Note on the Pectoral Fin of the Sole, Achirus capensis. 109 
the tendency in elongate forms for the pectorals to disappear. High speed 
is usually associated with some means of controlling it. 
The checking of forward progress in the primitive limbless vertebrates 
could be effected by the only available structures, the transverse opercular 
folds, and it may again be noted that the Cyclostomes which have no such 
transverse folds or opercula have apparently never advanced to the paired 
limb stage. 
(4) It may be objected that there is no evidence of the free opercular 
part of the gill septa becoming separated from the more internal part with 
its supporting skeleton. The evidence against that part of Gegenbaur's 
theory dealing with the origin of girdles and radials from gill arches 
and rays is in part a reply to this, and need not be repeated. It has 
carried so much weight that his whole hypothesis of the origin of 
paired limbs (from the vertical opercular folds as w T ell as the gill arches) is 
generally viewed with less favour than the lateral fold hypothesis. His 
theory might still be partly true, even though the girdles be derived 
from some one of the other sources which have been suggested — ribs and 
other organs. (The extra-branchial cartilages, which are well developed in 
some present-day sharks (e. g. Odontaspis) and form the main supports for 
the gill flaps, are possible sources of the girdles, more especially as they are 
situated in the somatopleure like the girdles, and not in the splanchnopleure 
like the visceral arches.) To the evidence already adduced of the possible 
persistence of the opercula, when gills, gill-clefts and the rest of the gill 
septum have disappeared, may be added that the operculum of living Teleostei 
may find other or additional supports, as, for instance, in the Scleroparsi, and 
that in the many cases of reduction of gill clefts the gill arch may be as 
greatly reduced as the gill cleft. 
It might be suggested that the fact that the pectoral fin can become an 
epidermal fold supports the suggestion that an opercular fold may have 
become a pectoral fin, as this is merely a reversal of the process. This 
argument might have had more weight at one time, but recent investigations 
seem to indicate that such reversals do not occur in the evolution of animal 
life. This has been inferred from the fact that no clear cases are known, 
but this inference does not, however, imply the existence of any law, and 
cannot be advanced as an argument against the above suggestion, if indeed 
the particular case of the Achirus may not point to the possibility of such 
reversal, at least in measure. The argument, however, is not a good one. 
It might be used also to indicate that the opercular fold disconnected from 
the gill septum, for in the case of the Achirus the fold has lost its attachment 
by radials to the shoulder girdle. It might be used also in favour of 
another suggestion, that paired fins have arisen from median fins, for in 
several soles the pelvic fin of one side may assume a median position and 
become continuous with the anal. 
