Evolution does a wonderful job on eyes. In the matter of seeing in dim light, for example, we are not just supplied with a good tool, but with the very best the system - the rest of the body - will allow. A recent paper in Nature describes work on human and toad perception. Humans are very sensitive - a dozen or so photons are enough to trigger dim sensation; but toads will make a strike at a moving target at light levels where humans can see nothing. The best explanation of the difference between ourselves and toads seems to lie in our higher blood temperature. This sets the level of random change in the photoreceptor molecules - the level of background 'noise' - which in turn determines the level below which seeing is impossible.

LRB 8 December 1988 | PDF Download