34 Mr. H. G. Seeley on the Origin 



tween the segments is maintained by senses which are not 

 repetitions of each other, that the brain has a terminal sense 

 anteriorly, and that by the bones touching each other on 

 every margin, along all of which they can grow, there is in 

 the skull an exercised facility for kinetic growth, which ren- 

 ders it impossible that potential growth should be manifested. 

 If, for instance, a potential epiphysis of the frontal segment 

 of the skull were to be formed, it could only be developed 

 between that segment and the parietal segment ; and it could 

 only reproduce, to mark its division, a new pair of eyes behind 

 the old pair. And it is impossible to conceive of such a change 

 taking place except as the only way in which the energy of the 

 animal could be manifested. While, therefore, the bones of each 

 segment remain separate from each other, and permit growth 

 within, it is impossible that any cerebral increase, supposing 

 for a moment it were competent for such an end under any 

 circumstances, could result in the formation of a new segment. 

 Then (no matter how the mammalian skull originated), being 

 segmented by the sense-capsules, it must ever have been sub- 

 jected with greater and increasing influence to the potential 

 power of the vertebral column, which will be manifested in 

 bringing the plan of the segments of the skull more and more 

 into harmony with the plan of the vertebne, and so will obli- 

 terate any differences due to origin or number that tliere may 

 have been, in an earlier condition, between the structures of 

 the different segments. 



Neglecting for the present the jaws of the potential skull 

 and the whole question of the nature of the inferior arches to 

 the segments, I would draw attention to the question whether 

 the potential character is always an induced one. 



In most sharks there is no differentiation Avhatever of tlie 

 brain-case into constituent bones. In a specimen of the angel 

 shark in the Museum of the Hoyal College of Surgeons, there 

 appears on the base of the skull to be a faint indication of a 

 transverse division. And it might be presumed that the seg- 

 ments would originate first, and then that each segment would 

 put on the divided condition ; but I doubt whether the ten- 

 dency to potential increase is the same in the neural arch and 

 centrum ; for in many sharks the neural arch appears to be 

 double, to have been formed originally at each end of the 

 centrum, though often one of these arches has more the aspect 

 of a supplementary arch introduced between two centrums ; 

 moreover the fact that in palaeozoic fossil fishes the centrum is 

 rarely ossified would lead us to anticipate that in the skull the 

 base-bones would be the last formed and least well defined ; 

 so that in conceiving of a skull induced potentially upon the 



