664 



THE EYE IN EVOLUTION 



Marmot 



form-discrimination is relatively poor (Karli, 1954 ; and others). On 

 the other hand, the care-free agility of the arboreal squirrel necessitates 

 an unusually keen vision, while the marmots in the Alps with their 

 j)ure-cone retina will whistle as they spot a climber long before he can 

 see them. In dogs, Pavlov (1911-27) found that conditioned reflexes 

 could be developed depending on the discrimination between ellipses 

 and a circle with a differentiation of the semi-axes of only 8 : 9 — a 

 very high standard of efficiency. Among the Ungulates the acuity 

 is higher than would be expected in a rod-rich, afoveate eye, possibly 

 because their eyes are usually large ; the horse or the deer, although 







Fig. 796. — The Visual Responses of the Robin. 



On the left is a mounted young robin with a dull brown breast ; on the 

 right a tuft of red feathers. The territory-holding male threatens the bundle 

 of red feathers rather than a complete robin which lacks red feathers (from 

 Lack ; Tinbergen, Sfudy of Instinct ; Clarendon Press). 



relying largely on movement, has excellent sight, while the acuity of the 

 higher Primates (and man), althougli not equal to that of Birds, is 

 sufficientl}^ high for vision to become the dominant sense in regulating 

 conduct. 



In any appreciation of the visual capacity of animals, however, 

 whether Fishes, Reptiles, Birds, or Mammals, it is to be remembered 

 that their visual perceptions often differ from our own in that they are 

 limited to one or a few relatively simple " sign-stimuli " of form, colour 

 or movement, and not to all the visual elements of the situation. For 

 this reason the pattern of innate behaviour can be released by the exhi- 

 bition of crudely coloured models in which resemblances of form are very 

 inexact. The threat -display in the male robin, for example, is elicited 

 by an isolated bundle of red breast feathers having little resemblance 

 to the bird's usual rival (Fig. 796) (Lack, 1943), or that of the lizard, 



