CHAPTER VIII. 



THE ORIGIN OF THE CHORDATA, CONSIDERED IN ITS RELATION TO 



PELAGIC INFLUENCES. 



SECTION 1. The Ancestral Chordata. 



I shall now attempt to study the origin and significance of the 

 structure of appendicularia in accordance with those conditions which 

 must, as Dohrn has pointed out (Studien, etc., VIII, p. 79), direct all 

 inquiry into the genealogy of animals. 



All biologists will agree with Dohrn that no amount of morphological 

 information, or of exhaustive microscopical study of the structure and 

 development of animals, can suffice, in the absence of comprehensive 

 knowledge of their mode of life and of the conditions of their existence, 

 for the institution of inquiries into their phylogenetic relationship. 



Unquestionably the first condition for genealogical inquiry is, as 

 Dohrn says, the establishment of a direct connection between our mor- 

 phological studies and the facts of physiology and biology. 



"The homologies which are established by comparative anatomy, 

 and the primititive identities which are established by comparative 

 embryology, are only the means for this end. They are in themselves 

 valuable in phylogenetic inquiry only so far as they furnish us the 

 opportunity to pass from the consideration of the structure of organs as 

 they now exist, and of the functions of these organs at the present time, 

 to the consideration of conditions which have passed away ; to the study 

 of the history of the modifications which have come between these struc- 

 tures and functions and those which we must attribute to the same 

 organs at an earlier genealogical stage." 



Keeping these conditions of genealogical inquiry in view, let us try 

 to study the structure of appendicularia in relation to the conditions of 

 its life, so far as these are known to us, and let us see what functions 

 we must, according to the principle of change of function, attribute to 

 the organs of the remote ancestors of the tunicates, and what are the 

 paths these organs have traversed in reaching their modern structure. 



If the reader of the following pages should think that I wander too 



