18641869] POLYMORPHISM 265 



To A. R. Wallace. Letter 189 



Down, January 22nd, 1866. 



I thank you for your paper on pigeons, 1 which interested 

 me, as everything that you write does. Who would ever 

 have dreamed that monkeys influenced the distribution of 

 pigeons and parrots ! But I have had a still higher satis- 

 faction, for I finished your paper yesterday in the Linnean 

 Transactions? It is admirably done. I cannot conceive that 

 the most firm believer in species could read it without 

 being staggered. Such papers will make many more converts 

 among naturalists than long-winded books such as I shall 

 write if I have strength. I have been particularly struck 

 with your remarks on dimorphism ; but I cannot quite 

 understand one point 3 (p. 22), and should be grateful for 



1 "On the Pigeons of the Malay Archipelago" (The Ibis, October, 

 1865). Mr. Wallace points out (p. 366) that "the most striking super- 

 abundance of pigeons, as well as of parrots, is confined to the Australo- 

 Malayan sub-region in which . . . the forest-haunting and fruit-eating 

 mammals, such as monkeys and squirrels, are totally absent." He points 

 out also that monkeys are " exceedingly destructive to eggs and young 

 birds." 



2 Linn. Soc. Trans., XXV. : a paper on the geographical distribution 

 and variability of the Malayan Papilionidae. 



3 The passage referred to in this letter as needing further explanation 

 is the following : " The last six cases of mimicry are especially instruc- 

 tive, because they seem to indicate one of the processes by which 

 dimorphic forms have been produced. When, as in these cases, one sex 

 differs much from the other, and varies greatly itself, it may happen that 

 individual variations will occasionally occur, having a distant resemblance 

 to groups which are the objects of mimicry, and which it is therefore 

 advantageous to resemble. Such a variety will have a better chance of 

 preservation ; the individuals possessing it will be multiplied ; and their 

 accidental likeness to the favoured group will be rendered permanent by 

 hereditary transmission, and each successive variation which increases 

 the resemblance being preserved, and all variations departing from the 

 favoured type having less chance of preservation, there will in time result 

 those singular cases of two or more isolated and fixed forms bound 

 together by that intimate relationship which constitutes them the sexes 

 of a single species. The reason why the females are more subject to this 

 kind of modification than the males is, probably, that their slower flight, 

 when laden with eggs, and their exposure to attack while in the act of 

 depositing their eggs upon leaves, render it especially advantageous for 

 them to have some additional protection. This they at once obtain by 



