238 BONY FISHES vm. 6 



process of evolution than has been possible from consideration of the 

 more ancient and less perfectly known groups of fishes. First of all 

 we may emphasize the persistence of change. No actinopterygian 

 fish living in the Devonian is to be found today. Polypterns may be 

 regarded as a living palaeoniscid, showing features that were common 

 in the Carboniferous, but it has undergone many changes since that 

 time. Similarly the living sturgeons show the stage of organization 

 present in the Jurassic Chondrostei, but they also have changed much. 

 Lepisostens is in general structure similar to a Triassic holostean such 

 as *Lepidotes and Amia to a Jurassic one such as *Caturus, but both 

 have their own more recent specializations. It is not until we come to 

 the Tertiary history of fishes that types belonging to recognizable 

 modern genera can be found. 



The Actmopterygii have therefore been changing slowly, but con- 

 tinuously, throughout the period of their existence. Was this change 

 dictated in some way by a change in their surroundings? Unfortu- 

 nately we cannot answer this question very clearly. The sea is a 

 relatively constant medium, though not as 'unchanging' as is some- 

 times supposed. In particular the relative extent of sea and fresh- 

 water changes frequently and perhaps because of such a change the 

 early freshwater actinopterygians took to the sea. Probably the life 

 in the sea is not constant over long periods. Almost certainly the 

 available nitrogen, phosphorus, and other essential elements change 

 in amount. We have reason to suspect that there was an increase in 

 the extent and productivity of the sea during the Triassic period. It 

 may be that such gradual changes in the life in the water, depending 

 ultimately on climate or inorganic changes, have been responsible for 

 the continual change of the fish population. However, it cannot be 

 said that we can detect evidence of any such relationship ; there is no 

 clear proof that the changes in the fish populations follow changes in 

 the environment. For the present we can only note the fact that change 

 occurs, even in animals living in the relatively constant sea. 



Very striking is the fact that as evolution proceeds not merely does 

 each genus change but whole types disappear and are replaced by 

 others. Thus the palaeoniscid type of organization had disappeared 

 almost completely by the Trias and become replaced by the holostean. 

 A few members retained the old organization and still survive today 

 as Polypterus and the sturgeons. Similarly the Holostei, with their 

 abbreviated heterocercal tails, hardly survived into the Tertiary, but 

 were replaced by the Teleostei, only Lepisosteas and Amia remaining 

 to show the earlier organization. 



