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THE AMERICAN NATURALIST [Vol. XL 



be admitted that the gap between amphioxus and Bdellostoma 

 is very great, both as regards time and the amount of the trans- 

 formation of structure which it has undergone, and that in many 

 things we cannot yet satisfactorily explain the manner in which 

 this transformation has come about. 



On the other hand, we may with certainty say nearly as much 

 about the gap between the Cyclostomes and the other Gnatho- 

 stomes, but the absence of complete knowledge has never been 

 permitted to blind unprejudiced minds to the just estimate of 

 the known facts in any zoological problem. 



Recent advances in our knowledge of the Marsipobranchs 

 render Haeckel's estimate of the amount of difference between 

 the Cyclostomes and vertebrates above them quite untenable. 



He says {Syst. Phylog., vol. 3): "Eine tiefe morphologische 

 Kluft und ein entsprechend 1 anger historischer Zwischenraum 

 trennt beide Classen der Agnathonen nicht bloss von den echten 

 Fischen, sondern auch von alien iibrigen Vertebraten die wir in 

 der Hauptgruppe der Amphirhinen oder Gnathostomen zusam- 

 menfassen." 



There is no reason why we should, at the present time, follow 

 partisans of any genealogical theory of the origin of the verte- 

 brates in ignoring the many anatomical and embryological facts 

 which we now possess and which clearly enough establish the 

 genetic relations of tlie.sc fonns. 



Some writers are too pn.nf to a->iinie the existence of large 

 groups of extinct forms iiitcriiicdiate l)etween the groups of exist- 

 ing vertebrates and between these and those forms from which 

 the vertebrates have descended — which have disappeared with- 

 out leaving any trace of their structure in recent forms. 



There is neither anatomical nor embryological ground for the 

 removal of amphioxus from the vertebrate class, and we may 

 express our view of the relation of existing forms thus:— 



Amphioxus Lcptocanlia - Acraiiia 

 All others — Pachycardia — Craniata 

 and by the following table: — 



