated by the onset of the 
brooding period, which is indicated in the figure by the slight indentation 
during May. The third period, June, July and August, is characterized 
by a falling off in mean monthly egg production. This of course is the 
period during which chicks are reared, and it represents also a prolonga- 
tion of the normal spring cycle. Finally, following the summer cycle, 
there is a period of 2 months, September and October, during which 
molting normally occurs. It is a period of minimal egg production. 
Pearl and Surface have made a careful and detailed investigation of each 
of these periods of laying, only the main results of which are indicated 
in the above account. However, one practical result of the investigation 
which has been employed extensively by them in subsequent investiga- 
tions is the determination of the nature of the winter cycle of egg pro- 
duction, and its value as an index of egg laying capacity. This is a cycle 
added to those normally found in wild fowls, it is fairly sharply marked 
off from other cycles, and it represents an invariable characteristic of 
high laying individuals. It is not participated in by all individuals, and 
as we shall show later it has been subjected to definite Mendelian formu- 
lation. Accordingly there is abundant justification for using winter 
egg production as an index of egg producing capacity rather than using 
the longer total yearly production as such an index. The work of record 
taking in practical breeding operations having in mind the production of 
strains of superior laying ability has been simplified and made more accu- 
rate by these investigations. It is thus plainly to be seen how very im- 
portant it is that as much detailed knowledge as possible be collected 
about any character with which the breeder is working. 
Correlation and Selection——Many of the “points to be observed in 
judging”’ or “‘aids to selection”’ of domestic animals are based upon 
empirical knowledge of the correlation which exists between conforma- 
tion and performance. These have been mentioned in earlier chapters 
and are presented in various texts and manuals on judging, selecting 
and breeding, and need not be discussed here. 
The practical value of a statistical knowledge of correlation between 
somatic characters and functional variations was mentioned in Chapter 
III; also the importance of biological soundness in the material used in 
investigating such correlations. Very few investigations have been 
conducted in this field of scientific research, but the increasing utilization 
of statistical methods is an earnest of future progress in this direction. 
A fine illustration of what the biometrician can do in this line is found 
in the recent work on the correlation between body pigmentation and 
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