THE ORIGIN OF THE VERTEBRATES. 215 



THE ORIGIN OF THE VERTEBRATES. 



The question as to the ancestors of the vertebrates is one of 

 the most vexed problems of zoology. It has seemed at times 

 as if the solution were near at hand. The recognition of chor- 

 date affinities in the tunicates, and, later, in Balanoglossus, at 

 the time when these were regarded as invertebrates, raised 

 hopes that were disappointed when it was found that these 

 forms were chordates, and that only superficial resemblances 

 had caused their association with the non-vertebrate groups. It 

 would seem that to-day we are not much nearer the answer to 

 the question than we were when the theory of evolution was new. 



Apparently the problem must be solved, if solved it ever 

 be, upon the basis of comparative anatomy and embryology. 

 Paleontology has never thrown the slightest light upon the 

 matter, and it seems as if it never could, because it is more 

 than probable that the ancestral chordate was a soft-bodied 

 animal of small size, incapable of leaving any definite impress 

 in the rocks. 



The three most important characteristics of the vertebrates, 

 and of all chordates, are the presence of gill slits, the existence 

 of a notochord, and the occurrence of a central nervous system 

 placed entirely upon one side of the alimentary canal. These 

 features are found in no invertebrate, and we can only speculate 

 upon the way in which they have arisen ; for it is one of the 

 canons of evolution that no organ arises de novo, but only by 

 modification of some pre-existing structure. 



At present the greater weight of evidence, such as it is, 

 points toward an annelid ancestry. Annelids and vertebrates 

 agree in the possession of metamerism, and the homologies of 

 the metameric structures can be traced with some detail. Mus- 

 cular system, coelomic pouches, and nephridia agree in their 

 general features, while the fact that the nephridial ducts in both 



