460 THE WONDER OF LIFE 



and the leaping of the falls all implying efforts which are 

 the more remarkable since there seems to be no evidence 

 that the adult salmon ever feeds in fresh water. Few salmon 

 seem to spawn more than once, and some die of spawning. 

 It is of interest to contrast the eel and the salmon, for the 

 former is a marine fish which has taken secondarily to a 

 life in the rivers and ponds, while the latter is primarily a 

 freshwater fish which has taken to the exploitation of 

 the sea. 



In the case of the Pacific salmon (Oncorhynchus) the 

 general facts are the same, but a simplification is implied 

 in the fact that the adults die after spawning once. They 

 do not return to the sea. The run up the rivers to the 

 spawning grounds several hundred miles off is very 

 remarkable ; it may occupy two or three months ; after 

 tidal waters are passed the fish continues, according to Pro- 

 fessor C. W. Greene's ' marking ' experiments, at an average 

 speed of not less than 7 miles a day ; all the work is done 

 on an empty stomach, for feeding stops absolutely in fresh 

 water ; the work often includes jumping six or seven feet 

 in height and then continuing against a swift rush of water ; 

 and all the time the reproductive organs are growing rapidly 

 at the expense of other parts of the body. It is a remarkable 

 performance. 



Frogs. Out of the frog's egg, in the midst of its 

 enveloping sphere of jelly, there emerges a ciliated larva, 

 which has already had an embryonic development of about 

 a fortnight. It is mouthless and limbless ; the eyes growing 

 out from the brain have not yet reached the surface ; there 

 are the beginnings of external gills ; and there is a glandular 

 cement organ on the under surface of the head, by means 

 of which the larva attaches itself to water -weed and other 



