in] THE FACTORS OF DISTRIBUTION 67 



successfully ' planted ' to carry on the species in un- 

 diminished numbers. Moreover the eggs are usually 

 so liberated by the parent that the currents or tides 

 will carry them to suitable places. If the parent is 

 sessile in habit it is usually found in shallow water 

 near the shore, and the tidal streams flow, as a rule, 

 along the land, or surge backwards and forwards to 

 and from the land ; therefore the larvae may inhabit 

 shallow water during the whole of their free-swim- 

 ming period. If the parent animal is a nektic one it 

 usually performs a spawning migration, that is to say 

 it migrates when about to spawn to such a place as 

 will ensure that its eggs will not all be lost. The 

 majority of fishes produce buoyant eggs which float 

 at or near to the surface of the sea, but the larvae 

 hatched from these eggs prefer to inhabit shallow 

 water near the shore. When the fish is about to 

 spawn it migrates out to sea to some distance from 

 the land and then spawns, and the spawning place is 

 so selected that the currents at that time of year, or 

 the prevailing winds will drift the eggs towards the 

 land. The cod, plaice, flounder, sole and many other 

 fishes behave in this way they migrate offshore to 

 spawn, and then the eggs drift inshore, completing 

 their embryonic development during the transporta- 

 tion, so that, barring accidents, the larvae are just 

 about to undergo metamorphosis by the time they 

 reach shallow water. The spawning takes place 



52 



