THE STIAl ES OF VERTEBRATE SKELETONS. 197 



process. It is also true ttiat tlie actions and reactions de- ■ 

 scribed would not of themselves give to the median portion 



a cylindrical shape, like that of the cartilaginous rod run- 

 ning along the back of the A?)?phioxiis. But what we have 

 here to note in the first place is, that these habitual alternate 

 flexions have a tendency to mark ofi" from the outer parts an 

 unlike inner part, which may be seized hold of, main- 

 tained, and further modified, by natural selection, should 

 any advantage thereby result. And we have to note in the 

 second place, that an advantage is likely to result. The con- 

 tractions cannot be efiective in producing undulations, un- 

 less the general shape of the body is maintained. External 

 muscular fibres unopposed by an internal resist ent mass, 

 would cause collapse of the body. To meet the require- 

 ments there must be a means of maintaining longitudinal 

 rigidity without preventing bends from side to side ; and such 

 a means is presented by a structure initiated as described. 

 In brief, whether we have or have not the actual cause, we 

 have here at au}^ rate "a true cause." Though there are 

 difficulties in tracing out the process in a specific wnj, it may 

 at least be said that the mechanical genesis of this rudiment- 

 ary vertebrate axis is quite conceivable. And even the 

 difficulties may, I think, be much more fully met than 

 woidd at first sight seem possible. 



What is to be said of the other leading trait which the 

 simplest vertebrate animal has in common with all higher 

 vertebrate animals — the segmentation of its lateral mus* 



