16 ON THE AFFINITIES OF THE BRAIN 



the development of its convolutions. This last of the seven brains 

 will allow us to apply our second canon to test the value of the 

 absence of this structure in the particular relation of superficial 

 position as a mark of serial degradation. 



But a structure which exhibits so much variability, as to conform 

 to the rule in but three, and to diverge more or less from it in four, 

 out of seven brains chosen at haphazard for examination, as being 

 all at that moment which a particular museum contained, will 

 scarcely seem to merit a high place as a zoological differentia. With 

 reference to the 'premier pli de passage' in the orang, a careful 

 comparison of the relations of the parts lettered a a, in Fig. 3, with 

 the same relations in Fig. 4, will show that this convolution is by 

 no means superficial in its entire extent on the left side of that 

 brain. And, secondly, in our second specimen of an oranges brain, 

 this convolution is concealed on both sides within the fissure; and 

 the cerebral hemispheres in this specimen present, in consequence, 

 as perfectly wave-like an opercular edge as in any other monkey. 

 In confirmation of this, I would appeal to Tiedemann's n and Wag- 

 ner's 2 figures, already referred to, as giving typical representations 

 of an external perpendicular fissure in the brain of orang utangs, in 

 which, according to M. Gratiolet, it should be invariably half-filled 

 up by his ' premier pli de passage.' 



Lastly, with reference to the chimpanzee : our specimen possesses 

 on its right side a well-marked, superior bridging convolution, 

 coming for a considerable part of its length nearly or quite to 

 a level with the lobes it connects. Tiedemann's figure of the 

 chimpanzee's brain leads us, by its imperfectly marked operculum, 

 to the same conclusion as its sharply drawn one did in the case of 

 the orang. The law of correlation of forms is a safe guide to us, 

 when we have to predict what will be found in the lower organisms 

 of well-marked families ; it loses its inflexibility, and becomes but 

 a leaden rule, when we come to examine the most perfectly evolved 

 species in such families. In the higher species of the order, apes, 

 as in the higher varieties of the species, man, we find variability 

 the rule, uniformity the exception ; in the lower species, as in the 



1 Wagner, 'Icones Zoot.' viii. 3. 



2 Ap. Wagner, ' Icon.' Taf. viii. fig. 2. But Bischoff, p. 77 of his paper on the 

 Grosshirnwindungen, says that Wagner had not taken away the pia mater, and that 

 when he took it away a first pli was visible only with its posterior segment hidden in 

 the external fissure. 



