354 
PHYSIOLOGY: R, PEARL 
bers of the fauna and the flora. There has been very probably a dis- 
tribution from the north and there may have been an antarctic center of 
dispersion, though there appears to be no evidence within the group 
itself which would justify a definite opinion as to this. 
1 McCuUoch, Econ. Ent. J., 10, 1917, (162-168). 
*Ibid., 10, 1917, (170-176). 
FERTILITY AND AGE IN THE DOMESTIC FOWL 
By Raymond Pearl 
BIOLOGICAL LABORATORY. MAINE AGRICULTURAL EXPERIMENT STATION 
Communicated March 22, 1917 
It has been shown by Marshall,' Pearl,^ and King,^ that in a variety 
of different mammals fertility changes with the age of the animal in a 
definite way. The nature of this change is that, starting at a low point 
at the beginning of the sexual Hfe, the rate of fertility rises with ad- 
vancing age to a maximum, and then declines with further increase in 
age, until total sterility is reached. Marshall inclined to the view, 
on the basis of statistics which I have elsewhere^ shown to be wholly 
inadequate, that the domestic fowl exhibited the same sort of change 
in fertihty with age. There has been no thorough or careful investi- 
gation of this matter in the fowl, based on adequate statistical material. 
The writer has lately studied^ the change in fertility with age in 
poultry, on the basis of 1114 matings of Barred Plymouth Rock fowls, 
covering in point of time a period of nine years. For the purposes of 
this investigation, and generally, the writer has defined fertility as the 
total net reproductive capacity of pairs of organisms, male and female, 
as indicated by their ability to produce viable individual offspring. 
As a working measure of fertihty may be taken a reproductive or fer- 
tility index which expresses the percentage which the number of viable 
offspring actually produced from a particular mating or pair of parents 
is of the maximum number which would be physiologically possible 
within the time limits during which the mating endures. This states, 
in most general terms, the form of index developed for the special case 
of poultry breeding in the present investigation. The same idea can 
be adapted to the measurement and biometrical study of fertihty in 
other sorts of animals, and probably in plants as well. 
Specifically, the reproductive, or fertihty index used in the work 
for poultry has the following form : 
RI = mc/E^ 
