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 Chimeeroids, and the Palzozoic 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 generalized 
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, 
