EVOLUTION AND SIGNIFICANCE OF FOVEA 115 



Elliott-Smith also attributes profound effects in cortical 

 development to macular vision. In this connection he says : 

 ''In the case of the tarsier and the marmoset, two creatures 

 not very dissimilar in size, the brain of the monkey is much 

 bigger, perhaps as much as five times the weight of that of 

 Tarsius. The chief reason for this contrast is the develop- 

 ment of the macula and in association with it the large 

 number of new fibers which pour into the lateral geniculate 

 body and through it stimulate a series of profound changes 

 in the neopallium." 



If the development of the macula is responsible for the 

 greatly increased size of the marmoset brain over that of 

 Tarsius, then might we not expect profound changes in 

 the diurnal lacertilian brain as compared with that of the 

 chelonians, for the lacertilians possess highly developed 

 foveas with more elements per unit area than in the human 

 fovea. Might we not also expect similar evolutionary 

 advances in the brain of those birds possessing a highly 

 developed macular area with a fovea, or especially in those 

 containing two macular areas. To be sure, in the forms 

 below the mammals there are no optic radiations and also 

 no neopallium to stimulate, but even here the greatly 

 increased number of fibers pouring into the primary optic 

 centers from the fovea should produce an equivalent effect 

 upon these centers connected with the primary optic stations 

 — if the assumed principle of stimulation is valid. Both 

 in the fovea-bearing lizards and birds, the numbers of 

 ganglion cells in the region of the macula are increased just 

 as they are in anthropoids and man, therefore, the augmenta- 

 tion of ingrowing fibers to the brain has the same chance of 

 producing profound advances in the brain of these forms 

 as obtains in the marmoset or any other monkey. Experi- 

 ments designed to give evidence pro or con on the alleged 

 stimulating effect of ingrowing opticus fibers upon prolifera- 

 tion in the lateral geniculate body, and in turn upon the 

 cortex, have been undertaken by my colleague, Doctor 

 Chace. His results (in press) show that unilateral removal 



