7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
341
ment from an insect eye. It has A max about 440 πΐμ, close therefore in
the spectrum to the human violet receptor (75).
Brown and Brown (27) have examined the visual pigments of the
European cuttlefish (Sepia officinalis) and octopus (Octopus vulgaris).
Sepia rhodopsin has A max 492 ηΐμ, and on irradiation yields metarhodopsin (acid form, A max 497 τημ; alkaline A max 380 τημ). Octopus
rhodopsin has A max 475 τημ, like that of a deep-sea fish. On irradiation
this goes to metarhodopsin (acid form, A max 502 τημ; alkaline A max 380
m/*).
To summarize these results: all these animals contain visual pigments based upon vitamin Ai and retinenei, comparable in their fundamental chemistry to vertebrate rhodopsins, and hence referred to here
as rhodopsins. Ultimately these pigments must be derived from vitamin
Ai itself, and the organisms must possess mechanisms in the eye or elsewhere for oxidizing this to retinenei, and hence also for reducing the
latter. Yet no active alcohol dehydrogenase system has yet been found
in an invertebrate eye. Nor has any one of these invertebrate systems—
except apparently that of the bee—been found under physiological conditions to bleach so as to yield free retinenei. On a short-term basis,
they appear normally to operate with the simple reversible system:*
Light
Rhodopsin ^
— Metarhodopsin
XI. Summary and Conclusions
Virtually all these observations can be brought together in a diagram
such as shown in Fig. 15.
It must be emphasized again that as a statement of the distribution
of vitamins A among the visual systems of contemporary animals, and
its relations to taxonomic and ecological groupings, this diagram correctly represents the observations. Its suggestion of phylogenetic sequence raises more stubborn questions, and will continue to do so.
Nevertheless, even in this sense such a diagram has much to tell us.
It is sometimes argued that natural selection, based as it is upon chance
variations, could not possibly have led to such intricate and effective
structures as organisms possess; and a favorite structure cited in support of this view is the eye. "Think of the eye!" one hears it said, "Can
you imagine chance variations ever giving rise to an eye?" The question
is meant to be rhetorical. The proposition is intended to be obviously
absurd.
My reading of evolution goes quite differently. An eye is possible
* Note added in proof: R. Hubbard and G. Wald (Nature, 1960, in press) have
extracted a rhodopsin from the eye of the horseshoe crab, Limulus polyphemus, with
Xmax about 520 ιημ, and which bleaches in the light to retinenej and opsin.
341
ment from an insect eye. It has A max about 440 πΐμ, close therefore in
the spectrum to the human violet receptor (75).
Brown and Brown (27) have examined the visual pigments of the
European cuttlefish (Sepia officinalis) and octopus (Octopus vulgaris).
Sepia rhodopsin has A max 492 ηΐμ, and on irradiation yields metarhodopsin (acid form, A max 497 τημ; alkaline A max 380 τημ). Octopus
rhodopsin has A max 475 τημ, like that of a deep-sea fish. On irradiation
this goes to metarhodopsin (acid form, A max 502 τημ; alkaline A max 380
m/*).
To summarize these results: all these animals contain visual pigments based upon vitamin Ai and retinenei, comparable in their fundamental chemistry to vertebrate rhodopsins, and hence referred to here
as rhodopsins. Ultimately these pigments must be derived from vitamin
Ai itself, and the organisms must possess mechanisms in the eye or elsewhere for oxidizing this to retinenei, and hence also for reducing the
latter. Yet no active alcohol dehydrogenase system has yet been found
in an invertebrate eye. Nor has any one of these invertebrate systems—
except apparently that of the bee—been found under physiological conditions to bleach so as to yield free retinenei. On a short-term basis,
they appear normally to operate with the simple reversible system:*
Light
Rhodopsin ^
— Metarhodopsin
XI. Summary and Conclusions
Virtually all these observations can be brought together in a diagram
such as shown in Fig. 15.
It must be emphasized again that as a statement of the distribution
of vitamins A among the visual systems of contemporary animals, and
its relations to taxonomic and ecological groupings, this diagram correctly represents the observations. Its suggestion of phylogenetic sequence raises more stubborn questions, and will continue to do so.
Nevertheless, even in this sense such a diagram has much to tell us.
It is sometimes argued that natural selection, based as it is upon chance
variations, could not possibly have led to such intricate and effective
structures as organisms possess; and a favorite structure cited in support of this view is the eye. "Think of the eye!" one hears it said, "Can
you imagine chance variations ever giving rise to an eye?" The question
is meant to be rhetorical. The proposition is intended to be obviously
absurd.
My reading of evolution goes quite differently. An eye is possible
* Note added in proof: R. Hubbard and G. Wald (Nature, 1960, in press) have
extracted a rhodopsin from the eye of the horseshoe crab, Limulus polyphemus, with
Xmax about 520 ιημ, and which bleaches in the light to retinenej and opsin.