SALPA IN RELATION TO EVOLUTION OF LIFE. 171 



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 morpho- 

 logical 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 con- 

 ditions 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 

 morphological studies and the facts of physiology and biology. 



" The homologies which are established by comparative anatomy, 

 and the primitive identities which are established by comparative 

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

 selves 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 structures 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 con- 

 ditions 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 tuni- 

 cates, and what are the paths these organs have traversed in reach- 

 ing their modern structure. 



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

 too far from the beaten paths of observation, I must plead as my 



