When an animal frequents two different haunts, in one of which it
breeds, it is very often safe to say that the breeding-place represents
the original home. The flounder is quite comfortable far up the rivers,
but it has to go to the shore-waters to spawn, and there is no doubt
that the flounder is a marine fish which has recently learned to
colonise the fresh waters. Its relatives, like plaice and sole, are
strictly marine. But it is impossible to make a dogma of the rule that
the breeding-place corresponds to the original home. Thus some kinds of
bass, which belong to the marine family of sea-perches, live in the sea
or in estuaries, while two have become permanent residents in fresh
water. Or, again, the members of the herring family are very
distinctively marine, but the shad, which belong to this family, spawn
in rivers and may spend their lives there.
So there are two different ways of interpreting the life-history of the
salmon. Some authorities regard the salmon as a marine fish which is
establishing itself in fresh water. But others read the story the other
way and regard the salmon as a member of a freshwater race, that has
taken to the sea for feeding purposes. In regard to trout, we know that
the ranks of those in rivers and lakes are continually being reinforced
by migrants from the sea, and that some trout go down to the sea while
others remain in the freshwater. We know also in regard to a related
fish, the char, that while the great majority of kinds are now permanent
residents in cold and deep, isolated northern lakes, there are Arctic
forms which live in the sea but enter the rivers to spawn. These facts
favour the view that the salmon was originally a marine fish. But there
are arguments on both sides, and, for our present purpose, the important
fact is that the salmon is conquering _two_ haunts. Its evolution is
going on.