14 A HISTORY OF FISHES 



sharp, thin keels, and, although these appear prominent when 

 viewed from the side they are much less so in the front aspect 

 of the fish (Fig. 5A). The paired fins are of two kinds only, the 

 pectorals and pelvicSy corresponding to the fore and hind limbs 

 of land vertebrates. 



Further evidence that the characteristic form of a typical 

 fish has been determined by its environment is provided by the 

 study of the evolution of other aquatic vertebrates. The 

 Cetaceans, mammals which have forsaken the land and re- 

 turned to a life spent entirely in the water, have revived the 

 fish-like shape, a result which has been brought about by 

 important anatomical and physiological changes. At the same 

 time, their mammalian ancestry is reflected in the diflferent 

 arrangement of the muscles used in swimming, as well as in 

 the swimming movements themselves. Again, in the extinct 

 reptiles, the Ichthyosaurs, we find the same fusiform shape, 

 powerful tail, wedge-like dorsal fin and paddle-like limbs 

 (Fig. 3), clear evidence of the adoption of an aquatic life. The 

 same external conditions acting over an immense period of 

 time on countless generations has produced much the same 

 result in three totally different groups of vertebrate animals, 

 an excellent example of what is known as convergence in 

 evolution. 



So much for the form of a typical pelagic fish. Departures 

 from this ideal shape of body are both numerous and varied, 

 but it must be remembered that many which at first sight 

 would appear to be anything but streamline shapes will prove 

 to be excellent ones when the particular mode of locomotion of 

 their possessors is considered. One fact is quite obvious: any 

 radical departure from the ideal form must inevitably lead to 

 a loss in swimming efficiency, or, at least, to a marked re- 

 striction of speed, and this becomes more and more evident the 

 further the fish departs from the typical shape. A fish like the 

 Mackerel depends on its speed, not only to obtain its food, but 

 as a means of escape from enemies, and any marked restriction 

 of its activities would leave it liable to the danger of extinction. 

 It is only where rapid mobility ceases to be of primary im- 

 portance to the life of the species, and is replaced by some other 

 compensating factor, such as heavy armour, that a fish is able 

 to dispense with the fusiform shape and survive in the struggle 

 for existence. 



Three examples selected from the class of Selachians will 

 serve to illustrate this point. The Blue Sharks and their allies 



