THE PERCEPTION OF FORM 665 



Lacerta viridis, by a crude clay model so long as it has a blue throat 

 (Kitzler, 1941).^ The feeding reactions of young herring-gulls are 

 initiated by crude models simulating only in a rough and ready manner 

 the red patch on the parents' mandible which forms the normal 

 stimulus (Tinbergen and Perdeck, 1950) ; and despite their remarkable 

 visual acuity birds show incubation responses to objects other than 

 eggs so long as they are small and round (Kirkman, 1937), or exhibit 

 escape reactions to a crude dummy as if it were an enemy bird of prey, 

 no matter what the colour or the shape of its wings and tail may be, so 

 long as the neck is short (Lorenz, 1940). So also will the male stickle- 

 back, Gasferosteus aculateus, react differently to a crude model of a 

 fish : in the head-up position it will exhibit mating activity, in the 

 head-down position it will exhibit fight (Tinbergen, 1948) and it will 

 be similarly stimulated by a truck passing outside its window provided 

 only that it is red as the belly of its natural rival. 



The differences between visual acuity in these members of the vertebrate 

 phyhim which have been experimentally investigated, and particularly the 

 difference between diurnal and nocturnal animals, are seen in the following 

 figures which refer to minutes of minimum visual angle : 



Diurnal — man, 0-44 to 0-83 ; chimpanzee, 0-47 ; rhesus monkey, 0-67 ; 



cebus monkey, 0-95 ; homing pigeon, 0-38. 

 Nocturnal — cat, 5-5 ; alligator, 11-0 ; opossum, 11-0 ; rat (pigmented) 

 26-0, (albinotic) 52-0. 



Abelsdorff. Arch. Anat. Physiol., Physiol. Dickerson.. The Frog Book, X.Y. (1906). 



Aht., 155 (1898). Diebschlag. Z. vergl. Physiol, 28, 67 



Alley and Boyd. The Ibis, 92, 46 (1950). (1940). 



Armstrong. Bird Display and Behaviour, Donner. Acta zool. Fenn., 66, 1 (1951). 



London (1947). van Eck. Arch, neerl. Zool., 3, 450 (1939). 



Ash. Brit. Birds, 45, 288 (1952). Engelmann. Z. TierpsychoL, 9, 91 (1952). 



Banta. Biol. Bull., 26, 171 (1914). Fincham. Trans, opt. Soc. Lond., 26, 239 



Beebe. Zoologica, 16, 149 (1934). (1925). 



Beer. Pfliigers Arch. ges. Physiol., 5Z, l"^ 5 Franz. Jetia. Z. Naturuiss., 40, 697 



(1893) ; 58, 523 (1894) ; 69, 507 ; 73, (1905). 



501 (1898). Zool. Jb., Abt. Zool. Physiol., 49, 323 



Beritoff. Pfliigers Arch. ges. Physiol., 21Z, (1931). 



370 (1926). Bolk's Hb. d. vergl. Anat. d. Wirbelthiere, 



Bourguignon and Verrier. Bull. Soc. Berlin, 2 (ii), 1093 ( 1934). 



ophtal. Paris, 273 (1930). Goldsmith. Bull. Inst. gen. Psychol., 14, 



Breder. Nat. Hist., 25, 325 (1925). 97 (1914). 



Buxton. Contribs. to Psychol. Theory, 2, Grundlach. J. comp. Psychol., 16, 327 



75 (1946). (1933). 



Casteel. J. aniiyi. Behav., 1, 1 (1911). Hartridge and Yamado. Brit. J. Ophthal., 



ten Gate. Arch, neerl. Physiol., 8, 234 6, 481 (1922). 



(1923). Helmholtz. v. Graefes Arch. Ophthal. 1 



Chard. J. e.vp. Psychol., 24, 588 (1939). (2), 1 (1855). 



Claparede. Arch, de Psychol., 20 (78), 178 Herter. Z. vergl. Physiol, 10, 688 (1929) ; 



(1926). 11, 730 (1930). 



Cousteau. The Silent World, London Die Fischdressuren u. ihre sinnes- 



(1953). physiologische Grundlagen, Berlin 



Czeloth. Z. vergl. Physiol, 13, 74 (1930). (1953). 



1 The literature on tliis subject is now comprehensive : see Russell (1934—43), 

 Lorenz (193.5-39), Noble (1936), Marshall (1936), Matthews (1938), Huxley (1938)! 

 Armstrong (1947), Tinbergen (1948-51), and others. 



