172 Origin of the Paired Limbs of Vertebrates 
a dozen papers dealing more or less directly with its various phases 
appearing since 1900. 
The work of the gill-arch theorists has been confined chiefly to drawing 
comparisons between the skeleton of the paired limbs and that of the 
gills, to studying the possibilities of migration in the paired fins, and to 
searching for connecting stages between the branchial and_ pectoral 
regions. But the study of the unpaired fins they have almost ignored. 
The studies of the fin-fold theorists, on the other hand, have been particu- 
larly directed toward the similarities between the median and paired fins 
and have aimed to prove the local and metameric development of the 
paired hmbs. ‘To the fin-fold theory the chief obstacle has been the fact 
that the paired limbs possess girdles while the median ones do not, a 
difficulty which the adherents of the gill-arch theory have considered in- 
surmountable. For the latter theory the most important stumbling- 
blocks have been: the lack of any intermediate stages between gill and 
fin, the absence of proof of migration sufficient to account for the posi- 
tion of the pelvic fins, and the difference in position between the gill- 
arch in the gut wall and the fin girdles in the body-wall. 
The various fields of anatomy, paleontology, and embryology have all 
been gleaned by various investigators for evidence to bear upon the 
problem and all have proved fruitful, though lack of more complete 
knowledge has at times caused apparent conflicts. The anatomical side 
of the question has been most thoroughly worked and our knowledge of 
the facts may be said to be fairly complete. Unfortunately, however, 
anatomical evidence is not the most conclusive in the study of phyloge- 
netic questions, and in the structures of the fins the adherents of both 
theories find what they consider good evidence in favor of their views. 
Paleontological data would obviously be of inestimable value in de- 
ciding doubtful points, if not the entire problem, but the evidence from 
fossils may always remain fragmentary and even conflicting. The 
Cladodont sharks are without doubt the oldest fossil forms which give 
us any light on the structure of the limbs, and they agree in possessing 
a fin-fold type of fin. Cladoselache, from the Devonian, is particularly 
enlightening in this respect, but the gill-arch theorists have consigned 
Cladoselache to the limbo of an aberrant group (Braus, 04). On the 
other hand, the Permian Pleuracanthus, while regarded by the gill-arch 
theorists as convincing evidence on their side of the question, is rejected 
by the adherents of the fin-fold theory. There seems no absolute ana- 
tomical reason why the pectoral fin of the modern shark might not have 
been derived (by different methods, to be sure) from either the biserial 
