224 Adaptations of Fishes 



evolution is to be traced in diverging characters that prove rarely 

 more than of family value ; one form, as an example, may have 

 become adapted for an active and predatory life, evolving 

 stronger organs of progression, stouter armoring, and more 

 trenchant teeth; another, closely akin in general structures, 

 may have acquired more sluggish habits, largely or greatly di- 

 minished size, and degenerate characters in its dermal investi- 

 ture, 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 be paralleled 

 among the carboniferous sharks, the Mesozoic Chimaeroids, and 

 the Palaeozoic lung-fishes and Teleostomes. Their environment 

 has retained their general characters, while modelling them 

 anew into forms armored or scaleless, predatory or defenseless, 

 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 pos- 

 sessors 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 armoring, 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 defenses, 

 medium powers of progression, omnivorous feeding habits, and 

 wide distribution, and these might be regarded as having pro- 

 vided 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 do not appear to 

 be again productive of more generalized groups. In all lines of 

 descent specialized forms do not appear to regain by regression 

 or degeneration the potential characters of their ancestral con- 

 dition. A generalized form is like potter's clay, plastic in the 



