THE ANCESTRY OF VERTEBRATES 



convert those who deny a priori the possibility of solving 

 a problem like that of the derivation of Vertebrates from 

 Invertebrates How wide once seemed the gulf between 

 Cryptogams and Phanerogams. Yet the bridge between them 

 has been found. 



Ascidians. — The first step towards the solution of the 

 problem of the origin of Vertebrates seemed to have been 

 done with KOWALEWSKY's (1866) researches on the develop- 

 ment of the Ascidians which revealed such remarkable 

 points of agreement between these animals, formerly con- 

 sidered as Invertebrates, and the Chordates. Pretty universally 

 the view was gaining ground that in the Ascidians we have 

 to look for the root of the Vertebrate genealogical tree. 

 Yet, though in recent days it has found again an advocate 

 in Brooks (1893), this first step soon proved to take 

 us no further than so m.any a mighty offensive in the 

 present European war carried those who undertook them ^). 

 There still remained an unbridged gap between Ascidians 

 and Invertebrates. 



Segmented Invertebrates. — While according to the above 

 conception the metameric structure of the Vertebrates could 

 have nothing to do with that of certain groups of Inverte- 

 brates, others especially laid stress on this point of agree- 

 ment between Vertebrates on the one side and Arthropods 

 and Annelids on the other. It is a well known fact that 

 already in the beginning of the nineteenth century Geoffroy 

 St-Hilaire (1822, p. 11) compared the Vertebrates to 

 Arthropods walking upside down, as was necessary to assume 

 in order to render possible the identification of the central 

 nervous system in both. The same idea was carried on by 

 LeydiG (1864, p. 185) who compared the supra- and infra- 

 oesophageal ganglia of Arthropods to a brain of Vertebrates 

 pierced by the gut between the crura cerebri. 



Annelids. — In the same year, but independently, SEMPER 

 and DOHRN (1875) then transferred the comparison from 

 Arthropods to Annelids, no doubt an important step forwards. 

 Thus was born the celebrated theory of the Annelidan af- 

 finity of Vertebrates, which for a long time has occupied 

 a dominating place in zoological work. DOHRN was led to 

 it by general considerations, SEMPER by the discovery that 



^) This book, though published later, has been written during 

 the great European war. 



