PHYSIOLOGY: R. PEARL 
355 
where RI denotes the index for any particular mating, C the number 
of chickens produced from that mating and ahve at the end of the third 
week after hatching, and the total number of days from the day 
when this mating began to the day when the last egg from this mating 
began its incubation. 
The reasoning on which this index is based is as follows: Maximum 
reproductive capacity, as represented by 100%, would be attained if 
during the period of the mating the hen laid one egg every day (max- 
imum fecundity), and if further every one of such eggs were fertile, 
and if each embryo hatched, and the hatched chick lived to be three 
weeks of age. There would then be one Hving chick three weeks of 
age for each day during which the mating existed. If the hen does 
not lay every day during the mating season this will cause some reduc- 
tion in the reproductive performance as measured by the index. Sim- 
ilarly a reduction in any of the other factors involved, prenatal or post- 
natal mortahty, will have the same sort of result. The final percentage 
value which one obtains by calculating the index will be a true measure 
of the reproductive capacity of that mating, including within its view 
all of the primary factors of reproduction in poultry. 
Applying this index to the problem of changing fertiUty with advanc- 
ing age we have the following results : 
Weighted mean reproductive indices for males of specified ages mated with females of all ages 
Ages Weighted mean BI 
Male = 1, mated with 9 9 of all ages 12.868 
Male = 2, mated with 9 9 of all ages 10.214 
Male = 3, mated with 9 9 of all ages 9.625 
Weighted mean reproductive indices for females of specified ages mated with males of all ages 
Ages Weighted mean BI 
Female = 1, mated with cf cf of all ages 12.765 
Female = 2, mated with cT cf of all ages 11 .660 
Female = 3, mated with cf cf" of all ages 10 . 722 
From these figures it appears that there is a decKne in net reproduc- 
tive ability or fertility, as measured by the reproductive index, with 
advancing age in both sexes. The rate of the decline is, however, 
more rapid in the male than in the female. 
It is desirable also to look at the matter from the standpoint of the 
mating. This may be done by taking means (weighted in proportion 
to the frequencies involved) of the reproductive indices for the ad- 
vancing combined ages of the two animals entering into each class of 
matings. If this is done we get the following results: 
