954 EVOLUTIONAL MODIFICATIONS 



ExOLLTIOXAL SIGNIFICANCE OF CeHEBRAL FiSSURES. Ill tlu' nuitttr of 



fissural patlcTii, tlir pinnate brain presents lew dillieulties in analysis. 

 As might he expected of a transitional form, the lenuir does not disclose 

 a pattern entirely harmonious with the primate lormula. The fissures 

 in lemur ha\c shown a consideral)le specialization of their own. Far 

 from being a lisseneephalie hemisphere awaiting, as it were, the impress of 

 adaptive inlluenees, the lemur brain manifests a distineti\e ellort in its 

 gyrencephalic specialization. Its fissural pattern is, to a degree at least, 

 rt'iiiiniscent of a carnivore type of brain. In it ma\' be seen some of that tend- 

 ency, peculiar to fissures m carnnores, to arrange themscKes in circum- 

 syKian arches. But e\'en if the brain of lemur is slightly suggestive of this 

 arcade-like arrangenu'iit, it has broken away Irom older influences m this 

 direction and established certain designs of its own. The anterior and pos- 

 terior rhinal lissuixs, eharaeleristic of the carnixores in general, haw now^ 

 disap])eared liom the lateral coiut'xitN due to the marked extensions in 

 the temporal lobe. The lemur thus indicates in its temporal region a distinct 

 advance o\er anj- possible subprimate prototype, and the Eocene 

 predecessor ol the lemurs (Adapis) had a brain e\en less developed. The 

 fissure of SyKius m lemur begins to assume the more oblicjue relations 

 notable m the ]irimates. The sulcus intraparietalis is well ck'fmed, as is also 

 the sulcus parallelus (superior temporal). The beginnings of a central sulcus 

 of Rolando may l)e discerned on the boundary l)et\\ecn the frontal and the 

 parietal lobes. W hat the lemur brain lacks, howexcr, is any extensive occipital 

 specialization upon tlu' comcxitx' of tlu' lu'mis[)here. 



The phyletic history of the Syl\ ian fissure contributes coinincing 

 evidence regarding the e\-olutional significance of the fissural j^attern of 

 the brain. As interpreted by Elliot Smith ( i()03 ), who doubtless has given this 

 subject the most extensive and aiithoritatn t' consideration, the fissure 

 of Sylvius is a composite whose elements xary considerably in different 



