266 EVOLUTION [Chap. IV 



Letter 189 an explanation, for I want fully to understand you. How 

 can one female form be selected and the intermediate forms 

 die out, without also the other extreme form also dying out 

 from not having the advantages of the first selected form ? for, 



acquiring a resemblance to other species which, from whatever cause, 

 enjoy a comparative immunity from persecution." Mr. Wallace has been 

 good enough to give us the following note on the above passage : "The 

 above quotation deals solely with the question of how certain females of 

 the polymorphic species (Pafillio Memnon, P. P amnion, and others) have 

 been so modified as to mimic species of a quite distinct section of the 

 genus ; but it does not attempt to explain why or how the other very 

 variable types of female arose, and this was Darwin's difficulty. As the 

 letter I wrote in reply is lost, and as it is rather difficult to explain the 

 matter clearly without reference to the coloured figures, I must go into 

 some little detail, and give now what was probably the explanation I gave at 

 the time. The male of Papilio Memnon is a large black butterfly with the 

 nervures towards the margins of the wings bordered with bluish gray 

 dots. It is a forest insect, and the very dark colour renders it con- 

 spicuous ; but it is a strong flier, and thus survives. To the female, 

 however, this conspicuous mass of colour would be dangerous, owing to 

 her slower flight, and the necessity for continually resting while depositing 

 her eggs on the leaves of the food-plant of the larva. She has accordingly 

 acquired lighter and more varied tints. The marginal gray-dotted stripes 

 of the male have become of a brownish ash and much wider on the fore 

 wings, while the margin of the hind wings is yellowish, with a more 

 defined spot near the anal angle. This is the form most nearly like the 

 male, but it is comparatively rare, the more common being much lighter 

 in colour, the bluish gray of the hind wings being often entirely replaced 

 by a broad band of yellowish white. The anal angle is orange-yellow, 

 and there is a bright red spot at the base of the fore wings. lietween 

 these two extremes there is every possible variation. Now, it is quite 

 certain that this varying mixture of brown, black, white, yellow, and red 

 is far less conspicuous amid the ever-changing hues of the forest with 

 their glints of sunshine everywhere penetrating so as to form strong 

 contrasts and patches of light and shade. Hence all the females — one 

 at one time and one at another — get some protection, and that is sufficient 

 to enable them to live long enough to lay their eggs, when their work is 

 finished. Still, under bad conditions they only just managed to survive, 

 and as the colouring of some of these varying females very much 

 resembled that of the protected butterflies of the P. coon group (perhaps 

 at a time when the tails of the latter were not fully developed) any rudi- 

 ments of a prolongation of the wing into a tail added to the protective 

 resemblance, and was therefore preserved. The woodcuts of some of 

 these forms in my Malay Archipelago (i., p. 200) will enable those who 

 have this book at hand better to understand the foregoing explanation." 



