ralaontolotjij to Biology. 3115 



also passed away since it was reco<2;iiized tliat within each 

 gronj) the lowest or most generalized members appeared 

 earliest, tiie highest, most specialized, or most degenerate 

 towards the end of the race. Modern research is concerned 

 only with the details of this succession and with the laws 

 which can now be deduced from the rapidly multiplying 

 available facts. 



Our present knowledge of the geological succession of the 

 fishes may be briefly summarized to show how Palaeontology 

 contributes to the solution of the fundamental ])roblem3 of 

 Biology. The earliest recognizable fish-like organisms, which 

 occur in Upper Hilurian formations, seem to have been mere 

 grovellers in the mud of shallow seas, nearly all with incom- 

 j)letely formed jaws and no paired fins, devoting most of their 

 growth-energy to the production of an efliective armour by the 

 fusion of dermal tubercles into plates (^Ostracodenni). W\\\\ 

 them were a 'icw true fishes which had completed jaws, but 

 "which possessed a pair of lateral fin-folds, variously sub- 

 divided, instead ot the ordinary two pairs ot fins {Diplacantk 

 Acanthodii). The main features of Silurian fish-lite were, 

 therefore, the acquisition of dermal armour, definite jaws, and 

 the begiTuiing of paired fins. Some of the lowly tyjjes thus 

 equipped survived and further evolved in the Devonian 

 })eriod ; but the multitude of new-comers which then formed 

 the majority were much higher in the scale of being [Crosso- 

 pterygii). They were still adapted for the most part to live 

 on the bottom of shallow water or in marshes, but they were 

 typical well-formed fishes in respect to their jaws, branchial 

 a))paratus, and two pairs of fins. Nearly all their bones were 

 external, very little of tiieir internal skeleton being ossified, 

 and the only changes they seem to have been undergoing 

 related to the fusion of some of the head-bones and the more 

 exact adaptation of their fins and tail to their enviroimient. 

 lishes more fitted for sustained swimming were also be- 

 ginning to appear, and these i^Pcd ceo nisei dee) formed the 

 large majority in the succeeding Carboniferous and Permian 

 jieriods. They were about equivalent in grade to the modern 

 sturgeons, and the tendency towards change in their structure 

 was in the direction of effective swimming, by the more 

 intimate correlation between the fin-rays and their supports 

 and by the shortening of the upper lobe of the tail. They 

 still exhibited scarcely any ossification of the internal skeleton. 

 As soon as the best type of balancing fin and the most 

 eftective type of propelling tail-fin had become universal 

 among the liighest fish-life of the Triassic period the internal 

 skeleton began to ossify and vertebral centra arose. In fact, 



