18 THE STORY OF FISH LIFE. 



living members of the herring tribe, which in- 

 deed trace their origin to this stock. With 

 these herring-like forms in the Cretaceous period 

 appeared numerous other familiar shapes, which 

 differ only in small points from their living 

 descendants of the seas and rivers of to-day. 



"The evolution of fishes," says Dr Bashford 

 Dean, "has been confined to a noteworthy de- 

 gree 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 of more than local importance, 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. 



"When members of any group of fishes be- 

 came extinct, those appear to have been the 

 first to perish which were the possessors of the 

 greatest number of widely modified or specialised 

 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 character — that is, whose 

 structures were in every way least extreme — 

 were naturally the best fitted to survive. Thus 

 generalised fishes should be considered those of 

 medium size, medium defences, medium powers 

 of progression, omnivorous feeding habits, and 



