viii. 6 EVOLUTIONARY CHANGES 241 



certain specified ways and even at a specified rate. Imperfect though 

 our knowledge still is, it enables us to approach towards the aim of our 

 study, to 'have in mind' all the fishes of actinopterygian type. 



This is to make the most of our knowledge: there remains a vast 

 ignorance. We cannot certainly correlate this tendency of the fishes 

 to change with any other natural phenomena. Put in another way, 

 we do not know why these changes have occurred. The sea certainly 

 did not stay the same, but it does not seem likely that its changes have 

 been responsible for those in the fishes. It would be very valuable to 

 be able to make a more certain pronouncement on this point, for the 

 case is one of crucial importance. On land the conditions are con- 

 stantly changing, and therefore we often find reason to suspect that 

 changes in the animals are following environmental changes. But 

 can this be so in the water? 



The evolutionary changes in the Actinopterygii certainly involve a 

 definite difference in the whole life. By development of the air- 

 bladder as a hydrostatic organ the animals have become able to 

 remain at rest at any level of the water, and thus, by suitable modifica- 

 tion of the shape of the body and fins, to dash about with remarkable 

 agility in pursuit of prey or avoidance of enemies. This has enabled 

 them to dispense with the heavy armour and thus further to increase 

 their mobility. But what made it necessary to adopt these changes? 

 Not surely any actual change in the sea itself. We must look then for 

 some factor imposed on the situation by the fishes themselves or the 

 neighbouring animals that constituted their biotic environment. Is 

 it the pressure of competition that has been responsible for the change 

 in fish form ? It may well be that the presence of an excess of fishes 

 has led them continually to search for food more and more actively, 

 and in new places, with the result that those types showing the greatest 

 ability have survived. Given the initial genetic make-up of the 

 palaeoniscids, further agility is most easily acquired by those fishes 

 in which competition tended to produce shorter tails, thinner scales, 

 and the other characteristics towards which the animals of this group 

 tend. 



The fact that the same set of changes can be produced independently 

 from several different populations of approximately similar type 

 (and presumably genetic composition) is strikingly shown by the 

 specialized creatures evolved for life in coral reefs. Animals with 

 rounded bodies and small mouths, sometimes with grinding teeth, 

 have appeared independently several times; in the Carboniferous, 

 *Amphicentrnm\ Permian, *Platysomus and *I)oryptents; Triassic 



