v.] IN THE THEORY OF NATURAL SELECTION. 287 



appeared in the different parts of different individuals, the 

 number of differences would increase with each sexually pro- 

 duced 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 com- 

 binations, and this latter consequence is as important as the 

 former. 



The former consequence can hardl}?- make itself felt in any 

 existing species, because in them every part already possesses 

 its peculiar character in all individuals. The second conse- 

 quence is, however, more important, viz. the production of new 

 combinations 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 breed- 

 ing, but that probably several, and perhaps very many charac- 

 ters, are simultaneously 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 uncon- 

 scious of what it is doing. Hence a mechanism must exist in 

 the insect's brain which compels it to assume this attitude, and 



