44 



THE EYE IN EVOLUTION 



Drosoph ila 



(iv) MENOTAXis {fiiveiv, to remain). Orientation is not directly 

 towards or away from the light but at an angle to it ; the animal 

 appreciates a definite distribution of the stimulus over its retina where 

 it retains the impression, and having evolved beyond the ability to 

 travel only in a straight line, it can orientate itself and accomplish 

 separate reactions with reference to different parts of its field of vision. 

 This activity is exemplified in the light-compass reactions of insects, 

 or the dorsal (or ventral) light reaction of fishes. 



(v) Kiihn's final category, mnemotaxis (/xvtj/mt^, memory), 

 wherein immediate orientation is aided by memory-images of past 

 experience, is associated with other methods as an adjuvant mechanism 

 of a higher type. 



In these responses to light three stages emerge in the evolutionary 

 process. In the simplest and most primitive response, the stimulus is 

 appreciated in an indeterminate manner and orientating movements 

 are corrective. In the next stage a more complicated but obviously 

 more efficient reflex mechanism ensures a directed and purposeful 

 orientation. The third and highest development involves the ability 

 to retain the impression made upon the receptor organ, to adjust the 

 response and utilize various means to gain the desired end should the 

 most obvious fail ; it is a purposive rather than a reflex response. This 

 more advanced development is exemplified in its simplest terms in the 

 continued ability of some worms to orientate themselves to light when 

 one eye has been removed, or in the compensatory modifications in the 

 responses of certain insects when some of the legs on one side have been 

 removed ; the same adaptability is seen in the complicated manoeuvres 

 of ants, backwards, sideways or forwards, to reach the desired goal, 

 and reaches its highest forms in the reactions of Vertebrates among 

 which its culmination is seen in the navigational ability of birds. 



All these reactions, however, whether simple or complex, have 

 certain features in common. In the first place, they are all innate and 

 show no evidence of being acquired ; thus Payne (1910-11) bred the 

 fruit -fly, Drosojjhila, in the dark and found that individuals of the 

 69th generation were normally photo -positive at the first trial ; while 

 the young bird may set out on its first migration to a new land 2,000 

 miles away and follow by a light-compass reaction approximately the 

 same route as its parents. It is true that the standard responses may 

 become altered by use, being either accentuated by habituation (as we 

 have seen even in Amoeba, Mast and Pusch, 1924),^ or diminished by 

 adaptation (as we shall see in some insects, Clark, 1928-33) ; but these 

 are physical processes. It is also true that their efficiency may be 

 increased with training, as is seen in the migration or homing of birds 

 (Rupp( '' and Schein, 1941 ; Matthews, 1953), or can be altered and even 



1 p. 36. 



