214 
BIOLOGY: PEARL AND PARKER Proc. N. A. S. 
All the progeny flies as they emerged as imagoes from the two series of 
bottles were counted and recorded as to sex. No account was kept of 
larvae or pupae. We are dealing here with net reproductive capacity, 
just as in certain of our poultry work (PearP°). Food and temperature 
conditions were constant for all bottles throughout the experiment. The 
only significant variable between the several series was the density of 
population. There occurred various accidents to some of the bottles 
in the course of the experiment. Some of the mated flies died; food 
occasionally rose in the bottles and killed a few individuals, etc. In re- 
ducing the results to a per 9 day basis allowance has been made for these 
accidents, and the final results set forth below represent the outcome for 
normal mated flies living under normal conditions. 
Inasmuch as only 16 day's progeny was counted the results cannot be 
directly stated in terms of total reproductive capacity. Instead they 
have been tabulated as the rate of reproduction per 9 day over a 16 day 
period after emergency. It is quite certain however that the absolute 
number of progeny per 9 in the first 16 days of life is highly correlated 
with the unknown total reproductive capacity. This we feel sure of from 
many observations in Drosophila and from analogous observations on 
fecundity in the domestic fow4. 
Results. — The results of the experiments are exhibited in Table 1, and 
graphically in figure 1. The table contains the following data: (a) the 
numbers of mated pairs per bottle at the start. The numbers of mated 
tiies per bottle were therefore twice these figures at the start ; (b) the mean 
population of mated flies over the whole period of 16 days. If no acci- 
dents had occurred these figures would have been 2, 4, 6, 8, 10, 12, 14, 
16, etc. But accidents and deaths did occur. The figures set down 
represent the actual average number of flies present in each bottle over 
the period of the experiments, taken as a whole, (c) The mean number 
of flies per cubic centimeter of free air space in the bottle. Since all the 
bottles were by careful measurement of the food made to contain the same 
number of cc. (130) of free air space, it follows that the entries in this col- 
umn are simply a constant (V130) times the entries in the preceding column. 
They are inserted merely to show what the actual density in these experi- 
ments was. (d) The mean number of flies per square centimeter of agar, 
growing yeast on its surface, in each bottle. The same remarks apply 
to this as to the cubic density figure. It corresponds exactly to the density 
of population figure used in human vital statistics. The food area per 
bottle was 23.76 cm.^ {e) The total female days, meaning the number 
got by adding together for all females the number of days that each was 
in the experiment. Thus if a female lived throughout the experiment 
she contributed 16 9 days (1X16) to the total; if she lived 7 days she 
contributed 7 9 days (1X7) and so on. (/) Total absolute number of 
