As populations grow and maintain themselves they must of necessity modify their environment. Conversely, it is logical to suppose that their environment must modify them. The biologist is well aware of this reversible relationship in a general, qualitative sort of way but frequently finds it difficult, largely for technical reasons, to get a more precise picture of the process. There has been a heightening of interest in this and closely related problems of late and notable contributions have been made, along with others, by Allee ( '31), Gause ( '34), Prenant ( '34), Pearl ( '30), Chapman ( '33), Elton ( '27), Johnson ( '33), Volterra and D'Ancona ('35), and Nicholson ('33). I n recent papers (Park, '34, '35, '36; Stanley, '34), an attempt has been made to approach this problem by analyzing the effects of conditioned flour on the beetle, Tribolium confusum Duval. As Tribolium populations grow, the flour, in which the beetles spend their entire life period, becomes altered through the activity of the forms themselves. By using experimentally * My best thanks are due to Prof. Raymond Pearl, of the Johns Hopkins University, for reading the manuscript. I am also greatly indebted to Miss Nancy Woollcott for assistance in making the fecundity and fertility counts.