v.] lA^ THE THEORY OE 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 hardly 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 

 nearl}^ 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 {Kalli ma para lecta), 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 



