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GEORGE WALD
clear; that these relations are determined genetically is also clear. What
is and will remain open to doubt is the arrangement of this information
in the form of a phylogenetic tree. All phylogenetic trees are far-fetched;
by reiteration they come to seem less far-fetched. The most they can
hope to do is to provide an orderly arrangement of observations, and
perhaps to stimulate new observations. I do not suppose that the above
arrangement represents more than this. It is not universally agreed that
the vertebrates arose in fresh water, and the weakness of equating ancestral vertebrates with present freshwater teleosts has already been
noted. One is on surer ground in believing that freshwater fishes gave
rise to amphibia, and those in turn to reptiles and other land vertebrates.
Could one arrange the same data otherwise and achieve equal order?
Suppose, for example, one assumes that the ancestral vertebrates used
vitamin Ai in vision. Then one must somehow get to freshwater fishes
with A 2 , from these to amphibia with A 2 and Ai, then back to A ± in the
land vertebrates. This has the unattractive feature of vacillating where
the former scheme presents a continuous development, yet otherwise
represents the observed biochemical and ecological relationships equally
well.
VII. Lampreys
An opportunity to come further with these problems is offered by
the cyclostomes. These members of the ancient subclass Agnatha, the
jawless vertebrates, include the most primitive living vertebrates. Among
them the hagfishes are blind, and so cannot help us. The lampreys, however, have a reasonably well-formed eye.
Some years ago the author examined the retinas of the sea lamprey,
Petromyzon marinus, an anadromous form that has a life cycle like the
salmon. I found them to contain both vitamins A x and A 2 , with a great
predominance (about 90%) of A 2 (55). I concluded from this and from
earlier reports of the color of lamprey retinas that this animal possessed
primarily porphyropsin. It seemed to me that this observation lent support to the view that porphyropsin is the primitive vertebrate visual
pigment. Clearly the first scheme of vertebrate phylogeny presented
above would accept this datum smoothly, whereas it would be an added
embarrassment for the second.
Recently, however, Crescitelli (56) extracted the visual pigment
from retinas of this species of lamprey, and found the pigment to be
rhodopsin. Crescitelli noted that this observation favored the view that
rhodopsin rather than porphyropsin is the primitive type of vertebrate
pigment.
GEORGE WALD
clear; that these relations are determined genetically is also clear. What
is and will remain open to doubt is the arrangement of this information
in the form of a phylogenetic tree. All phylogenetic trees are far-fetched;
by reiteration they come to seem less far-fetched. The most they can
hope to do is to provide an orderly arrangement of observations, and
perhaps to stimulate new observations. I do not suppose that the above
arrangement represents more than this. It is not universally agreed that
the vertebrates arose in fresh water, and the weakness of equating ancestral vertebrates with present freshwater teleosts has already been
noted. One is on surer ground in believing that freshwater fishes gave
rise to amphibia, and those in turn to reptiles and other land vertebrates.
Could one arrange the same data otherwise and achieve equal order?
Suppose, for example, one assumes that the ancestral vertebrates used
vitamin Ai in vision. Then one must somehow get to freshwater fishes
with A 2 , from these to amphibia with A 2 and Ai, then back to A ± in the
land vertebrates. This has the unattractive feature of vacillating where
the former scheme presents a continuous development, yet otherwise
represents the observed biochemical and ecological relationships equally
well.
VII. Lampreys
An opportunity to come further with these problems is offered by
the cyclostomes. These members of the ancient subclass Agnatha, the
jawless vertebrates, include the most primitive living vertebrates. Among
them the hagfishes are blind, and so cannot help us. The lampreys, however, have a reasonably well-formed eye.
Some years ago the author examined the retinas of the sea lamprey,
Petromyzon marinus, an anadromous form that has a life cycle like the
salmon. I found them to contain both vitamins A x and A 2 , with a great
predominance (about 90%) of A 2 (55). I concluded from this and from
earlier reports of the color of lamprey retinas that this animal possessed
primarily porphyropsin. It seemed to me that this observation lent support to the view that porphyropsin is the primitive vertebrate visual
pigment. Clearly the first scheme of vertebrate phylogeny presented
above would accept this datum smoothly, whereas it would be an added
embarrassment for the second.
Recently, however, Crescitelli (56) extracted the visual pigment
from retinas of this species of lamprey, and found the pigment to be
rhodopsin. Crescitelli noted that this observation favored the view that
rhodopsin rather than porphyropsin is the primitive type of vertebrate
pigment.