Although still believing that Wallace's hypothesis in large part accounts for the facts briefly summarised above, the present writer has recently been led to doubt whether it offers a complete explanation. Mimicry in the male, even though less beneficial to the species than mimicry in the female, would still surely be advantageous. Why then is it so often entirely restricted to the female? While the attempt to find an answer to this question was haunting me, I re-read a letter written by Darwin to Wallace, April 15, 1868, containing the following sentences: "When female butterflies are more brilliant than their males you believe that they have in most cases, or in all cases, been rendered brilliant so as to mimic some other species, and thus escape danger. But can you account for the males not having been rendered equally brilliant and equally protected? Although it may be most for the welfare of the species that the female should be protected, yet it would be some advantage, certainly no disadvantage, for the unfortunate male to enjoy an equal immunity from danger. For my part, I should say that the female alone had happened to vary in the right manner, and that the beneficial variations had been transmitted to the same sex alone. Believing in this, I can see no improbability (but from analogy of domestic animals a strong probability) that variations leading to beauty must often have occurred in the males alone, and been transmitted to that sex alone. Thus I should account in many cases for the greater beauty of the male over the female, without the need of the protective principle."("More Letters", II. pages 73, 74. On the same subject--"the gay-coloured females of Pieris" (Perrhybris (Mylothris) pyrrha of Brazil), Darwin wrote to Wallace, May 5, 1868, as follows: "I believe I quite follow you in believing that the colours are wholly due to mimicry; and I further believe that the male is not brilliant from not having received through inheritance colour from the female, and from not himself having varied; in short, that he has not been influenced by selection." It should be noted that the male of this species does exhibit a mimetic pattern on the under surface. "More Letters" II. page 78.)The consideration of the facts of mimicry thus led Darwin to the conclusion that the female happens to vary in the right manner more commonly than the male, while the secondary sexual characters of males supported the conviction "that from some unknown cause such characters (viz. new characters arising in one sex and transmitted to it alone) apparently appear oftener in the male than in the female." (Letter from Darwin to Wallace, May 5, 1867, "More Letters", II. Page 61.)Comparing these conflicting arguments we are led to believe that the first is the stronger. Mimicry in the male would be no disadvantage but an advantage, and when it appears would be and is taken advantage of by selection. The secondary sexual characters of males would be no advantage but a disadvantage to females, and, as Wallace thinks, are withheld from this sex by selection. It is indeed possible that mimicry has been hindered and often prevented from passing to the males by sexual selection.
We know that Darwin was much impressed ("Descent of Man", page 325.) by Thomas Belt's daring and brilliant suggestion that the white patches which exist, although ordinarily concealed, on the wings of mimetic males of certain Pierinae (Dismorphia), have been preserved by preferential mating.
He supposed this result to have been brought about by the females exhibiting a deep-seated preference for males that displayed the chief ancestral colour, inherited from periods before any mimetic pattern had been evolved in the species. But it has always appeared to me that Belt's deeply interesting suggestion requires much solid evidence and repeated confirmation before it can be accepted as a valid interpretation of the facts. In the present state of our knowledge, at any rate of insects and especially of Lepidoptera, it is probable that the female is more apt to vary than the male and that an important element in the interpretation of prevalent female mimicry is provided by this fact.
In order adequately to discuss the question of mimicry and sex it would be necessary to analyse the whole of the facts, so far as they are known in butterflies. On the present occasion it is only possible to state the inferences which have been drawn from general impressions,--inferences which it is believed will be sustained by future inquiry.
(1) Mimicry may occasionally arise in one sex because the differences which distinguish it from the other sex happen to be such as to afford a starting-point for the resemblance. Here the male is at no disadvantage as compared with the female, and the rarity of mimicry in the male alone (e.g.
Cethosia) is evidence that the great predominance of female mimicry is not to be thus explained.
(2) The tendency of the female to dimorphism and polymorphism has been of great importance in determining this predominance. Thus if the female appear in two different forms and the male in only one it will be twice as probable that she will happen to possess a sufficient foundation for the evolution of mimicry.
(3) The appearance of melanic or partially melanic forms in the female has been of very great service, providing as it does a change of ground-colour.
Thus the mimicry of the black generally red-marked American "Aristolochia swallowtails" (Pharmacophagus) by the females of Papilio swallowtails was probably begun in this way.
