v.] IiY THE THEORY OF NATURAL SELECTIOX. 315 



fail to detect them when at rest, — from this very, point any 

 further improvement in the similarity to leaves would cease, 

 because the advantage to be gained from any such improve- 

 ment would cease at the same time. 



Such reciprocal intensification of adaptive characters appears 

 to me to have been one of the most important factors in the 

 transformation of species : it must have persisted through long 

 series of species during phj'logeny : it must have affected the 

 most diverse parts and characters in the most diverse groups 

 of organisms. 



In certain large butterflies of the Indian and African forests 

 — Kallima paralecta., K. inacJiis, and K. albofasciata — it has been 

 frequently pointed out that the deceptive resemblance to a 

 leaf is so striking that an observer who has received no hint 

 upon the subject believes that he sees a leaf, even when 

 he is looking at the butterfly very closely. The similarity is 

 nevertheless incomplete ; for out of sixteen specimens in the 

 collections at Amsterdam and Leyden, I could not find a 

 single one which had more than two lateral veins on one side 

 of the mid-rib of the supposed leaf, or more than three upon 

 the other side ; while about six or seven veins should have been 

 present on each side. But from two to three lateral veins are 

 amply sufficient to produce a high degree of resemblance ; in 

 fact so much so that it is a matter for wonder as to how it has 

 been possible for such a relatively perfect copy to have been 

 produced ; or how the sight of birds can have become so 

 highly developed that while flying rapidly they could perceive 

 the vein-like markings ; or to state the case more accurately, 

 that they could detect those individuals with a less number of 

 veins than others. It is possible that the process of increase 

 in resemblance is stiU proceeding in the species of the genus 

 Kallima ; at all events, I was struck by the rather strong 

 individual differences in the markings of the supposed leaf. 



On the other hand, the cause of the increase in length of the 

 tubular corolla and of the butterfly's ' tongue,' lies neither in 

 the flower nor in the butterfly, but it is to be found in those 

 other insects which visit the flower and steal its honey without 

 being of any assistance in cross-fertilization. It may be stated 

 shortly, that non-tubular corollas, with the honey freely ex- 

 posed—for it must be assumed the ancestral form was of this 



