MODE OF EVOLUTION 



II 



The evolution of groups of fishes must, accordingly, 

 have taken place during only the longest periods of time. 

 Their aquatic life has evidently been unfavourable to deep- 

 seated structural changes, or at least has not permitted 

 these to be perpetuated. Recent fishes have diverged in 

 but minor regards from their ancestors of the Coal Meas- 

 ures. Within the same duration of time, on the other 

 hand, terrestrial vertebrates have not only arisen, but have 

 been widely differentiated. Among land-living forms the 

 amphibians, reptiles, birds, and mammals have been 

 evolved, and have given rise to more than sixty orders. 



The evolution of fishes has been confined to a note- 

 worthy degree within rigid and unshifting bounds ; their 

 living medium, with its mechanical effects upon fish-like 

 forms and structures, has for ages been almost constant 

 in its conditions ; its changes of temperature and density 

 and currents have rarely been more than of local im- 

 portance, and have influenced but little the survival of 

 genera and species widely distributed ; its changes, more- 

 over, in the normal supply of food organisms, cannot be 

 looked upon as noteworthy. Aquatic life has built few 

 of the direct barriers to survival, within which the ter- 

 restrial forms appear to have been evolved by the keenest 

 competition. 



It is not, accordingly, remarkable that in their descent 

 fishes are known to have retained their tribal features, and 

 to have varied from each other only in details of structure. 

 Their 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 armouring, and more trenchant teeth ; another, 

 closely akin in general structures, may have acquired more 



