THE ANCESTRY OF THE CHOBDATA. 67 



The Ancestry of the Chordata. 

 By 



fVilliani Bateson, M.A,, 



Fellow of St. John's College, Cambridge. 



The Ancestry of the Chordata. 



Preface. — lu view of the facts relating to the structure of- 

 the Enter opneusta which form the subject of the accom- 

 panying paper and of those which have preceded it, it seemed 

 necessary to attempt some analysis of their import and bearing 

 upon morphological problems, and especially upon the vexed 

 question of the ancestry of the Chordata. 



But at the outset it was impossible to attempt such an 

 analysis without first clearing the way by a discussion of the 

 morphologic meaning of Segmentation. Since the Enterop- 

 neusta are essentially " unsegmented " animals and the Verte- 

 brata are " segmented/' this preliminary discussion was neces- 

 sary. Moreover, having shown reason for not accepting the 

 view that the vertebrate segmentation was of such a kind as to 

 necessitate the existence of a series of segmented ancestors to 

 account for it, it became also necessary to treat the whole 

 question of the origin of segmentations of this class upon a 

 wider basis. This must be the apology for the introduction 

 into this paper of some matter and speculation not otherwise 

 immediately relevant to the subject. 



The decision that it would be profitable to analyse the 

 bearing of the new fact in the light of modern methods of 

 morphological criticism, does not in any way prejudge the 

 question as to the possible or even probable error in these 

 methods. 



Of late the attempt to arrange genealogical trees involving 



