Quantitative methods have long been increasing in importance in ecological studies and certain types of population data may be confidently handled with standard statistical procedures. When dealing with the distribution of living organisms in space, however, the assumption of normality of distribution is often so questionable as to cast doubt on the conclusions reached by means of statistical analysis. Some of the most persistent difficulties encountered in ecological field work stem from the fact that populations of living organisms are very rarely distributed at random over the space available to them. Nor is random distribution, implying that the individual organisms are scattered by chance, to be expected of most biological material. When plants reproduce either vegetatively or by means of seeds there is a tendency for the offspring to be concentrated in the neighborhood of the parent plant. The same is true of animals which produce their young in litters and especially of the many forms which deposit masses of eggs thus temporarily leading to a heavy concentration of individuals within a small area. Most animals furthermore show some tendency toward active congregation. Even the sexual attraction which brings pairs of animals together is a departure from the theoretical conditions necessary to produce a randomly distributed population, while the social instincts which induce the formation of flocks of birds, herds of mammals, and colonies of social insects depart so radically from random processes that no one would expect the density of ants or deer to exhibit any uniformity over a wide area. Both plants and animals also exhibit nonsocial, environmentally-induced congregation which is really an expression of sample heterogeneity. To a greater or less extent the investigator can circumvent intersample variability by selecting as homogeneous areas as possible for sampling, but there is always the possibility that the organisms under investigation were responding to local factors of which the investigator was unaware. Whatever the cause of the departures from randomness, the empirical distributions of organisms belong to the general type which Polya ('31) has termed distributions, the ecological implication being that the presence of one or more organisms within a sample unit influences the probability of other organisms occurring in the same sample. In most recorded distributions this influence is positive in sign so that the organisms become concentrated into relatively few of the samples, leaving, by contrast with random distribution, an excess of unoccupied units and of densely occupied units, and a deficiency in the number of samples yielding isolated individuals or small numbers of individuals. There is, however, no reason for excepting from this classification those distributions which are negatively contagious I as a result of some type of disoperation or antagonism between individuals which causes the organisms to be spaced more regularly than would be expected to result from chance processes. Examples of negative contagiousness are furnished by certain bird colonies in which each nest occupies