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 vertebree. 
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 vertebra, 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 vertebree 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 vertebre are or are not modified into skulls, and whe- 
ther, if so modified, the segments of skulls can in any rational 
sense be called vertebre. 
Now vertebre consist of several ossifications, 7.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 
