OF TARSIUS FIIOM SARAWAK. 185 



dissected. The uterus has been presei'ved for Professor J. P. 

 Hill to examine. Mr. Le Gros Clark gives me the important 

 information that neither with the ophthalmoscope nor by direct 

 examination of the retina after opening the eye was he able to 

 detect any sign of a yellow spot (macula lutea). At the same 

 time he cautiously refrained from denying its presence. Dr. H. 

 Woollard has been investigating the specimens of Tarsius given 

 to me many years ago by Dr. Charles Hose for the purpose of 

 preparing a monogi'aph on the anatomy of Tarsius. He was 

 unable to find the macula, lutea in his preparations of the retina, 

 nor was he able to discover any trace of it in a series of excellent 

 histological preparations lent me by Dr. W. L. H. Duckworth of 

 Cambridge. 



The macula lutea is a ver}^ obtrusive feature of the retina in 

 monkeys, and can easily be seen even in badly preserved material. 

 Hence it could hardly have been overlooked in the fresh retina 

 of Tarsius, if it had been present. Hence Mr. Le Gros Clark's 

 observations, taken in conjunction with Dr. Woollard's work at 

 University College, make it practically certain that in Tarsius 

 this distinctive featui-e of the retina of the Anthropoidea (among 

 mammals) is lacking. In this respect Tarsius agrees with the 

 Lemurs, and differs from all monkeys. 



In the Anthropoidea the development of the macula lutea is 

 associated with a. distinctive ai'rangement of the fibres of the 

 optic tract. Instead of a, complete decussation of the optic 

 nerves there is only a semi-decussation, the fibres coming from 

 the lateral half of each retina remaining uncrossed. Dr. Woollard 

 tells me that he is unable to find any evidence of such an arrange- 

 ment in Tarsius, which, like the Lemurs, seems to have either a, 

 complete or almost complete decussation. 



These observations are of far reaching significance from the 

 light they shed on the cardinal factors that bi-ought the 

 Anthropoidea into existence. 



I have i^epeatedly called attention to the influence of the 

 adoption of an arboreal life upon the cerebral cortex and the 

 behaviour of mammals, which is so clearly displayed in the 

 Menotyphlous Insectivora by comparing Macroscelides with 

 Tupaia. In the Tree-Shrew the importance of the sense of smell 

 is diminished, and those of vision, touch, and hearing greatly 

 enhanced. Tlie further emphasizing of these adaptations brought 

 the Primates into existence. Among the Prosimise the reduction 

 of the size of the nose in one group, the Tarsioidea, allows the 

 eyes to come to the front of the face so that their fields of vision 

 overlap. The enormous enha.ncement of the importance of vision 

 which is thus efiected leads to the sudden expansion of the 

 cortical area lor vision and its further specialisation in structure. 

 The sense of sight now completely supplants that of smell as the 

 doininant guide to the animal, a,nd the alteration of the position 

 of the eyes enables the animal to look forward, both in the literal 

 a,nd the metaphorical senses of the expression. 



Proc. Zool. Soc— 1921, No. XIII. 13 



