SLAVE-MAKING INSTINCT. 239 



several hen birds unite and lay first a few eggs in one nest 

 and then in another ; and these are hatched by the males. 

 This instinct may probably be accounted for by the fact of 

 the hens laying a large number of eggs, but, as with the 

 cuckoo, at intervals of two or three days. The instinct, 

 however, of the American ostrich, as in the case of the. 

 Molothrus bonariensis, has not as yet been perfected ; for a 

 surprising number of eggs lie strewed over the plains, so that 

 in one day's hunting I picked up no less than twenty lost 

 and wasted eggs. 



Many bees are parasitic, and regularly lay their eggs in 

 the nests of other kinds of bees. This case is more remark- 

 able than that of the cuckoo ; for these bees have not only 

 had their instincts but their structure modified in accordance 

 with their parasitic habits ; for they do not possess the 

 pollen-collecting apparatus which would have been indis- 

 pensable if they had stored up food for their own young. 

 Some species of Sphegida? (wasp-like insects) are likewise 

 parasitic; and M. Fabre has lately shown good reason for 

 believing that, although the Tachytes nigra generally makes 

 its own burrow and stores it with paralyzed prey for its own 

 larvae, yet that, when this insect finds a burrow already 

 made and stored by another sphex, it takes advantage of the 

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

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

 culty in natural selection making an occasional habit perma- 

 nent, 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 (Polyerges) 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 certainly 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 larvaa. 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* 



