964 EVOLUTIONAL MODIFICATIONS 



all gradations to culminate in xhv intricate (issural design of tlie human 

 brain. There seems to Ix' no single step missing in this gradual transition. 



EXOLUTIONAL SIGNIFICANCE OF rilE CeKEBKAL LoBES IN I HI PlilMATE 



Brain. Nor is this process of grachial expansion confined tothelissuresalone. 

 Inasmuch as these landmarks serve to delimit the well-marked physiological 

 areas of the biain surface and thus produce lohation, as a concomitant of 

 the increasing richness of fissural pattern, the lolx's themselves become more 

 complicated. In the transitional form of lemur, the identilication ol the 

 quadrilobular condition characteristic of the primates is difficult, largely 

 because the central sulcus of Rolando is onl\- in an incipient state and the 

 boundaries of llu' frontal lobe must, therefore, be drawn largely by inference. 

 So also it is dillicult to distinguish I)et\\een the parietal and frontal lobes. 

 But the boundary between the temporal and parietal lobes is fairly well 

 defined by the lissure of Sylvius. 



Upon the lati'ral coiuexity the e\ idence of an occipital lobe is almost 

 entirely wanting and such expression as this lobe has is largely confined to 

 the mesial surface of the hemispheix'. This observation of the occipital lobe 

 in perhaps its most primitixe condition is borne out by the size ol the 

 superior colliculi which maintain much of lluir original j^roportion and also 

 not a little of their ]3rimiti\-e stratilication. In tarsius and marmoset, the 

 brains of \vhich are so largely lissence])ha!ic, tlu' dilliculty of determining 

 the lobes is etiuall\ pronounced. In tlu' absence of any sulcus centralis, no 

 distinct di\iding line between the frontal and i:)arietal lobes exists, and onl>- 

 in a general way is tlu' dixision between the temporal and parietal lobes 

 given by the fccblx de\t'lo])ed S\ l\ ian lissure. Nothing resembling the sulcus 

 simiarum is jjri'sent in these brains, although the dexelopment ol an occip- 

 ital lobe is much more pronounceti than in the lemur, as show 11 b\ tlu'lact 

 that the cerebellum is now almost eom|)lctt'l\ co\ered by tin' caudal ])ole 

 of the hemisphere. Thus, starting in the Tarsiodea with an anthropoid 



