466 THE WONDER OF LIFE 



area represents the old home. The sahnon is essentially 

 a freshwater fish, though its nutritive periods are mostly- 

 spent in the sea. Its spawning in the rivers is indicative 

 of its original home. The common eel, on the other hand, 

 which has its nutritive period in the fresh waters, goes 

 down to the Deep Sea to spawn, and is probably to be 

 regarded as essentially a marine fish with its old home 

 in the gTeater depths. A similar argument leads to the 

 view that the Sea-Lamprey is primarily a freshwater fish 

 which has secondarily taken to spending a nutritive period 

 in the sea. We have abeady spoken of the forms of 

 Petromyzon marinus in lakes of the State of New York, 

 which do not leave the fresh water at all, though they 

 migrate from lake to river to spawn. In the case of the 

 River Lampern {Lampetra fluviatilis), whose young are 

 also called ' niners ', some remain all their lives in fresh 

 water, while others go down to the sea. This is paralleled 

 by the Trout (Salmo trutta), some forms of which remain 

 in lakes and rivers, while others (distinguished nominally 

 as Sea-trout) go down to the sea. It must also be noted 

 that a number of species of lamprey, such as the Brook 

 Lamprey, never leave the fresh water, and this may be 

 taken as another argument in support of the view that 

 the Sea Lamprey is secondarily marine. Let us follow 

 them now on their return journey to their cradle-area. 

 The spawning of the Sea Lamprey has been weU described 

 by Dr. L. Hussakof . A circular depression is made, two to 

 three feet in diameter, in the river-bed. Large numbers of 

 pebbles and stones are carried out of the chosen area until 

 a shallow basin is formed, naturally with a floor of sand 

 and fine gravel. The adhesive disc around the mouth 

 acts hke a vacuum-sucker, and it can be made to ' work ' 



