112 



Biology in America 



from head to tail and ventrally as far as the anus, where it 

 divided to pass forward along the sides as a pair of folds 

 to the head. Such a condition we actually find in Amphi- 

 oxus, wh^e the fossil shark Cladoselache possessed paired 

 fins placed exactly as we should expect them to be had they 

 been derived fiom such hypothetical fin folds. In the 

 Japanese goldfish the anal fins, which are ordinarily single, 

 are paired, as would happen if the paired folds extended 

 further back than usual. 



These folds are supposed to have acted as balancing organs 

 originally, but later they became strengthened at their 

 anterior and posterior ends while their middle parts dropped 



(Above) A LUNG FISH 



From Pirsson and Schuchert 's Geology, by permission of John Wiley 

 & Sons. 



(Below) CLADOSELACHE 



A fossil shark, whose paired fins give evidence of the origin of these 

 structures from a pair of continuous fin folds. From Dean's " Fishes, 

 Living and Fossil, ' ' by permission of the Macmillan Company. 



out and the parts remaining were modified to form the paired 

 fins of the modern fish. 



The origin of the vertebrate limb is shrouded in the mists 

 of the past. Whence it came, and how, we may never know ; 

 for there are as -'vet no links to connect the fin of the fish 

 with the limb of the amphibians. True it is that the impress 

 of a foot has been found in Pennsylvania in sandstone rocks 

 of the Devonian period, when fishes were the dominant types 

 of life; which is supposed to have been made by some lowly 

 ancestor of the amphibians. This represents only the foot 

 however and tells us little or nothing as to the origin of the 

 limb as a whole. Various hypotheses have been advanced to 

 take the place of facts, but about the most that can be said 

 for any of them is that they are hypotheses, and one is per- 



