404 Causes and Course of Organic Evolution 



usually marine and breeding there at considerable depths, 

 the fifth or Haplomi — that includes the common pike — are 

 largely fresh-water though a fair number live and even breed 

 in the sea, and the remaining nine (Heteromi, Catosteomi, 

 etc.) are almost whollv marine with rarelv fresh-water inclu- 

 sive genera. 



But a comparative review of the number of fresh-water 

 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 

 fresh-water in their life histor\% and 902 genera are marine, 

 with occasional fresh-water representatives. But when such 

 statistics are compared with the geological record, another 

 and different relation is revealed. The oldest known groups 

 of the true fishes are the Crossopterygii, the Dipnoi, and the 

 Chondrostei, representatives of all of which have been foimd 

 in the lower devonian. All of these are fresh- water forms 

 at the present day, and many if not all of the fossil forms like- 

 wise were. But as these attained to the greatest climax of 

 their development in inland seas, in lacustrine and in fluviatile 

 situations, the group of the Selachii probably evolved from 

 the primitive acanthodeans, took to a marine life, and soon 

 became an increasingly powerful type of fish up to the period 

 of the carboniferous epoch or even later. Their aggressive 

 and voracious tendencies, 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 representatives 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 fresh-water, and 155 are marine. 

 Only during the period from the cretaceous and especially 

 from the beginnmg of the eocene onward do the teleostean 



