12 FISHES IN GENERAL 



sluggish habits, larger or greatly diminished size, and degen- 

 erate characters in its dermal investiture, teeth, and organs 

 of sense or progression. The flowering out of a series of 

 fish families seems to have characterized every geological 

 age, leaving its clearest imprint on the forms which were 

 then most abundant. The variety that to-day maintains 

 among the families of Bony Fishes is thus known to 

 have been paralleled among the Carboniferous Sharks, the 

 Mesozoic Chimaeroids, and the Palaeozoic Lung-fishes and 

 Teleostomes. Their environment has retained their gen- 

 eral characters, while modelling them anew into forms 

 armoured or scaleless, predatory or defenceless, great, 

 small, heavy, stout, sluggish, light, slender, blunt, taper- 

 ing, depressed. 



When members of any group of fishes became extinct, 

 those appear to have been the first to perish which were 

 the possessors of the greatest number of widely modified 

 or specialized structures. Those, for example, whose teeth 

 were adapted for a particular kind of food, or whose 

 motions were hampered by ponderous size or weighty 

 armouring, were the first to perish in the struggle for 

 existence ; on the other hand, the forms that most nearly 

 retained the ancestral or tribal characters — that is, those 

 whose structures were in every way least extreme — were 

 naturally the best fitted to survive. Thus ge7ieralised 

 fishes should be considered those of medium size, medium 

 defences, medium powers of progression, omnivorous feed- 

 ing habits, and wide distribution : and these might be re- 

 garded as having provided the staples of survival in every 

 branch of descent. 



Aquatic living has not demanded wide divergence from 

 the ancestral stem, and the divergent forms which may 

 culminate in a profusion of families, genera, and species, 



