238 EVOLUTION OF TO-DAY. 



entirely different modes of life, but the form of 

 their body is entirely different. Indeed, workers 

 differ more from the males and females of the same 

 colony than do different species from each other. 

 Nor is this all, for the workers may not only differ 

 from the males and females, but also from each 

 other to such an extent that several castes of work- 

 ers are sometimes found. In some cases there are 

 three castes of these workers, so that the colony 

 contains five sorts of individuals. Nor do these 

 castes, as a rule, graduate into each other ; they are 

 perfectly distinct, as much so indeed as any two 

 genera of the same family. 



Unquestionably all of this differentiation is of 

 advantage to the colony ; but how is natural selec- 

 tion to explain the origin of these castes of workers ? 

 The difficulty lies in the fact that the workers are 

 sterile, and the possibility of their transmitting 

 favorable variations to another generation is out of 

 the question. If these individuals had been fertile, 

 the explanation would have been that they were 

 acquired slowly by natural selection, which preserved 

 the favored individual and thus caused their peculi- 

 arities to be transmitted to other generations. But 

 the insects are not fertile, never have any descend- 

 ants, and consequently cannot transmit their peculi- 

 arities, and it would therefore seem that they could 

 not be preserved. How then could these castes 

 have arisen? The only answer is to consider the 

 whole colony as an individual, and the various castes 

 as organs. In other animals it is not difficult to see 

 how any organ can be modified. If it is of advan- 



