NATURAL SELECTION 249 



We believe that these experiments prove that birds have a 

 certain power of discrimination between insects of different 

 colour-pattern and that, on the whole, insects of a black and 

 yellow or red colour are unacceptable. This holds at least 

 for the area (Massachusetts) in which the experiments were 

 carried out. Whether the experiments can be used as evidence 

 that the colours and unacceptability have evolved as a result 

 of selection appears to us somewhat doubtful. The following 

 difficulties seem to be important. 



In Appendix C, p. 381, the author gives a tabulation of 

 the acceptability of each insect used. Unfortunately only the 

 mean acceptability is recorded, and there is no indication as to 

 whether the acceptability in different experiments was usually 

 of nearly the same value. Unless the acceptability rating is 

 found to be very constant, large numbers of each species are 

 required to substantiate anything like the true value. Actually 

 the mean number of specimens of each species used was 25 

 (5,000 specimens of 200 species) ; in only 12 species were more 

 than 100 specimens used, and in only 16 species more than 80. 

 It appears, therefore, quite possible that only the more extreme 

 differences in the assigned ratings may be of any significance. 

 Looked at in this way, the experiments show that birds usually 

 distinguish between very conspicuous and dull- coloured insects, 

 or between very nauseous and harmless or ' tasty ' insects. 

 On the other hand, the experiments scarcely indicate whether 

 birds have a power of discriminating between minor variations 

 in these properties. Probably most observers would agree 

 that birds recognise and avoid some of the very conspicuous, 

 evil-smelling insects. But if these properties have arisen as a 

 result of the selection of small variants, birds must be supposed 

 to have very much keener discriminating powers than can 

 actually be deduced from the experiments. Possibly further 

 experiments on the same lines, employing numerous specimens 

 of a species of variable colour-pattern, might throw some light 

 on this difficulty. 



McAtee (1932) has made another voluminous contribution 

 to the subject. He summarises the analyses of the contents 

 of 80,000 bird stomachs collected for the U.S. Biological 

 Survey. McAtee's main contention is that all types of animals 

 are preyed on in proportion to their numbers. It is not yet 

 possible to estimate the numbers of most animals accurately, 



