RELATIONS TO OTHER SCIENCES 559 



long been familiar in its general outlines. At least a century has 

 elapsed since it was made clear that the various organisms come into 

 existence at different times and in a definite order, according to their 

 grade in the scale of being, the lowest first, the highest latest. Several 

 decades have also passed away since it was recognized that within 

 each group the lowest or most generalized members appeared earliest, 

 the 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 paleontology contributes 

 to the solution of the fundamental problems of biology. The earliest 

 recognizable fish-like organisms, which occur in Upper Silurian 

 formations, seem to have been mere grovelers in the mud of shallow 

 seas, nearly all with incompletely formed jaws and no paired fins, 

 devoting most of their growth-energy to the production of an effective 

 armor by the fusion of dermal tubercles into plates (Ostracodermi). 

 With them were a few true fishes which had completed jaws, but 

 which possessed a pair of lateral fin-folds, variously subdivided, in- 

 stead of the ordinary two pairs of fins (Diplacanth Acanthodii). The 

 main features of Silurian fish-life were, therefore, the acquisition of 

 dermal armor, definite jaws, and the beginning of paired fins. Some 

 of the lowly types thus equipped survived and further evolved in the 

 Devonian period; 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 bot- 

 tom of shallow water or in marshes, but they were typical well-formed 

 fishes in respect to their jaws, branchial apparatus, and two pairs of 

 fins. Nearly all their bones were external, very little of their 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 environment. 

 Fishes more fitted for sustained swimming were also beginning to 

 appear, and these (Palceoniscidce) formed the large majority in the 

 succeeding Carboniferous and Permian periods. They were about 

 equivalent in grade to the modern sturgeons, and the tendency to- 

 wards 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 effective type 

 of propelling tail-fin had become universal among the highest fish-life 

 of the Triassic period, the internal skeleton began to ossify and ver- 

 tebral centra arose. In fact, the whole of the succeeding Jurassic 



