68 LIFE IN THE SEA [CH. 



during the spring when there is abundant food, 

 and when the temperature is rising rapidly in the 

 shallow water near the land ; therefore the young 

 larvae are carried to a land of plenty. The egg- 

 laying skates and rays behave in an analogous manner, 

 migrating towards parts of the sea where the bottom 

 is rough and stony and contains abundance of sea- 

 weed, and where there is little run of tide. The eggs 

 are laid on the sea-bottom where they develop with- 

 out much risk of being washed away by the tide, or 

 stranded on muddy beaches, or taken out into deeper 

 water. The herring makes a somewhat similar migra- 

 tion for the same purpose. The salmon was probably 

 originally a marine fish which adopted the habit of 

 migrating into rivers in order that its eggs might not 

 be exposed to the same risks of destruction by such 

 enemies as they would encounter in the sea. Thus 

 the spawning migrations of fishes are directed move- 

 ments carried out to achieve a definite purpose. We 

 need not suppose that they are conscious actions ; 

 and we may regard them as ' instinctive acts.' The 

 stimulus to the performance of the spawning migra- 

 tion is the development and ripening of the generative 

 organs, and the elaboration of an internal secretion 

 from the ovary or testis which produces an intoxi- 

 cation, and impels the fish to seek water of definite 

 physical conditions. What these conditions may be 

 depends on the former history of the species the 



