564 THE ORIGIN OF SPECIES 



and becomes for the occasion parasitic. In this case, as 

 with tliat of the Molothrus or cuckoo, I can see no diffi- 

 culty in natural selection making an occasional habit per- 

 manent, if of advantage to the species, and if the insect 

 whose nest and stored food are feloniously appropriated 

 be not thus exterminated. 



Slave-making instinct. — This remarkable instinct was 

 first discovered in the Formica (Poljerges) rufescens 

 by Pierre Huber, a better observer even than his 

 celebrated father. This ant is absolutely dependent 

 on its slaves; without their aid, the species would cer- 

 tainly become extinct in a single year. The males 

 and fertile females do no work of any kind, and the 

 workers or sterile females, though most energetic and 

 courageous in capturing slaves, do no other work. 

 They are incapable of making their own nests, or of 

 feeding their own larvaj. When the old nest is found 

 inconvenient, and they have to migrate, it is the slaves 

 which determine the migration, and actually carry their 

 masters in their jaws. So utterly helpless are the mas- 

 ters, that when Huber shut up thirty of them without a 

 slave, but with plenty of the food which they like best, 

 and with their own larv» and pupae to stimulate them to 

 work, they did nothing; they could not even feed them- 

 selves, and many perished of hunger. Huber then intro- 

 duced a single slave (F. fusca), and she instantly set to 

 work, fed and saved the survivors; made some cells and 

 tended the larvae, and put all to rights. What can be 

 more extraordinary than these well-ascertained facts ? H 

 we had not known of any other slave-making ant, it 

 would have been hopeless to speculate how so wonderful 

 an instinct could have been perfected. a 



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