440 SECTIONAL TRANSACTIONS.— D. 



Monday, August 22. 



Symposium on Sense perception and the evolution of colour and pattern 

 (lo.o). 



Dr. J. S. Huxley, F.R.S. — The bearing of allcesthetic characters on our 

 knowledge of sense perception in animals. 



Allsesthetic characters are those adapted for exerting an effect via the 

 (distance) receptors of another organism of the same or different species. 

 Criteria of various degrees of validity can be established for the allsesthetic 

 nature of characters. The nature of allsesthetic characters perm'its deduc- 

 tions as to the type of sense perception to be found in the organisms affected, 

 e.g. the commonness of bright colours in birds and entomophilous flowers 

 and their rarity in sub-primate mammals points to the existence of colour 

 vision in birds and insects, its absence in the mammals — a conclusion later 

 confirmed by experimental evidence. Similarly bird-pollinated flowers 

 tend to be of a different colour from insect-pollinated forms, owing to the 

 difference in the reception of red and ultra-violet in the two groups. 



Visual allassthetic characters are of various functional types. The first 

 division is into cryptic and sematic. Sematic characters fall into various 

 groups — warning, threat, recognitional, deflective, and display. Each has 

 its own features of pattern, etc. Consideration of the various types allows 

 us to draw interesting conclusions as to the receptor and perceptor faculties 

 of the organisms at which they are directed. 



A marked general similarity of visual perception in different groups of 

 animals is indicated, with some limited but striking exceptions. 



Dr. H. B. CoTT. — Adaptive appearance and interspecific relationships 

 (10.30). 



In the interrelationships between animals of the same, or of different 

 species, external appearance plays a considerable part. Broadly speaking, 

 the various phenomena of adaptive coloration fall into three main classes, 

 according to the visible results achieved — namely, concealment, advertise- 

 ment, and disguise ; and their respective functions, in the interspecific 

 relationships of animals, are to elude, to attract, or to deceive the eyes of 

 potential enemies or prospective prey. These visual effects are related to 

 the two primary needs of the individual — food and safety : in other words, 

 they are correlated with offence and defence — they facilitate the capture of 

 prey, or escape from the predator. The appearance produced may be 

 extremely elaborate in artistry, and highly effective in action — depending 

 not merely upon profound modifications of structure, colour, and pattern, 

 but of attitude and instinctive behaviour. 



The view that adaptive appearances have evolved in relation to the visual 

 perceptions of animals— that they appeal to the eye — is supported by a great 

 body of evidence, which is considered from various standpoints : (i) The 

 arrangements of colour and pattern which for optical reasons are best 

 adapted to pfoduce special visual effects, are those actually employed in the 

 coloration of different cryptic, aposematic, and mimetic species. (2) Dis- 

 ruptive patterns, especially those of the coincident type (Cott, 1935, Rept. 

 Brit. Assn., p. 384), are independent of, and frequently cut right across, 

 underlying structural elements — anatomical features becoming subordinate 

 to the illusionary appearance superimposed upon them. (3) Particular 

 colour schemes are largely independent of affinity, and furnish a special 



