Excepting the subtitle - why must so many of today's science writers suggest shortcomings in Darwin's analysis? - this is an excellent depiction of how natural selection has dealt with vision. The second volume of Parker's trilogy on the topic, this one shows how colour perception - or lack of it - has guided the track of natural selection. Vision issues have received little attention from zoologists. As the author demonstrates so well, there are many hidden facets to vision and perception. We tend to think we can see everything, but in fact, our visual acuity has serious limitations. Other creatures, such as bees, see in the ultraviolet range. Many ocean fish have colour limitations due to their habitat, particularly those in deep water. Parker describes how these abilities evolved by using seven - well, eight, actually - commonly detected colours.
"The" eye's complexity, which disturbed Darwin, is nearly always limited to the human version. Parker explains how eyes are structured. Certain types have differing colour detection abilities. Within the eye are organelles known as "rods" and "cones". The rods are the light intensity detectors, while the cones are colour selectors. As he explains, light is meaningless until the signals reach the brain where they are decoded. Eye and brain are thus closely linked, the arrangement having evolved with each species over time. Changes in habitat are reflected in changes in visual abilities. Eyes, and which colours they perceive, as Parker indicated in his first volume, are a major indicator of evolution's path. Some colours seem straightforward and unambiguous, like leaves or fruit. Others, however, are generated by pattern or movement. Moth and bird wings have delicate shifts of colour from rest state to flapping, for example. Birds and many insects have evolved superior colour perception as a result.
Not all colour is simply variations of reflectivity. Some creatures produce light. Parker's chapter on "Blue" takes us through the realm of "bioluminscence". Sailors tell of sparkling lights in a ship's wake. Fireflies are well known in many places, and the range of tactics in their mating and seeking prey is delightfully described. Some fish, which seem to have special abilities in generating light actually host and manipulate colonies of bacteria in their bodies. The fish control the luminescence with a flap of skin, seemingly providing the fish with "headlights" to illuminate the surroundings.
Mimicry is a common means of defense against predators. If an animal blends into its surroundings, it's difficult to see. This ploy is used by many insects, such as the "Stick Insect" or the "Spicky Leaf Insect", to avoid birds. It's also effective to look like a prey species which is toxic to many predators. A fly that looks like a wasp is likely to be left alone. Another trick Parker describes is more subtle. The tree frog is blue and should show up plainly against a green-leafed tree. Instead of mimicry, it adopts a hemispherical shape, resulting in its casting almost no shadow to betray its presence. Parker's collected information on these variations in protection make fascinating reading. It will be even more fascinating when his third volume in this set is released. In the meantime, take up this book to see what evolution has produced. Darwin would be delighted to see what has been learned about that organ he felt carried the greatest challenge to his theory of natural selection. [stephen a. haines - Ottawa, Canada]