Dear Darwin

I saw Bates a few days ago & he mentioned to me this difficulty of the catterpillars.² I think it is one that can only be solved by special observation. The only probable solution I can imagine is something like this   Catterpillars are very similar in form & there are hundreds of species that are only to be distinguished by colour.

Now great numbers are protected by their green colours assimilating with foliage or their brown colours resembling bark or twigs. Others are protected by prickles and long hairs—which no doubt render them distasteful to birds, especially to our small birds which I presume are the great destroyers of catterpillars. Now supposing that others, not hairy, are protected by a disagreeable taste or odour, it would be a positive advantage to them never to be mistaken for any of the palatable catterpillars, because a slight wound such as would be caused by a peck of a bird’s bill almost always I believe kills a growing catterpillar. Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable catterpillars, would enable birds to recognise them easily as a kind not fit for food, & thus they would escape seizure which is as bad as being eaten. ³

Now this can be tested by experiment, by any one who keeps a variety of insectivorous birds. They ought as a rule to refuse to eat and generally refuse to touch gaudy coloured catterpillars, & to devour readily all that have any protective tints. I will ask Mr. Jenner Weir of Blackheath about this, as he has had an aviary for many years & is a very close and acute observer, & I have no doubt will make the experiment this summer.⁴

When our discussion on Mimicry took place a most interesting little fact was mentioned by Mr. Stainton. After mothing he is accustomed to throw all the common species to his poultry & once having a lot of young turkeys he threw them a quantity of moths which they eat greedily, but among them was one common white moth (Spilosoma menthastri) One of the young turkeys took this in his beak, shook his head & threw it down again, another ran to seize it and did the same, and so on, the whole brood in succession rejected it! Mr. Weir tells me that the larva of this moth is hairy & is also rejected by all his birds, which sufficiently accounts for the insect being very common. ⁵ But what is still more curious, another moth much less common (Diaphora mendica) has the female also white, (although the male is quite different) and might at night be easily mistaken for the other! So here we have a case of British mimicry exactly analogous in all its details to that of the Heliconidæ & Danaidæ; and it is particularly valuable because it is a direct proof that Lepidoptera do differ in flavour, & that certain flavours are distasteful to birds.⁶

My female mimetic butterfly is much more beautiful than the male, being metallic blue while the male is dull brown.⁷ I sometimes doubt whether sexual selection has acted to produce the colours of male butterflies. I have thought that it was merely that it was advantageous for the females to have less brilliant colours, & that colour has been produced merely because in the process of infinite variation all colours in turn were produced. Undoubtedly two or three male butterflies do often follow a female, but whether she chooses between them or whether the strongest & most active gets her is the question.⁸ Cannot this also be decided by experiment? If a lot of common butterflies were bred, say our “brimstone” or better, the “orange tip”, & the males and females separated & then a certain number of the males discoloured by rubbing the wings carefully;—and we were then to turn out a female along with a coloured and a discoloured male into a room or greenhouse, would the female always or in the majority of cases choose the best coloured male.? A series of experiments of this kind carefully carried out would I think settle the question. I will suggest these two classes of experiment at the next meeting of the Entomological & perhaps some country residents may be induced to carry them out.⁹

I hope you will take care of your health, & not work too hard when you get a little better.

I often wish I lived in the country, & was able to carry out some of these most interesting observations but I do not know whether I shall be able to manage it.

Yours very faithfully | Alfred R. Wallace

C. Darwin Esq.