The Wallace-Darwin Correspondence 



write does. Who would ever have dreamed that monkeys 

 influenced the distribution of pigeons and parrots! But 

 I have had a still higher satisfaction ; for I finished yester- 

 day your paper in the Linnean Transactions ^ It is admir- 

 ably 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 (p. 

 22), and should be grateful for 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, as I understand, 

 both female forms occur on the same island. I quite agree 

 with your distinction between dimorphic forms and varieties ; 

 but I doubt whether your criterion of dimorphic forms not 



1 "The Geographical Distribution and Variability of the Malayan Papi- 

 lionidse," Linn. Soc. Trans., xxv. 



« The passage referred to in this letter as needing further explanation is 

 the following : " The last six cases of mimicry are especially instructive, 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 be 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 lilceness to the favoured group will be rendered permanent by 

 hereditary transmission, and each successive variation which increases the 

 resemblance being preserved, and all variation 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 arc 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 additional protection. This they at once obtain 

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

 a comparative immunity from persecution." 



167 



