ORIGIN OF VERTEBRATES. 347 



(6) Segmental Symmetry. — Like Annelids and Arthropods, 

 Vertebrates are segmented animals, as is evidenced by the 

 distribution of nerves and ganglia, by the gill-slits and their 

 supporting arches, by the muscle-segments of the embryo at 

 least, by the vertebral bodies and skull-segments, and by the 

 arrangement of the nephridia which compose the embryonic 

 kidney. There is, however, no very distinct segmentation 

 in Balanoglossus, nor in Tunicates. The precise meaning 

 of segmentation is uncertain. It secures reproduction in 

 some worms, etc., by the liberation of posterior segments ; 

 in other cases, it probably increases the co-ordinating power, 

 and it may make the reparation of injuries more possible. 

 It seems to depend on conditions of growth, of which we 

 have very little understanding. Segmentation occurs in so 

 many ways and in so many classes, that it is likely that it 

 originated repeatedly and independently at different stages 

 in the evolution of animals. 



Origin of Vertebrates. — It is not at present possible to trace the path 

 along which Vertebrates have evolved. There are several rival theories, 

 but it is not certain that even any one of them need be correct. Only 

 very sanguine people talk about the ancestor of all the Vertebrates, as 

 if he would be almost certainly discovered next year, but except those 

 who are easily satisfied, and those who have staked heavily on one 

 theory, all acknowledge that the pedigree of Vertebrates is still obscure. 

 It seems possible that among various sets of Invertebrates, there were 

 convergences towards' Vertebrate structure, and perhaps it is better to 

 regard these not as inconsistent but simply as manifold affinities of which 

 one or none may represent the line of descent. 



I take it as proved that ^mphioxus is a Vertebrate, that the Tunicata, 

 especially in their youth, are Vertebrates, and that Balanoglossus has at 

 least more remarkable affinities with Vertebrates than have the other 

 claimants, viz. , Annelids, Nemerteans, and perhaps even some Arthro- 

 pods. In regard to the relative simplicity of Tunicates and Amphioxus, 

 it must be remembered that degeneration seems to have been by no 

 means uncommon in the history of animals. There is no doubt that the 

 Tunicates are victims of degeneration, for few of them fulfil the promise 

 of their youth, and some authorities believe that Amphioxus is also a 

 degenerate form, " a weed in the vertebrate garden." 



In accounting for the origin of a Vertebrate from an Invertebrate 

 type, one of the greatest difficulties to be faced is, that the nerve-cord is 

 dorsal in the former, while it is ventral, or at least never dorsal in the 

 latter.- The suggestion naturally occurs that a Vertebrate may corres- 

 pond to an inverted Invertebrate, to a worm wallowing back downwards, 

 to an insect swimming feet upwards. De Blainville and Geoffroy St. 

 Hilaire suggested this possibiUty early in the century, and it has never 

 since been far from the thoughts of speculative morphologists. 



