There are lots of fascinating connections one can uncover by browsing the history of science. In my search for Lord Rayleigh’s invisibility research, I happened across a paper titled, “Insects and the colours of flowers,” published in Nature 11 (1874), 6. This comment by Rayleigh is short enough to be reproduced in full:
There is one point connected with Mr. Darwin’s explanation of the bright colours of flowers which I have never seen referred to. The assumed attractiveness of bright colours to insects would appear to involve the supposition that the colour-vision of insects in approximately the same as our own. Surely this is a good deal to take for granted, when it is known that even among ourselves colour-vision varies greatly, and that no inconsiderable number of persons exist to whom, for example, the red of the scarlet geranium is no bright colour at all, but almost a match with the leaves.
Rayleigh is apparently commenting on the work of Charles Darwin post-On the Origin of Species. After publishing his most famous work, Darwin turned his energies to understanding the mechanisms by which flowers reproduced. As discussed in this nice article in the New York Review of Books, the prevailing wisdom in his time was that flowers — which possess both male and female reproductive organs — were self-fertilizing, in essence having sex with themselves. Darwin set out to show that insects play a crucial role in cross-pollinating plants, carrying pollen from one flower to another, and he published his results on orchids in a book in 1862, Fertilisation of Orchids. Among those results is the insightful observation that the colors and scents of flowers had evolved to attract insects and optimize the cross-pollination process.
This observation is what Rayleigh seems to be commenting on, and mildly criticizing. Rayleigh suggests that Darwin’s argument leads to the conclusion that insects must have vision similar to human vision — otherwise, why would a flower which is pretty to us be pretty to an insect? Rayleigh argues that this is a rather large assumption to make based on the limited evidence available in that era.
Rayleigh’s argument is a bit too simplistic, however, as it implicitly assumes that all insects are attracted to all flowers. From the NY Review of Books article:
What had once been a pretty picture of insects buzzing about brightly colored flowers now became an essential drama in life, full of biological depth and meaning. The colors and smells of flowers were adapted to insects’ senses. While bees are attracted to blue and yellow flowers, they ignore red ones, because they are red-blind. On the other hand, their ability to see beyond the violet is exploited by flowers which use ultraviolet markings—the so-called honeyguides which direct bees to their nectaries. Butterflies, with good red vision, fertilize red flowers, but may ignore the blue and violet ones. Flowers pollinated by night-flying moths tend to lack color, but to exude their scents at night. And flowers pollinated by flies, which live on decaying matter, may mimic the (to us) foul smells of putrid flesh.
In short, different insects have different color sensitivities in their vision — and each type of flower has developed colors that attract the ‘optimal’ pollinating insect.
It is curious that Rayleigh’s letter appeared some ten years after Darwin’s book was published; I’m not sure if the book had been the subject of discussion for that long a period of time, if it had simply not gotten attention until a few years after publication, or if Darwin had published a more recent paper in one of the journals that drew more attention. Any Darwin-scholars out there have any ideas?
In any case, it is interesting to note that Darwin’s evolutionary researches were “on the radar” of at least one preeminent physicist of the time.