Introduction 5 



"But a comparative review of the number of freshwater 

 and marine genera of the present day, in the light of their 

 known first appearance in geologic time, brings out some 

 rather remarkable results. Thus, of the entire group of 

 the Gnathostomata or toothed fishes, 453 living genera 

 are purely freshwater in their life history, and 902 genera 

 are marine, with occasional freshwater representatives. 

 But when such statistics are compared with the geological 

 record, another and different relation Is revealed. The old- 

 est known groups of the true fishes are the Crossopterygii, 

 the Dipneustei, and the Chondrostel, representatives of 

 all of which have been found in the lower Devonian. All 

 of these are freshwater forms at the present day, and many 

 if not all of the fossil forms likewise were. But as these at- 

 tained to the greatest climax of their development In inland 

 seas, In lacustrine and in fluviatlle situations, the group of 

 the Selachii probably evolved from the primitive acantho- 

 deans, took to a marine life, and soon became an increasing- 

 ly powerful type of fish up to the period of the carboniferous 

 epoch or even later. Their aggressive and voracious ten- 

 dencies, their often lithe movements, and their inclination 

 to feed on dead as well as living animal matter, seem to have 

 enabled them to contend successfully with the giant repre- 

 sentatives of the cephalopod molluscs that at that time 

 largely held the seas. But the highest or teleostome fishes 

 gradually evolved from the Permian period onward In ever 

 Increasing numbers. It Is a striking fact however that, if 

 we compare the number of living genera of fishes whose 

 ancestry dates from the period of the Cretaceous or further 

 back, 320 living genera are freshwater, and 155 are marine. 

 Only during the period from the Cretaceous and especially 

 from the beginning of the Eocene onward do the teleo- 

 stean fishes attain that enormous marine development 

 which now characterizes them; so that, while 133 genera of 

 recently evolved living teleosteans are freshwater, 747 

 genera are marine. This remarkable development seems 

 to have been correlated with a gradual dying out alike of 

 the dominant groups of cephalopod molluscs and of sela- 

 chean fishes. So, when we sum up all of the living genera 

 of gnathostome fishes, and find that 453 are freshwater 



