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GEORGE WALD
freshwater fishes use vitamin A 2 in vision; all known marine fishes with
the exception of a few Labridae and closely related Coridae use vitamin
Ai. All known mammals and birds also use vitamin Ai in vision, whereas
the few reptiles examined divide in this regard: the alligator, rattlesnake, geckos, and marine turtles use Ai; some freshwater turtles, A 2 .
In every case the visual pigment has been found to be formed from
whatever vitamin A the retina possesses; but owing to rods and cones
possessing different opsins, and the opsins of different species sometimes
introducing wide departures of A max from the usual positions, a degree
of scatter and consequent confusion may be introduced by the visual
pigments themselves that is avoided if one regards simply the retinal
vitamins A. Yet the visual pigments also for the most part present us
with a simple distribution. The few aberrations that have appeared
among them have taken on exaggerated importance as workers grow
bored with the prevalent forms, and concentrate their attention and
energies on discovering exceptions.
IV. Euryhaline Fishes
The almost precise division between freshwater and marine fishes,
the former using exclusively vitamin A 2 and the latter almost exclusively
vitamin Ai in vision, raised acutely the question of those fishes which
are neither wholly freshwater nor marine, but migrate between both
environments. Before considering the distribution of visual systems in
these forms, it will be necessary to make plain their biological
status (30).
All aquatic animals can be divided into forms restricted to narrow
ranges of salinity, called stenohaline, of which there are two groups:
freshwater and marine; and forms that range as adults through wide
ranges of salinity, called euryhaline. The euryhaline fishes in turn are
divided into two groups, called anadromous and catadromous, meaning
literally upstream and downstream, referring to the direction of the
spawning migration. Thus a typical anadromous fish is the salmon,
which having grown up in the sea, migrates upstream to spawn; whereas
the classic example of a catadromous fish is the "freshwater" eel,
Anguilla, which having matured in fresh water migrates seaward to
spawn.
Too much emphasis has been placed in the past upon the migrations
of these animals. So far as known none of them needs to leave its spawning environment to complete a normal life cycle. The spawning environment itself is in every instance fixed; but migration away from it among
adult fishes is nothing more than a privilege which some forms exploit
frequently and others rarely. It is more meaningful to define an ana-
GEORGE WALD
freshwater fishes use vitamin A 2 in vision; all known marine fishes with
the exception of a few Labridae and closely related Coridae use vitamin
Ai. All known mammals and birds also use vitamin Ai in vision, whereas
the few reptiles examined divide in this regard: the alligator, rattlesnake, geckos, and marine turtles use Ai; some freshwater turtles, A 2 .
In every case the visual pigment has been found to be formed from
whatever vitamin A the retina possesses; but owing to rods and cones
possessing different opsins, and the opsins of different species sometimes
introducing wide departures of A max from the usual positions, a degree
of scatter and consequent confusion may be introduced by the visual
pigments themselves that is avoided if one regards simply the retinal
vitamins A. Yet the visual pigments also for the most part present us
with a simple distribution. The few aberrations that have appeared
among them have taken on exaggerated importance as workers grow
bored with the prevalent forms, and concentrate their attention and
energies on discovering exceptions.
IV. Euryhaline Fishes
The almost precise division between freshwater and marine fishes,
the former using exclusively vitamin A 2 and the latter almost exclusively
vitamin Ai in vision, raised acutely the question of those fishes which
are neither wholly freshwater nor marine, but migrate between both
environments. Before considering the distribution of visual systems in
these forms, it will be necessary to make plain their biological
status (30).
All aquatic animals can be divided into forms restricted to narrow
ranges of salinity, called stenohaline, of which there are two groups:
freshwater and marine; and forms that range as adults through wide
ranges of salinity, called euryhaline. The euryhaline fishes in turn are
divided into two groups, called anadromous and catadromous, meaning
literally upstream and downstream, referring to the direction of the
spawning migration. Thus a typical anadromous fish is the salmon,
which having grown up in the sea, migrates upstream to spawn; whereas
the classic example of a catadromous fish is the "freshwater" eel,
Anguilla, which having matured in fresh water migrates seaward to
spawn.
Too much emphasis has been placed in the past upon the migrations
of these animals. So far as known none of them needs to leave its spawning environment to complete a normal life cycle. The spawning environment itself is in every instance fixed; but migration away from it among
adult fishes is nothing more than a privilege which some forms exploit
frequently and others rarely. It is more meaningful to define an ana-