242 BONY FISHES vm. 6 



*Cleithrolepis\ Jurassic *Microdon and *Dapedius, and in some modern 

 teleosteans such as parrot fishes (Scaridae) and butterfly fishes 

 (Chaetodontidae). This is very valuable evidence of the way in which a 

 common stimulus can work on genetical constitutions that are simi- 

 lar but not identical. In this example the stimulus is a particular set of 

 environmental conditions; in other cases a similar effect may be pro- 

 duced by the stimulus of competition between animals, which was 

 probably the 'cause' of the common changes that affected so many 

 descendants of the palaeoniscids. 



The history of these fishes therefore gives plausible ground for the 

 belief that the driving 'forces' that have produced evolutionary change 

 are the tendencies of living things to do three things: (i) to survive 

 and maintain themselves, (2) to grow and reproduce, (3) to vary from 

 their ancestors, all of these operating under the further stress of any 

 slow change in the environment. 



Finally we must consider whether this change in the fishes can in 

 any way be considered to be an advance. Several times we have found 

 ourselves implying that this is so, that the later teleosts are 'higher' 

 than their Devonian ancestors. We shall be wise to suspect this judge- 

 ment as a glorification of the present of which we are part. However, 

 perhaps this danger is less marked when we are dealing with fishes not 

 ancestral to ourselves, whose 'advance' does not therefore bring them 

 nearer to man. The judgement can be put into quite specific terms: 

 the later Actinopterygii are 'higher' than the earlier ones because they 

 are more mobile, quicker, and can live free in the water with lesser 

 expenditure of energy than their ancestors. Unfortunately we have 

 no means of estimating the total amount or biomass of fish matter 

 that is supported by the teleostean organization, but it seems possible 

 that it is absolutely greater than that of any previous type, say the 

 holostean or palaeoniscid. If this is true, the change in plan of struc- 

 ture has perhaps led to an increase not only in fish biomass but in the 

 total biomass of all life in the sea. 



The teleostean plan has certainly allowed for the development of a 

 great range of specializations, fitting the animals to all sorts of situa- 

 tions in the sea and fresh water. We must therefore not forget this 

 adaptability in judging the status of the group: it seems likely that 

 modern teleosts are more varied than any of their ancestors. This 

 power to enter a wide range of habitats not previously occupied is 

 perhaps the clearest sign of all that a group has 'advanced', and we 

 have already suggested that it is in this sense of suiting animals to 

 new modes of life that there has been a progress in evolution. It is 



