T H HE study of populations of late years has acquired a particular interest. A great stimulus to the work along this line has been given by the discovery of the law of population growth made by Raymond Pearl. "A population may be defined as an aggregation of individual organisms of the same species, living together in a limited and defined universe" (z8, p. 5I5). In the case of human population it is the population of a town or of a country that possesses these characters. As an example of population among insects we can take a group of grasshoppers of a certain species inhabiting a given locality. The density of such a population may be considered as its most important characteristic. It can be expressed by the number of individuals occupying a certain unit of space. In field ecology this notion is designated as the abundance of a species. For terrestrial animals like grasshoppers the density of population may be expressed by the number of grasshoppers dwelling on the space of one square meter. The density of population is very important because of its influence on the trend of the main biological processes. Raymond Pearl has shown that it has a pronounced influence on the longevity and fertility of organisms. The rate of growth is also connected with the density of population, which in its turn depends on different factors of environment. We observe it on examining the distribution of organisms in natural conditions. Every organism occupies usually a definite vital zone with most favorable conditions. At the same time, according to field investigations, the density of population of a given species is not the same in different parts of its vital zone. The density of population reaches its highest value in definite optimal conditions, decreasing in such habitats where the conditions deviate from the optimum. The study of the correlation between the density of population and environment represents the leading problem of the ecology of populations. Field investigation is one of the methods for such a study. It requires a detailed examination, on the one hand, of the environment in which a given organism dwells, and on the other, of the density of population of this organism under different conditions of environment. Unfortunately the relationships existing in nature are not easy to elucidate. First of all, we are dealing here, not with separate physico-chemical and social factors of environment, but with their combinations, which are variable in time. Moreover it is often rather difficult to obtain accurate determinations of the population size and of the factors of environment. All this brings us to the necessity of an experimental study of the influence of ecological factors on the density of population. As an example of such an experiment we can take the growth of population of a species of insect under different temperatures. For this purpose jars equal in shape and size are filled with the same quantity of food, each populated by a pair of insects, one male and one female, and kept