As the result of a previous investigation, involving the analysis of environmental factors controlling population increase in nature, it became apparent to the writer that population density constituted an environmental factor which had been comparatively neglected, doubtless on account of its obscurity, but whose effects were nevertheless of greater significance than generally realised. It seemed desirable, therefore, that this relatively obscure phenomenon should be investigated, and the writer commenced work at the Laboratory of General Physiology, Harvard University, U. S. A. It has long been known that crowding of animals produces definite and peculiar effects on the various vital processes, and even the morphology of the individual. For instance, as far back as 1854, Hogg produced evidence to show that a snail kept in a small cell would only grow to such a size as would enable it to move about freely. Later, Semper (1874) came to the conclusion that there was a relationship between volume and the ultimate size of the individual. As regards the effect of crowding upon reproductive rate the first observation appears to have been made by Balbiani (1860), who reported that
Paramecium
must be placed in not less than 2-3 c. c. of medium to bring about maximum productivity. Again, Farr (1843) showed that there existed a definite relationship between the density of the human population and the death-rate, and anticipated the trend of modern biology to the extent of elaborating a mathematical formula which conformed to his findings.
The effect of population density upon rate of reproduction with special reference to insects
