STRLC" ' • ^ ~ -: 951 



In a similar manner, the diametric indices of the brain reveal a general 

 tendency toward expansion in p>assing from the lower to the upp>er extremity" 

 of this order. There are, however, certain notable exceptions in this regard 

 which need explanation. Thus, marmoset and lemur occupy the lowest 

 jx>srtions, but, departing from what might be exp>ected, mycetes representing 

 the majority" of the Cebidae, develojjs the highest figure of all the primates 

 with a diametric value of 92 p>er cent. This undoubtedly is due to the great 

 expansion in the parietal lobe incident to the increment in somesthetic 

 sensibilitA" produced by the sp>eciaIization of the prehensile tail and extreme 

 quadrumanal differentiation. Among the intermediate primates, the gibbon 

 stands as high as any of the great anthrop>oids. This again is accounted for 

 by the extreme sp>eciaIization of its brachiating arms incident to its peculiar 

 tA"pe of locomotion and exquisitely arboreal life. Ofthe three great anthropwids, 

 the orang has the highest p>osrtion according to this index, higher even than 

 that of man. The relatrvely low figure attributed to gorilla and man is explained 

 by the pronounced expansions in the frontal lobe of both of these species. Thus 

 while the diametric index may not be considered to furnish the reicord of a stead- 

 ily progressive evolutionary." process, it indicates certain adaptrse modifications 

 to which the cerebral axes have resp>onded. in ojnseqtience of specialized 

 exp>ansions in particular regions of the brain. The condition of broad brained- 

 ness as contrasted with that of long brainedness is due to the <^>eration 

 of several factors. In the fir?t place, the general mammalian tendenc>- toward 

 the development of a long brain depends up>on the degree to which the rhinen- 

 cephalon dominates the process of growth. The recession in prominence of 

 this oIfactor\" pwrtion of the brain tends to shorten the long axis and thus 

 raise the value of the diametric index. Another influence producing the same 

 result is the combined recession of the rhinencephalon with marked acces- 

 sions in the pxarietal lobe. A third factor may alter the values of this axial 

 index. This is the lensrthenini: of the loni: axis of the brain while the trans- 



