IN a recent issue of NATURE,1 “B. P. U.” makes the following challenging statement : “The whole of the book is full of examples—most carefully collected and thoroughly analysed, and represents an array of arguments of which selectionists will find it very difficult to dispose.” Unfortunately, the book2 to which reference is made is not yet available here, but it would appear from the information given by “B. P. U.” that the claim made in this quotation is not supported by the facts. While it is evident that the book contains a vast array of statistical data on the contents of birds' stomachs, no mention is made of a similarly exhaustive survey of the food that was available to the birds, and, in fact, it is inconceivable that such a survey could be made adequately. When the statistical data deal only with the contents of birds' stomachs, and there is no quantitative information about the available food, how is it possible to claim that “The principle of proportional predation is exhibited so clearly and forcefully that a discrimination in the choice of prey by birds—is shown to be simply non-existent”? The examination of the stomach contents of birds, by itself, can give no evidence for the existence of “proportional predation”, and so such work can throw no light upon the process of natural selection.