Complex Organs

Darwin’s own consideration of the concrete case argument focused on some complex organs and structures that he rightly believed to be hard to understand in terms of natural selection. Two examples are prominent in the Origin, the eye and the electric organs found in some fish. The latter exam- ple disconcerted Darwin because the fish with electric organs are of very dif- ferent types, and have their organs in different parts of their bodies. Much to his relief, research on electric fish carried out by a contemporary who was not sympathetic to evolutionary ideas—”McDonnell of Dublin (a first-rate man)”*—revealed that, for each type of fish with an electric organ, there is a related fish with a similar organ (not functionally electric) in the same posi- tion. What had initially appeared to be the challenge of understanding how different electric organs had been built from scratch, became the much sim- pler question of how a similar change had occurred in each instance.

Darwin himself offered a tentative proposal about the evolution of the eye. He supposed that sensitivity to light might come in degrees, and that it might be possible to find, among existing organisms, some with a crude ability to respond to light, others with a more refined capacity and so on in something like a series. Perhaps, he speculated, research on these creatures might expose reasons why the different levels of sensitivity provided an ad- vantage over rival organisms who had less, thus providing a way of answer- ing (or sidestepping) the creationist quip, “What use is half an eye?”

It has taken more than a century of research on a wide variety of organisms to demonstrate that Darwin’s hunch was basically right. Appearances to the contrary, organs and structures sensitive to light can be assembled piecemeal, with the intermediates enjoying some advantage over the competition. Biolo- gists have studied organisms that respond to the light that impinges on their surfaces, organisms with indentations of the superficial layer that are able to acquire information about the direction of the light, organisms with deeper in- dentations whose light detection is more fine grained, organisms that have a structure resembling a pinhole camera, organisms that interpose a translucent medium between the surface and the aperture through which the light comes—and so on. By studying this sequence of organisms, they have been able to explore the transitions through which relatively crude abilities to detect light were successively refined.** One feature of the story deserves emphasis. Darwin didn’t start with a comparison between the fully formed eye—in a hu- man being or an octopus, say—and then think of the component parts as being introduced, one at a time. He resisted the challenge to explain first the advan- tage of an eighth of an eye, then the advantage of a quarter of an eye, and so on, and focused instead on a function, light sensitivity, that might have been refined from an initial state of absence. To put it more bluntly, he didn’t allow his envisaged challengers to define the sequence of “intermediates” for him.