FRESH- WATER ANIMALS 95 



in the sea. It is the species and not the individual 

 that here benefits, for while the salmon on running 

 up the rivers in the winter months is fat and in 

 good condition, on its return journey, after laying 

 eggs, it is in wretched condition, and very many 

 die on their way back to the sea. Many other 

 fish, such as lampreys and sturgeons, have similar 

 habits, ascending rivers during the spawning 

 season. On the other hand many characteristically 

 fresh-water fish, as the stickleback, frequently 

 descend the rivers to the sea, in which they are 

 perfectly at home. 



The large size of the eggs of fresh-water fish, as 

 the trout, or such fish as breed in fresh water, as 

 the salmon, is worthy of notice. In consequence 

 of the larger supply of food contained in the eggs, 

 as compared with those of marine fish, the young 

 hatch of larger size and greater strength, and are 

 therefore better able to cope from the time of their 

 birth with the downward currents of the streams 

 and rivers. 



The Amphibians may be briefly alluded to in 

 conclusion. They are a characteristically fresh- 

 water group of animals, the chief interest about 

 which is that during their development, at least in 

 the higher forms, such as the frogs, they pass from a 

 gill-breathing aquatic fish-like stage, to a lung- 

 breathing terrestrial condition, and show us very 

 clearly steps by which air breathing vertebrates have 

 been evolved from the more primitive water-breath- 

 ing forms. 



Summarising the results arrived at, we may say 

 that fresh-water animals have almost certainly 



