INSTINCT 219 



ated instincts, but as small consequences of one general law, 

 namely, transition?" 



Various birds, as has already been remarked, occasionally lay 

 their eggs in the nests of other birds. This habit is not very un- 

 common with the Gallinaceae, and throws some light on the sin- 

 gular instinct of the ostrich. In this family 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 remarkable 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 indispensable if they had stored up food for their own 

 young. Some species of Sphegidae (wasp-like insects) are likewise 

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

 ing 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 difficulty in natural selection making an 

 occasional habit permanent, 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 ex- 

 tinct in a single year. The males and fertile females do no work of 

 any kind, and the workers or sterile females, though most ener- 

 getic and courageous in capturing slaves, do no other work. They 

 are incapable of making their own nests, or of feeding their own 

 larvae. When the old nest is found inconvenient, and they have to 



