196 ON THE ORIGIN OF SPECIES 



The occasional habit of birds laying their eggs in 

 other birds' nests, either of the same or of a distinct 

 species, is not very uncommon with the Gallinaceae ; 

 and this perhaps explains the origin of a singular 

 instinct in the allied group of ostriches. For several 

 hen ostriches, at least in the case of the American 

 species, 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 in the case of the cuckoo, at intervals of two or three 

 days. This instinct, however, of the American ostrich 

 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 always lay their eggs in 

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

 markable than that of the cuckoo ; for these bees have 

 not only their instincts but their structure modified in 

 accordance with their parasitic habits ; for they do not 

 possess the pollen-collecting apparatus which would be 

 necessary if they had to store food for their own young. 

 Some species, likewise, of Sphegidae (wasp-like insects) 

 are parasitic on other species ; and M. Fab re 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 to feed 

 on, 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 the supposed case of the cuckoo, I can 

 see no difficulty in natural selection making an occa- 

 sional habit permanent, if of advantage to the species, 

 and if the insect whose nest and stored food are thus 

 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 



