236 THE ORIGIN OF SPECIES 



Sir J. Lubbock made drawings for me, with the camera lucida, of 

 the jaws which I dissected from the workers of the several sizes. 

 Mr. Bates, in his interesting "Naturalist on the Amazons," has de- 

 scribed analogous cases. 



With these facts before me, I believe that natural selection, by 

 acting on the fertile ants or parents, could form a species which 

 should regularly produce neuters, all of large size with one form 

 of jaw, or all of small size with widely different jaws; or lastly, and 

 this is the greatest difficulty, one set of workers of one size and 

 structure, and simultaneously another set of workers of a different 

 size and structure; a graduated series having first been formed, as 

 in the case of the driver ant, and then the extreme forms having 

 been produced in greater and greater numbers, through the sur- 

 vival of the parents which generated them, until none with an in- 

 termediate structure were produced. 



An analogous explanation has been given by Mr. Wallace, of 

 the equally complex case of certain Malayan butterflies regularly 

 appearing under two or even three distinct female forms; and by 

 Fritz Miiller, of certain Brazilian crustaceans likewise appearing 

 under two widely distinct male forms. But this subject need not 

 here be discussed. 



I have now explained how, I believe, the wonderful fact of two 

 distinctly defined castes of sterile workers existing in the same nest, 

 both widely different from each other and from their parents, has 

 originated. We can see how useful their production may have been 

 to a social community of ants, on the same principle that the di- 

 vision of labor is useful to civilized man. Ants, however, work by 

 inherited instincts, and by inherited organs or tools, while man 

 works by acquired knowledge and manufactured instruments. But 

 I must confess, that, with all my faith in natural selection, I should 

 never have anticipated that this principle could have been efficient 

 in so high a degree, had not the case of these neuter instincts led 

 me to this conclusion. I have, therefore, discussed this case, at 

 some little but wholly insufficient length, in order to show the 

 power of natural selection, and likewise because this is by far the 

 most serious special difficulty which my theory has encountered. 

 The case, also, is very interesting, as it proves that with animals, 

 as with plants, any amount of modification may be effected by the 

 accumulation of numerous, slight, spontaneous variations, which 

 are in any way profitable, without exercise or habit having been 

 brought into play. For peculiar habits, confined to the workers of 

 sterile females, however long they might be followed, could not 



