IN his review in NATURE1 of a publication by W. L. McAtee, entitled “Effectiveness in Nature of the so-called Protective Adaptations in the Animal Kingdom”2, “B.P.U.” apparently expresses agreement with that author in interpreting what he believes to be ‘proportional predation’ as evidence of indiscrimination in the choice of food by birds and other predatory animals. Mr. A. J. Nicholson has pointed out that the examination of birds' stomach contents, by itself, can give no evidence of proportional predation.3 There is, however, another aspect of the subject to which I would direct attention.