48 THE BIOLOGY OF THE SEASONS 



an eel. An overland excursion is sometimes required, 01 

 a drain-pipe may serve. In passing from the rivers to 

 the sea the males are said to lead the way in some cases, 

 as statistics of capture seem to bear out. They go down 

 to the depths of the sea, from which there is certainly 

 no return. But whether they die after spawning, as is 

 probable enough, no one knows with certainty. 



A life-history so weird and circuitous as that of the 

 eel raises many questions, and the need of some general 

 interpretation is obvious. To some extent that is supplied 

 by the theory to which many facts point, that the common 

 eel is originally a deep-water fish which has secondarily 

 taken to fresh waters, just as the salmon is a freshwater 

 fish which has secondarily taken to feeding in the sea. 



Part of the argument is based on the fact that animals 

 usually go back to their original home to spawn. The toad 

 is in the main a terrestrial animal, but it spawns in the 

 water. The land-crab often lives far from the sea, but it 

 journeys to the shore at the spawning season. 



Again, from the fact that most members of the eel 

 family (Anguillidae) are marine fishes and only a few enter 

 fresh water, we may infer that the habitat of the exceptional 

 minority is secondary. It is, furthermore, very signifi- 

 cant that, out of a total of about 150 species, about fifty 

 are found in the Deep Sea in the stricter sense, going down 

 to 2500 fathoms. 



The deep-sea eel (Synaphobranchus pinnatus) of the 

 North-East Atlantic keeps to the abyssal habitat, which 

 was probably primitive for the common eel. Its larvae 

 have never been taken so near the surface as the Lepto- 

 cephali of the common eel, never, indeed, above about 50 

 fathoms. There is nothing to suggest a movement towards 

 the shore. On the contrary, the larvae sink during metamor- 

 phosis towards the floor of the sea, where they become mature. 



