340 EDWIN S. GOODRICEI. 



region ; but as they pass inwards to become attached to the 

 base, or the girdle, the muscles no longer preserve the " con- 

 cordance." On the other hand, nothing is so striking on 

 examining sections through the developing fins of Elasmo- 

 branchs, whether paired or unpaired, as the extraordinarily 

 regular " concordance " ; it is obvious on the very first appear- 

 ance of the procartilagiuous radial (p. 358, figs. 5, 8, 9, 18). 



These attempts to undermine the lateral fold theory, by 

 showing that the adult muscles are compound and polyneurous, 

 and that the concordance is secondary, are not borne out by 

 the evidence. Moreover, even if it could be proved that the 

 metamerism of the fin elements has been lost, the lateral fold 

 theory would scarcely be affected, since it only claims that 

 the muscles and skeletal radials formed a longitudinal series 

 of metameric origin in the beginning. No one doubts that 

 the metamerism has been obscured, or lost, in the higher 

 vertebrates; it matters little, theoretically, whether it still 

 persists in modern fish. 



The Gill- Arch Theory. 



Let us now pass to the rival theory. It is claimed that the 

 initial stages in the phylogenetic history of the paired fins 

 are more easily accounted for on the gill-arch theory of 

 their origin. Now, according to the lateral fold theory 

 the paired fins appeared, as they do in ontogeny, as longi- 

 tudinal ridges, which, from their very first appearance, may 

 have been useful as balancing and directing organs. Even 

 in modern fish the paired fins are used not so much for pro- 

 gression as for guidance and balancing. 



On Gogenbaur's theory the direction of the paired fins must 

 at first have been dorso-ventral across the long axis of the 

 body ; such folds would probably be a hindrance to progres- 

 sion, and both the pectoral and pelvic fins would have been 

 placed close together behind the head in a most unfavourable 

 situation. 



The position of the pelvic fin is accounted for by supposing 



