AIR-BREATHERS OF THE COAL PERIOD. 293 



the interval to be bridged over is wide and the differences 

 are by no means those which we should expect. Were the prob- 

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

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

 acters 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 vertebrae, 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 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. 



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 

 Hylnnomus grew out of Dendrerpeton or Baphetes, and Eosaurus 

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

 grew out of Rhizodus, or Hylotiomus out of JPalceoniscus. It 

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

 fishes put on the reptilian guise in the same period, or else the 

 vast lapse of ages required for the production of a reptile from 

 a fish, must be indefinitely increased for the production of many 

 dissimilar reptiles from each other ; or on the other hand we 

 must suppose that the limit between the fish and reptile being 

 once overpassed, a facility for comparatively rapid changes 

 became the property of the latter. Either supposition would, I 

 think, contradict such facts bearing on the subject as are known 

 to us. 



We commenced with supposing that the reptiles of the coal 

 might possibly be the first of their family, but it is evident from 

 the above considerations, that on the doctrine of natural selection, 

 the number and variety of reptiles in this period would imply that 

 their predecessors in this form must have existed from a time 

 earlier than any in which even fishes are known to exist; so that 

 if we adopt any hypothesis of derivation, it would probably be 



