Integration of Problems Concerning Protozoan Populations with those of General Biology

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Abstract
This paper, a summarizing discussion of a symposium on protozoan populations, discusses problems raised by a study of such populations in connection with general knowledge of the mechanics of the control of population size in insects and other animals as well as in Protozoa. Following the entomologists, density-independent and density-dependent control factors are recognized and the latter are seen to consist of direct and inverse density-dependent factors. Such direct factors take a larger % toll the greater the population while inverse density-dependent factors take a higher % toll the smaller the population. Population problems are discussed in connection with a simplified version of Clements and Shelford''s ecological action system. The conclusion is reached that survival values in population physiology present an essential dichotomy into beneficial (cooperative) and harmful (disoperative) effects. Competition may be either beneficial or harmful, depending in part on criteria of value. From the existence of under-crowding, as contrasted with over-crowding, and the resulting optimum population of some intermediate size, it is argued that there is a tendency toward non-conscious cooperation which is a basic and wide-spread phenomenon. Among other implications, this suggests that the population is a unit upon which natural selection can act to bring about the higher phases of social living. Still more general implications are suggested.

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