346 Mr. H. G. Seeley on a Theory 



new. And hence, until an attempt is made to discover how far 

 these theories may be different ways of presenting the same 

 truth, and how far they fall short of being true, it will be im- 

 possible to compare the facts which they attempt to explain. 

 When one master examines the lowest form of Vertebrate life, 

 the cranium appears to be a continuation of the vertebral column; 

 when another master expounds the highest forms of life, it ap- 

 pears to be a distinct structure, and not to consist of vertebrae. 

 I do not propose to offer anything in this paper which shall be 

 in antagonism with either of these theories. 



But it is no less remarkable than patent that, although one 

 Professor has long battled to show that the skull is so many 

 vertebrae, and another Professor battles to show that it is not, 

 no one has discussed the nature of vertebrae, or considered whe- 

 ther it were possible to have a theory of the skull founded in 

 truth before making a theory of a vertebra. In all these specu- 

 lations a vertebra is regarded as a fundamental principle, as ele- 

 mentary as hydrogen or oxygen to the chemist; and though 

 Professor Owen has classified it into exogenous parts and auto- 

 genous parts, no attempt is made to show why it has these 

 parts ; and until this is done, I fail to see how it is possible to 

 effect any kind of comparison between a vertebra and a skull ; 

 for vertebrae from different animals and from different parts of 

 the same animal vary so much among themselves, that until the 

 principle of the law of variation as well as the law of persistence 

 in structures is known, it will be hard to say whether the ele- 

 ments of vertebrae are or are not modified into skulls, and whe- 

 ther, if so modified, the segments of skulls can in any rational 

 sense be called vertebrae. 



Now vertebrae consist of several ossifications, i. e. of bones 

 which in various degrees grow. This change and substitution 

 of structure is obviously due to force, and must either be of the 

 kind which assists the first development of seeds (in which 

 case it may perhaps at present fitly be called the embryonic or 

 developmental force), or it must be due to the mechanical force 

 of the atmosphere or water, or of one structure or function of 

 the animal modifying another. If it is found, from abstract 

 mechanical principles, that growth must take place under the 

 influence of certain mechanical forces, and if it is found, from 

 pathological observation, that growth does occur under these in- 

 fluences, then, should it be found in healthy structures that in- 

 tensity of growth varies with the intensity of the forces, it will 

 be proved that their action is a cause of normal growth. Then 

 it would be possible, from morphology, to show that the same 

 causes which developed the bones originally called them into 

 existence. And therefore, if it is found, in the development of an 



