280 THE SIGNIFICANCE OF SEXUAL KEPKODUCTION 



of differences would increase with each sexually produced generation, 

 until all the parts in which the variations occurred had received 

 a peculiar character in all individuals. 



Moreover sexual reproduction not only adds to the number of 

 existing differences, but it also brings them into new combina- 

 tions, and this latter consequence is as important as the former. 



The former consequence can hardly make itself felt in any 

 existing species, because in them every part already possesses its 

 peculiar character in all individuals. The second consequence is, 

 however, more important, viz. the production of new combina- 

 tions of individual characters by sexual reproduction ; for, as 

 Darwin has already pointed out, we must imagine that not only 

 are single characters changed in the process of breeding, but that 

 probably several, and perhaps very many characters, are simul- 

 taneously modified. No two species, however nearly allied, differ 

 from each other in but a single character. Even our eyesight, 

 which has by no means reached the highest pitch of development, 

 can always detect several, and often very many points of difference ; 

 and if we possessed the powers necessary for making an absolutely 

 accurate comparison, we should probably find that everything is 

 different in two nearly allied species. 



It is true that a great number of these differences depend upon 

 correlation, but others must depend upon simultaneous primary 

 changes. 



A large butterfly (Kallima paralecta), found in the East Indian 

 forests, has often been described in its position of rest as almost 

 exactly resembling a withered leaf ; the resemblance in colour 

 being aided by the markings which imitate the venation of a leaf. 

 These markings are composed of two parts, the upper of which is 

 on the fore-wings, while the lower one is on the hind wings. The 

 butterfly when at rest must therefore keep the wings in such a 

 position that the two parts of each marking exactly correspond, 

 for otherwise the character would be valueless ; and as a matter of 

 fact the wings are held in the appropriate position, although the 

 butterfly is of course unconscious of what it is doing. Hence a 

 mechanism must exist in the insect's brain which compels it to 

 assume this attitude, and it is clear that the mechanism cannot have 

 been developed before the peculiar manner of holding the wings 

 became advantageous to the butterfly, viz. before the similarity 



