338 SPECIAL INSTINCTS. [Chap. VHI. 



few esjcjs 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 ot the Molothrus bona- 

 riensis, 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 e^srs. 



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 believing that, although the 

 Tachytes nigra generally makes its own burrow and 

 stores it with paralysed prey for its own larvae, yet that, 

 when this insect finds a burrow alreadv 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 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 



