CHAPTER 7
The Distribution and Evolution of Visual Systems
GEORGE WALD*
Biological Laboratories of Harvard University, Cambridge, Massachusetts
I. Introduction
311
II. Visual Systems of Vertebrates
313
III. Habitat Relations: Freshwater Fishes versus Marine Fishes and Land
Vertebrates
320
IV. Euryhaline Fishes
324
V. Amphibia: Metamorphosis of Visual Systems
328
VI. Vertebrate Visual Systems: Habitat Relations and Phylogeny
.
.
330
VII. Lampreys
332
VIII. Deep-Sea Fishes
334
IX. Eels
337
X. Invertebrates
338
XI. Summary and Conclusions
341
References
343
I. Introduction
All life on this planet depends upon sunlight. It is of the utmost
consequence therefore that photosynthetic plants find the light, that
herbivores find photosynthetic plants, and that carnivores find herbivores.
These are the objects of phototropism in plants and vision in animals. Central as they are among the conditions for survival, these processes have been subjected throughout the course of evolution to the
most searching selection. No accidents or casual variations are to be expected here. The tasks are too important for that, the animals and plants
too dependent upon them. Here is a realm in which adaptation has
done its utmost, and we can look for significance in every manifestation. The arrangements which organisms have evolved for vision and
phototropism make sense, because they must.
A first manifestation of this fitness involves the wavelengths of radia* The investigations from our laboratory reviewed in this paper were supported
in part by grants from the Rockefeller Foundation and the Office of Naval Research.
311
The Distribution and Evolution of Visual Systems
GEORGE WALD*
Biological Laboratories of Harvard University, Cambridge, Massachusetts
I. Introduction
311
II. Visual Systems of Vertebrates
313
III. Habitat Relations: Freshwater Fishes versus Marine Fishes and Land
Vertebrates
320
IV. Euryhaline Fishes
324
V. Amphibia: Metamorphosis of Visual Systems
328
VI. Vertebrate Visual Systems: Habitat Relations and Phylogeny
.
.
330
VII. Lampreys
332
VIII. Deep-Sea Fishes
334
IX. Eels
337
X. Invertebrates
338
XI. Summary and Conclusions
341
References
343
I. Introduction
All life on this planet depends upon sunlight. It is of the utmost
consequence therefore that photosynthetic plants find the light, that
herbivores find photosynthetic plants, and that carnivores find herbivores.
These are the objects of phototropism in plants and vision in animals. Central as they are among the conditions for survival, these processes have been subjected throughout the course of evolution to the
most searching selection. No accidents or casual variations are to be expected here. The tasks are too important for that, the animals and plants
too dependent upon them. Here is a realm in which adaptation has
done its utmost, and we can look for significance in every manifestation. The arrangements which organisms have evolved for vision and
phototropism make sense, because they must.
A first manifestation of this fitness involves the wavelengths of radia* The investigations from our laboratory reviewed in this paper were supported
in part by grants from the Rockefeller Foundation and the Office of Naval Research.
311