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BIOLOGY: PEARL AND PARKER Proc. N. A. S. 
1-pair bottles held there. The result is 1904.6. But this is 200 or more 
less than the absolute number produced in various other bottles. Therefore 
the drop in the rate from the 1-pair to the 2-pair bottles cannot be ex- 
plained on the supposition of larval crowding and elimination. Possibly 
this factor comes into play in the higher densities, though there is no evi- 
dence as yet that such is the fact. 
That density of population influences fecundity in the same sense that 
it is here shown to affect rate of reproduction was demonstrated with 
fowls more than a decade ago.^^ It was found that there was a steady 
decline in mean annual fecundity in Barred Rock pullets as one increased 
the number of birds per year from 50 to 100 and to 150. The experiments 
were so arranged that the square feet of floor space per bird (that is to 
say the density of population in Farr's sense) was substantially identical 
in all three sets — exactly so in the case of the 50 and 100 bird pens, and 
only slightly less so in the 150 bird pens. We cannot go into the details 
of these careful and critical experiments with poultry here, but would 
point out that they indicate that there is apparently another element 
involved in the case besides mere physical density, which element in our 
ignorance we may perhaps tentatively call psychological. Apparently 
there is a physiological effect of keeping large numbers of individuals 
together in a confined area or space, even though the amount of space or 
area per individual is identically the same in the large crowds of individuals 
as in the small crowds. 
In general there can be no question that this whole matter of influence 
of density of population, in all senses, upon biological phenomena, de- 
serves a great deal more investigation than it has had. The indications 
all are that it is the most important and significant element in the bio- 
logical, as distinguished from the physical, environment of organisms. 
Summary. — In this paper it is shown that the rate of reproduction of 
Drosophila during the first 16 days of its imaginal life varies inversely 
with density of population, in an extremely exact manner, in accordance 
with the following equation which is essentially an inverse form of Farr's Law, 
log y = a — bx — c log x. 
1 Papers from the Department of Biometry and Vital Statistics, School of Hygiene 
and Public Health, Johns Hopkins University. No. 57. 
2 Semper, K. The Natural Conditions of Existence as they Affect Animal Life. Fourth 
Edit. London 1890. (Other references to detailed papers are given in this book.) 
^ Cited from Semper, loc. cit. 
4 Bilski, F. Pfluger's Arch. 188, pp. 254-272, 1921. 
^ Farr, Wm. Decennial Suppl. Reg. Gen. 1861-70. 
6 Brownlee, J. /. Roy. Stat. Soc. 82, pp. 34-77, 1919, and 83, pp. 280-283, 1920. 
7 Drzwina, A., and Bohn, G., C. R. Soc. Biol. Paris, 84, pp. 917-919, 1921. 
8 Pearl, R. and Parker, S. L. Amer. Nat. 56, pp. 174-187. 1922. 
