TRANSMUTABLE. 167 



ants, individuals differing not only from the fertile 

 ones, but from themselves, and that in a remark- 

 able degree, and that they do not graduate into 

 each other, but are well-defined. Some carry 

 shields on their heads, others have differently- 

 developed jaws; one caste never leaves the nest, 

 being fed by the workers of another caste! 



All these difficulties Mr. Darwin gets over by 

 a series of assumptions. He supposes, beginning 

 at the separation of the neuter from the fertile 

 species, that the variation did not all come 

 in the neuter at the same time, but only in a 

 few. Thus they would appear as he finds them 

 with a gradation of form, which is the fact, 

 according to his researches; and he ultimately 

 comes to the conclusion that "natural selection, 

 by acting on the fertile parents, could form a 

 species which should regularly produce neuters, 

 either all of large size, with one form of jaw, 

 or all of small size, with jaws having a widely 

 different structure; or, lastly, and this is our 

 climax of 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 been first formed, 

 as in the case of the driver ant; and then the 

 extreme forms, from being the most useful to 

 the community, having been produced in greater 

 and greater numbers, through the natural selec- 

 tion of the parents which generated them ; until 

 none with an intermediate structure were pro- 

 duced." (Page 241.) 



