April 1929 – vol. 22, issue 11–323 want of satisfactory data regarding the numbers of animals in relation to space and time is an obstacle of which biology is becoming acutely aware. It is clear that until accurate statistics are secured on a sufficient scale research must be restricted, if not actually held up, at a great many points. The small number of observers who are available for any such task, and the obvious difficulties i... This article is only available to registered users.To read more, please log in or purchase a subscription.