214: MORPHOLOGICAL DEVELOPMENT. 



a cylindrical shape, like that of the cartilaginous rod running 

 along the back of the Amphioxus. But what we have here 

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



flexions have a tendency to mark off 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 

 contractions cannot be effective in producing undulations, 

 unless the general shape of the body is maintained. External 

 muscular fibres unopposed by an internal resistant 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 any rate " a true cause." Though there are 

 difficulties in tracing out the process in a definite way, it 

 may at least be said that the mechanical genesis of this rudi- 

 mentary vertebrate axis is quite conceivable. And even the 

 difficulties may, I think, be more fully met than at first 

 sight seems 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 muscular 

 masses? Is this, too, explicable on the mechanical hypo- 

 thesis? Have we, in the alternating transverse strains, a 

 cause for the fact that while the rudimentary vertebrate axis 



