64 



THE EYE IN EVOLUTION 



Homing pigeon 



polarization pattern of the sky (Montgomery and Heinemann, 1952). 

 The evidence would seem incontrovertible that these birds can 

 orientate themselves by an innate ability to estimate the sun's arc by 

 observation of its movement over a small distance and, by extrapola- 

 tion, to navigate automatically over great distances with extreme 

 accuracy even when flying is continued during the night. Flight 

 throughout the journey is governed by a number of factors developed 

 by individual experience in respect of which considerable variations 

 exist, but the fundamental basis of the method of orientation is an 

 innate form of sun-navigation depending on an appreciation and 

 memory of the angle of incident light and an ability to make appropriate 

 corrections according to the 24-hour rhythm of a reference system (an 

 internal clock) operating in the brain (Ruppell and Schein, 1941 ; 

 Lack, 1943 ; Saint Paul, 1953 ; Matthews, 1953-55 ; Kalmus, 1954 ; 

 Pratt and Thouless, 1955). 



This theory had its origin in the observations of Ising (1945), Varian (1948), 

 Davis (1948) and Wilkinson (1949), but the most satisfying evidence came 

 from the experiments of Matthews (1951-55) on homing pigeons, gulls and 

 Manx shearwaters. He found (as have others) that birds released in a strange 

 or clueless environment (such as over the sea) rapidly orientated themselves in 

 the correct direction for home as they soared to jfly, and maintained their direction 

 over long, direct flights over unknown country ; but they were able to find the 

 correct direction only when the sun was up and their initial accuracy in flight 

 depended on a clear sky ; in cloudy or overcast weather they were helpless 



Figs. 39-40. — Navigation by Birds. 

 To illustrate the initial orientation of the Manx shearwater when 

 released in a strange environment. The home direction is vertically upwards. 

 The length and breadth of the rays is proportional to the number of birds 

 that orientated in the direction indicated. 



Fig. 39. 



Fig. 40. 



Fig. 39. — Orientation under a cloudless sky. It is seen that the great 

 majority of the birds orientated themselves initially in approximately the 

 right direction. 



o"ra. 40. — Orientation under heavily clouded skies. The ability to 

 orieniate correctly has been lost (G. V. T. Matthews). 



