IN THE THEORY OF NATURAL SELECTION. 307 



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 indi- 

 viduals with a less number of veins than others. It is possible that 

 the process of increase in resemblance is still 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 exposed for it must be 

 assumed the ancestral form was of this kind gradually developed 

 into corollas with the honey deeply concealed. The whole process 

 was presumably first started by the flower, for the gradual with- 

 drawal of the honey to greater depths conferred the advantage of 

 protection from rain (Hermann Miiller), while larger quantities of 

 honey could be stored up, and this would also increase the num- 

 ber of insects visiting the flower and render their visits more 

 certain. As soon as this withdrawal occurred, the mouth-parts of 

 insects began to be subjected to a selective process whereby these 

 organs in some of them were lengthened at the same rate as that at 

 which the honey was withdrawn. When once the process had 

 begun, its continuance was ensured, for as soon as flower- frequent- 

 ing insects were divided into two groups with short and with long 

 mouth-parts respectively, a further increase in the length of the 

 corolla-tube necessarily took place in all those flowers which were 

 especially benefited by the assured visits of a relatively small 

 number of species of insects, viz., those flowers in which cross- 



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