d hardly explain in this age of enlightenment, are
not fish; but their actions help to throw a side-light on the migratory
instinct in salmon, eels, and so many other true fish which have
changed with time their aboriginal habits. The salmon himself, for
instance, is by descent a trout, and in the parr stage he is even now
almost indistinguishable from many kinds of river-trout that never
migrate seaward at all. But at some remote period, the ancestors of the
true salmon took to going down to the great deep in search of food, and
being large and active fish, found much more to eat in the salt water
than ever they had discovered in their native streams. So they settled
permanently in their new home, as far as their own lives went at least;
though they found the tender young could not stand the brine that did
no harm to the tougher constitutions of the elders. No doubt the change
was made gradually, a bit at a time, through the brackish water, the
species getting further and further seaward down bays and estuaries
with successive generations, but always returning to spawn in its
native river, as all well-behaved salmon do to the present moment. At
last, the habit hardened into an organic instinct, and nowadays the
young salmon hatch out like their fathers as parr in fresh water, then
go to the sea in the grilse stage and grow enormously, and finally
return as full-grown salmon to spawn and breed in their particular
birthplace.

Exactly the opposite fate has happened to the eels. The salmonoids as a
family are freshwater fish, and by far the greater number of
kinds--trout, char, whitefish, grayling, pollan, vendace, gwyniad, and
so forth--are inhabitants of lakes, steams, ponds, and rivers, only a
very small number having taken permanently or temporarily to a marine
residence. But the eels, as a family, are a saltwater group, most of
their allies, like the congers and muraenas, being exclusively confined
to the sea, and only a very small number of aberrant types having ever
taken to invading inland waters. If the life-history of the salmon,
however, has given rise to as much controversy as the Mar peerage, the
life-history of the eel is a complete mystery. To begin with, nobody
has ever so much as distinguished between male and female eels; except
microscopically, eels have never been seen in the act of spawning, nor
observed anywhere with mature eggs. The ova themselves are wholly
unknown: the mode of their production is