THE OLDEST AIR-BREATHERS 305 



evidences of transition from the gills of the Prosobranchiate 

 and the Crustacean to the air sac of the Pulmonate and the 

 tracheae of the millipede. It is also to be observed that many 

 other structural changes are involved, the aggregate of which 

 makes a Pulmonate or a millipede different in every particular 

 from its nearest allies among gill-bearing Gasteropods or 

 Crustaceans. 



It may be said, however, that the links of connection be- 

 tween the coal reptiles and fishes are better established. All 

 the known coal reptiles have leanings to the fishes in certain 

 characters ; and in some, as in Archegosaurus, these are very 

 close. Still the interval to be bridged over is wide, and the 

 differences are by no means those which we should expect. 

 Were the problem given to convert a ganoid fish into an 

 Archegosaurus or Dendrerpeton, we should be disposed to 

 retain unchanged such characters as would be suited to the 

 new habits of the creature, and to change only those directly 

 related to the objects in view. We should probably give little 

 attention to differences in the arrangement of skull bones, in 

 the parts of the vertebrae, in the external clothing, in the micro- 

 scopic structure of the bone, and other peculiarities for serving 

 similar purposes by organs on a different plan, which are so 

 conspicuous so soon as we pass from the fish to the Batrachian. 

 It is not, in short, an improvement of the organs of the fish that 

 we witness so much as the introduction of new organs. 1 The 

 foot of the batrachian bears, perhaps, as close a relation to the 

 fin of the fish as the screw of one steamship to the paddle 

 wheel of another, or as the latter to a carriage wheel ; and can 

 be just as rationally supposed to be not a new instrument, but 

 the old one changed. In this connection even a footprint in 

 the sand startles us as much as that of Friday did Robinson 



1 An ingenious attempt by Prof. Cope, to deduce the batrachian foot 

 from the fins of certain carboniferous fishes, will be found in the Proceed- 

 ings of the Philos, Academy of Philadelphia for the present year. 



