It is possible to think of organisms as having a certain limited amount of time or energy available for expenditure, and of natural selection as that force which oper- ates in the allocation of this time or energy in a way which maximizes the contribution of a genotype to following generations. This manner of treatment of problems con- cerning the adaptation of phenotypes is called the "Principle of Allocation" (Levins and MacArthur, unpublished), and one of its applications might be the formulation of a general theory to account for clutch size in birds. At this stage we will assume that clutch size is a hereditary phenotypic characteristic which can be affected to a greater or lesser extent by the prevailing environmental conditions and which ex- hibits the normal variability of such char- acteristics. Lack (1954) discusses the validity of several hypotheses which' at- tempt to account for clutch size and its variation under different circumstances and conditions, all of which were rejected in favor of his now widely accepted theory that clutch size is adapted to a limited food supply. This paper is an attempt to show that this and other existing hypotheses when taken singly are inadequate in some respect to account for all the data, that each holds for some particular set of con- ditions, and that each is but a part of the complete explanation. The theories will be dealt with individually and it will be shown that as environment varies so will the fac- tors which determine clutch size. PRESENTATION OF THE THEORY