140 THE STORY OF FISH LIFE. 



to this exchange wc owe the preservation of 

 some very interesting and archaic forms, such 

 as the bony-pike (Lepidosteus), bichir {Polypterus), 

 and barramunda {Ceratodus), for instance. 



There can be no doubt but that fish life 

 originally commenced in the sea, and spread 

 thence to the brackish and fresh water by a 

 series of sporadic migrations such as we have 

 just instanced. Possibly this migration was due 

 to pressure and competition amongst the species 

 involved in this migration, just as amongst our- 

 selves, overcrowded populations seek relief by 

 emigration. 



The causes of the migration of the mackerel 

 seems to be an exception to the rule suggested — 

 that migration in fishes is probably due to the 

 desire to secure a safe harbourage for the young 

 fry. Mackerel swim in shoals, and spawn in 

 the open sea. Periodically, however, they ap- 

 pear ofi* shore, apparently, as Dr Gunther sug- 

 gests, in pursuit of other fishes on which they 

 feed. They prey upon the young and adults of 

 the herring-tribe, the pilchard and sprat. These 

 guide the movements of the mackerel. 



The perils of the migrating adults of such 

 species as the herring or sprat, for instance, are 

 many, for not only are they subjected to an 

 unceasing attack from hordes of their predaceous 

 relatives, but toll is taken by numberless others 

 besides, such as "schools" of porpoises, and 

 countless flocks of birds, who seize them from 

 above. Besides these we have to reckon the 

 millions captured annually by our fishing-fleets. 

 In spite of all this persecution, wonderful though 



