32 Mr. H. G. Seeley on the Origin 



eventually its potential energy would influence their arrange- 

 ment, and gradually bring the structure of the brain-case into 

 harmony with the vertebral plan. Thus there are three pos- 

 sible ways of formation for a skull: — 1st, potential repetition 

 of the vertebrate plan ; 2ndly, independent ossification ; and, 

 3rdly, independent ossification modified by potential repetition. 

 The facts of the case are such that it is quite possible to select 

 examples which would sustain each of these views. Thus 

 among the shark tribe, the bony cerebral envelope is made up 

 of homogeneous osseous particles which show no indication 

 whatever of segmentation. And, in the absence of evidence 

 of division of the head into separate bones, it would be an 

 unwarrantable use of the imagination to suppose that the 

 divisions had once existed and have become obliterated. This 

 would seem to be a type of those examples of the skull which 

 have originated independently of the vertebral column and 

 before it extended the whole length of the animal. The ser- 

 pent might be taken as a type in which the skull might have 

 originated as a natural consecutive part of the vertebral sys- 

 tem ; while for the third type we might instance fishes like 

 the sturgeon or animals like the Chelonians, where the brain 

 is first sheathed in homogeneous cartilage which may have 

 been formed independently of the vertebral system, then this 

 is covered with osseous plates, which reproduce with some 

 modifications the vertebral elements. 



Thus there must always be a conflict between potential 

 energy, in organization, leading to uniformity and simplicity, 

 and kinetic energy, leading to variety ; and the longer any 

 type endures in time, the more closely its cerebral region will 

 approximate to the vertebral structure, so far as the grouping 

 of the bones is concerned,* thus in the human subject the 

 structure of the brain-case is more simple and the segments 

 are better marked than is the case with fishes ; so that a 

 theory of the skull will depend upon the organization of the 

 animal, which determines the relative influence of kinetic and 

 potential ossification. 



The human brain-case, being almost entirely a potential 

 ossification, is one of the simplest. It consists of some (three) 

 bones at the base, in the median line, called in sequence basi- 

 occipital, basisphenoid, and presphenoid, the basisphenoid 

 and presphenoid in the adult being united together as one 

 bone. The basioccipital immediately follows the centrum of 

 a vertebra ; and these bones are to the skull what the centrums 

 would be to three segments of the vertebral column. On each 

 side of this row of skull-bones are placed three other bones (a 

 side bone to each base-bone), which rise up to embrace the 



