DARWIN AND BERGSON ON EVOLUTION 



unpalatable Model with a white bar crossing the fore wing, such a 

 variety as that represented in Fig. 1 1 might well form the starting- 

 point for a new mimetic form. If, in part of the range, such a 

 model predominated over Planemce of the type of the male PI. alcinoe 

 (Fig. 2) we should expect the new form more or less completely to 

 supplant the ordinary western female (Figs. 9 and 10). No such 

 Model exists in the Lagos district, and therefore the variety repre- 

 sented in Fig. 11 is at a disadvantage as compared with the ordinary 

 females and is extremely unlikely to escape extinction. 



The comparison between the eastern and western forms of a 

 single mimetic Acrcea, therefore, leads to important conclusions. 

 The essential cause of the detailed likeness of Mimicry is Natural 

 Selection — " mechanism," as it is called by Bergson. We do not 

 know how or why the first essential variation arose, nor do we know 

 the origin of the later variations out of which the detailed mimetic 

 pattern was built. We do not know, and therefore the " elan vital," 

 the " rush of life " — an origin that can only be disproved by finding 

 an origin that is capable of proof — may be accepted by those to 

 whom it appeals. The majority of scientific men will probably 

 prefer to follow Darwin and rest content with an admission of 

 ignorance. But when ignorance is replaced by the fullest knowledge 

 the role of variation will remain the same ; it provides the rough 

 stones out of which the finished building is erected by selection — to 

 use a metaphor employed by Darwin in Variation under Domestica- 

 tion. Everything that excites our wonder and admiration in the 

 close and detailed mimicry of the male of a very different species by 

 the female of alciope has been built up out of small differences by 

 the accumulative power of selection. Even the first essential varia- 

 tion which produced a rough likeness to the Model, is, as a variation, 

 a very small affair, — a minute transformation, probably chemical 

 in nature, which involved the change from fulvous to white over 

 part of the hind wing. And this variation would have been of no 

 more significance to the species than the western variety shown in 

 Fig. 11 if the pattern of the male Planema macarista had not existed 

 at Entebbe. 



The beginnings of a mimetic resemblance between species that 

 were originally very unlike has been a great difficulty to many 

 naturalists, but I believe that the example discussed in the present 



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