PEOPLING OF LAND AND SEA 175 



Saccobranchs and Amphipiwits. The organs we call lungs in 

 the Dipnoid Fish differ in no way from these pouches in their 

 vascular qualities. They are themselves exactly equivalent to 

 the lungs of the Batrachian, which are provided in their early 

 stages, and sometimes throughout their whole life, with 

 external branchiae. There is certainly no genealogical relation- 

 ship between the Fish of the Silurid family and the Batrachians ; 

 it has not even been definitely established that the latter are 

 directly descended from the Dipnoi. If, however, we admit 

 a principle which has been so frequently demonstrated, namely 

 the same mechanisms acting on organisms of the same fundamental 

 constitution produce the same effects, then the arrangements we 

 have just compared permit us to assume that the Batrachians 

 owe their external branchiae and their lungs to the fact that their 

 ancestors had for a long time lived in waters frequently 

 polluted, i.e. in swamps or muddy rivers, as the Dipnoi 

 certainly did. The principle just invoked, moreover, is 

 the same that has brought about those resemblances, due to 

 causes other than heredity, which are found among different 

 animals, and which have recently been called phenomena of 

 convergence — a term far less exact than Isidore Geoffroy Saint- 

 Hilaire's expression parallelism. 



Thenceforth the Vertebrates were provided with an apparatus 

 permitting them to brave the danger of desiccation, and to 

 breathe in the open air, but they could not move over the 

 ground by means of their fins ; they needed feet. How did 

 feet develop from fins, which they most certainly replaced ? 

 For can it be doubted that the amphibious Batrachians are 

 descended from Fish, and form the link uniting them with the 

 first definitely terrestrial Vertebrates, the Reptiles ? Here 

 we remain in the dark, but still we must know the reason why. 

 There are some Fish which walk with the aid of their fins, 

 but, unfortunately, these Fish are very different from the 

 primitive forms, and their comparatively recent attempts at 

 walking are far from perfect. Indeed, their fins are so little 

 adapted to walking that Anabas, which has special 

 arrangements in its branchial chamber permitting it 

 to live for a certain time in the open air, prefers, when climbing 

 trees, to use the spines of its operculum and the rays of its 

 caudal fin rather than its pectoral fins. However, the 

 Pteriophthalmidae, which live more out of water than in it, do 



