THE EVOLUTION OF THE EYEBROW REGION OF THE FOREHEAD. 293 



human skull, they must be looked upon as being merely peculiar to certain individuals 

 and as occurring sporadically in several different genera and species of ape. 



Anyone who studies the ape skull can readily satisfy himself that within one species 

 several different forms of the supraorbital region may be encountered. To some extent 

 this is apparently the result of the vagaries of individual development, but it is probably 

 more frequently due to changes which occur with the advance of age. As adult life is 

 approached, there appears to be a tendency towards a partial or complete fusion of the 

 three elements, and thus amongst the apes the same individual may present very 

 different types of eyebrow region at different periods of life. 



Type II. — In Type II. are included those skulls which exhibit that condition of the 

 supraorbital region which Schwalbe has described as being peculiar to recent man, and 

 also to the mandrill and other species of ape. The superciliary projection has coalesced 

 with the part of the supraorbital margin which lies to the inner side of the supraorbital 

 notch. On the outer side of the notch the eminence extends outwards with a varying 

 degree of prominence and for a varying distance towards the trigonum supraorbitale. 

 From the latter it is separated by a faint groove which ascends obliquely upwards and 

 outwards from the supraorbital notch. The trigonum supraorbitale, which varies in its 

 extent according to the degree of development of the arcus superciliaris, is, as a rule, 

 depressed and flattened. This form of the supraorbital region occurs in all races, past 

 and present, with the exception of the Neanderthal race, and is undoubtedly the con- 

 dition which is most distinctive of man. 



In the group of 25 Australian skulls to which I have already referred, Type II. of 

 the supraorbital region occurred no less than 18 times, or in 72 per cent. 



Natives of Victoria — 8 males . . .5 times 



5 females . . . 3 ,, 



Natives of Queensland — 10 males . . . 8 „ 



2 females . . . 2 ,, 



As Schwalbe has shown, Type [I. of the supraorbital region also appears in the 

 mandrill (6 and 10), but the form which he has figured (fig. 1, p. 286) only occurs 

 in young specimens. In the adult skull there is a tendency towards the coalescence of 

 the different elements and the formation of a torus which is morphologically equivalent 

 to what is seen in the gorilla and the chimpanzee. Still, this is not by any means the 

 invariable result of advancing age in the mandrill. In the largest and most character- 

 istic skull of this ape in the British Museum, the form presented by the supraorbital 

 region falls clearly within Type II., although the massive superciliary eminence is 

 restricted to the inner part of the region and does not stretch outwards in the form 

 of an elongated ridge, as in the young specimens of the same species. 



It is in the genus Cynocephalus that we find the closest approximation to Type II. 

 as it is exhibited in the human skull. Probably this is the only group of apes in which 

 this type of eyebrow region is almost invariably present. Young specimens may be 

 found in which Type I. occurs (PL I., fig. 17) ; these are rare. Again, as age advances 



