CHAPTER IX 



ON THE SUPPOSED EVOLUTIONARY RELATION- 

 SHIPS AND THE FUNCTIONAL SIGNIFICANCE 

 OF THE FOVEA 



In an address by the late G. Elliott-Smith (1928) on the 

 subject ''The New Vision," he attributes the changes which 

 occur in the structure of the parts of the anthropoid brain 

 concerned with vision, as due to the origin of the macula 

 which is also, according to him, responsible for the profound 

 evolution of the nature of vision in apes and man. In this 

 connection he says, ''The crucial importance of the spectral 

 tarsier for the study of these problems is now widely recog- 

 nized. This interesting creature has almost, though not 

 quite, developed into a monkey. The essential and funda- 

 mental factor in the evolution of a real monkey is the 

 development of a macula lutea and all that this involves 

 in a series of profound changes in the nervous system, and 

 an even more striking influence on the animal's behavior 

 and capabilities." He cites Woollard (1925), who studied 

 the retina of Tarsius spectrum, and who found there what 

 must be considered as an area centralis retinae — a local 

 thickened area with an increase in the number of visual 

 elements. Elliott-Smith speaks of this as a primordium 

 maculae and contrasts it with the true fovea such as occurs 

 in a primitive monkey, e.g. the marmoset. He regards the 

 development of the true macula as intimately associated 

 with the altered position of the eyes and the extensive 

 overlapping of the visual fields. The position of the eyes 

 has already changed in Tarsius, but according to him the 

 rearrangement of the optic tracts in this creature does not 

 seem to have attained a stage that is essential before the 

 macula can develop. He says further, "Before this important 



108 



