Modern biologists are entangled in such numerous problems that their total staff is far too small to tackle but a fraction of them. It seems reasonable, therefore, that we should concentrate on the solution of those problems which may help most to widen our general view. This is the reason why I propose to call your attention to the advantage of studying marine level-bottom communities in the sense described by C. G. Joh. Petersen (1911, 1913, 1918). It is a field which now seems to be ripe for world-wide teamwork and which at the same time has economic implications, since it concerns the feeding grounds of most of our bottom-dwelling food fish, especially of the cod- and and flounder-groups.