Beviews and Notices of Books. 



139 



so far as we know, introduced so early. But negative evidence 

 must not be relied on ; the record is very imperfect, and such 

 creatures may have existed though unknowai to us. It may be 

 answered that they could not have existed in any considerable 

 numbers, else some of their shells would have appeared in the 

 coal formation beds, so rich in crustaceans and bivalve moUusks, 

 Further, the little Pupa remained unchanged during a very long 

 time, and shows no tendency to resolve itself into any thing higher 

 or to descend to any thing lower. Here, if anywhere, in what 

 appears to be the first introduction of air-breathing invertebrates, 

 we should be able to find the 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 gilh bearing gas- 

 teropods or crustaceans. 



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

 the coal reptiles and the fishes are better established. All the 

 known coal reptiles have leanings to the fishes in certain cha- 

 racters, and in some, as in Archegosaurus, these are very close. 

 Still the interval to be bridged over is wdde, and the differences 

 are by no means those wdiich we should expect. Were the pro- 

 blem given to convert a ganoid fish into an Archegosaurus or Den- 

 drerpeton, we should be disposed to retain unchanged such cha- 

 racters 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 arrange- 

 ment of skull bones, in the parts of the vertebrse, in the external 

 clothing, in the microscopic 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. The foot of the batrachian bears perhaps as close a 

 relation to the fin of the fish as the screw of one steam-ship to 

 the paddle wheel of another, or*as the latter to a carnage wheel, 

 and can be just as rationally supposed to be not a new in- 

 strument but the old one changed. 



" Again, our reptiles of the coal do not constitute a continuous 

 series, nor is it possible that they can all, except at widely different 

 times, have originated from the same source. To suppose that 

 Hylonomus grew out of Bendrerpeton or Baphetes^ and Eosaurus 

 out of either, startles us almost as much as to suppose that Ba- 

 phetes grew out of Rhizodus^ or Hylonomus out of Baloeoniscus . 

 It either happened, for some unknown reason, that many kinds of 



