EVOLUTION AND PALEONTOLOGY 251 



Old Red Sandstone was originally given, have many fresh-water 

 characteristics. It does not follow, however, that all the for- 

 mations were fresh water. Pteraspis and Cephalaspis occur in 

 the Lower Old Red in Perthshire ; considering that no shells 

 or corals have ever been found in the Old Red Sandstone of 

 Scotland, it would seem from this evidence that the Ostraco- 

 derms were fluviatile. They may of course have lived also on 

 the sea-coast, or if estuarine their bodies may have been some- 

 times carried down to the sea and this may be the explanation 

 of their occurrence in the marine deposits mentioned above. 

 Higher than the Devonian they do not occur, and we have no 

 evidence that any more recent forms are descended from them. 

 They appear to have become extinct, leaving no descendants. 



With regard to the other principal types of fishes, we may 

 postpone the special consideration of the Teleosteans, which 

 we know to have arisen by gradual transformation of the 

 original Ganoids in the Cretaceous period, but of the four chief 

 divisions of the fishes proper, those provided with jaws, namely 

 Sharks, Chimaeroids, Lung-fishes, and Teleostomes, we find 

 that all were in evidence contemporaneously in the earliest 

 period of which we have any record. All occurred in the 

 Devonian. Thus there is only a single group of importance 

 which has been evolved since the beginning of the geological 

 record, namely the Teleostei. During the same time all the 

 widely divergent types which inhabit the land have arisen, 

 Amphibia from Dipnoans, Reptiles from Amphibia, and Birds 

 and Mammals from Reptiles. Evolution is thus the history 

 of the invasion of new media: the watery medium was already 

 occupied at the beginning ; amphibia partly, reptiles entirely 

 acquired the power of living on dry land, flying reptiles invaded 

 the air, and that sphere was afterwards more fully occupied by 

 the birds, while the mammals again, having like the birds be- 

 come independent of external variations of temperature made 

 a more complete conquest of the land. 



That fishes of the shark type existed in the Upper Silurian 

 is proved by the so-called ichthydorulites, or fish-spines of 

 the Upper Ludlow bed. 



Similar spines of Elasmobranchs occur in the Lower Old 

 Red Sandstone of Scotland ; but few complete skeletons or 

 remains showing the shape and character of the entire fish 



