934 The American Naturalist. [November, 
general attention from zoologists for a quarter of a century 
past. The full and able defense of it by W. K. Brooks in his 
Salpa monograph is so recent and well-known that it will suf- 
fice to recall that the theory assumes that Appendicularia is 
near the ancestral pelagic type of the Chordata; that it gave 
rise to a form with multiple gill openings leading to Tunicates, 
and by the further acquisition of segmental structure leading 
to Amphioxus, and from this last on to the true Vertebrata. 
It must be carefully noted that by the Appendicularia theory 
the prime morphological importance and significance is attri- 
buted to the notochord and the branchial pharynx, and a 
secondary importance to the metamerism, while the Annelid 
theory reverses this and attributes prime importance to the 
metamerism, and secondary to the notochard and pharynx. 
E. The Annelid theory was founded independently by Dohrn 
and Semper, and has always been supported in my opinion by 
far more numerous and sounder arguments than any other 
theory of Vertebrate descent, with possibly the exception of 
the Appendicularia theory. The fullest presentation known 
to me, of the Annelid theory, is that by Hugo Eisig in his 
monograph of the Capitellide. Those, who have had theories 
of their own to defend, have never attempted (ought we not 
say, have never dared tc attempt?) to confute the Annelid 
theory. Sedgwick, Bateson, Willey, Gaskell, Patten, Morgan 
and others have either left the subject alone, or at most, have 
spoken slurringly and in disparagement of it, so that there are 
literally no important direct criticisms against the theory 
known tome. I take this to signify that the writers in ques- 
tiod have attacked the problem of the ancestry of Vertebrates 
not in a judicial but in a somewhat partisan spirit. 
4. APP ARLE VS. ANNELIDA AS THE ANCESTORS OF 
THE CHORDATA. 
The most essential difference between the two theories under 
consideration is formulated above in the paragraph on the 
Appendicularia theory. The alternative of choice between 
the two theories hinges upon the interpretation of Amphioxus 
which we accept, i. e.: 
