AND AFFINITIES OF TARSIUS. 



467 



development of the different physiological areas, and especially to 

 the rate and order of their growth during foetal life. The ground 

 plan of this pattern of the sulci was already laid down in the 

 Eocene ancestors of most of the Orders ; and it is so peculiar to 

 each of them as to afi'ord a sure criterion of the right of any 

 creature to be regarded as a true member of an Order. The fact 

 that tlie Lemuroidea strictly conform in evei-y respect to this 

 distinctive test of membership of the Primates is conclusive proof 

 of their riafht to be included in the Order. 



Text-fie-ure 1. 



Perodicticus. 



Pithecia. 



A comparison of the cerebral liemispheres of a Lemuroid {Perodicticus) and a 

 New- World Monkey (Pithecia) — natural size. 



F. — Inferior frontal sulcus. T. — Superior temporal sulcus. 



-Postcentral (intraparietal) sulcus. 



C — Central sulcus. 

 S. — S3'lvian fissure. 



P. 



L. — Lunate sulcus. 



In the Primates it was the precocious expansion of the brain 

 and the simultaneous cultivation of the visual, auditory, tactile, 

 and motor a,reas of the cerebral cortex that first differentiated 

 the earliest Primates from all other mammals, and provided them 

 with the germs of the capabilities and the means of attaining the 

 supreme position expressed in the name of the Ordei'. 



The pattern formed by the cerebral sulci, equally in the 

 Lemuroidea and the Anthropoidea (see text-figure), is the direct 

 expression of the factors I have already mentioned. The pre- 

 cocious expansion of the visual cortex causes this area to become 

 folded along its axis, so that the major portion of the area striata 

 is bent in to form the retrocalcarine fuiTow in a way that is 

 peculiarly distinctive of the Primates, and is found equally in all 

 three Suborders*, but in no other mammal. The simultaneous 

 expansion of the tactile and auditory areas leads to the develop- 

 ment of the equally distinctive Sylvian fissure (text-figure, S.), 



* I have discussed this at some length in mj- Linnean memoir (ojj. cit. supra). 



Proc. Zool. Soc— 1919, No. XXXIT. 32 



