ANTS — SLAVE-MAKING. 67 



leaving the nest, and marched along the same road to a 

 tall Scotch fir tree, twenty-five yards distant, which they 

 ascended together, probably in search of aphides or cocci.' 

 And, according to Huber, the principal office of the slaves 

 in Switzerland is to search for aphides. 



Mr. Darwin also made the following observation : — 

 * Desiring to ascertain whether F. sanguinea could dis- 

 tinguish the pupae of F. fusca, which they habitually 

 make into slaves, and which are an unwarlike species, 

 from F. flava, which they rarely capture, and never 

 without a severe fight,' he found ' it was evident that 

 they did at once distinguish them ; ' for while ' they 

 eagerly and instantly seized the pupae of F, fusca, they 

 were much terrified when they came across the pupae, or 

 even the earth from the nest, of F. flava, and quickly ran 

 away ; but in about a quarter of an hour, shortly after 

 the little yellow ants had crawled away (from their nest 

 having been disturbed by Mr. Darwin), they took heart 

 and carried off the pupae.' 



Concerning the origin of this remarkable instinct, 

 Mr. Darwin writes : — 



As ants which are not slave-makers will, as I have seen, 

 carry off pupae of other species if scattered near their nests, it 

 is possible that such pupae originally stored as food might be- 

 come developed, and the foreign ants thus unintentionally 

 reared would then follow their proper instincts, and do what 

 work they could. If their presence proved useful to the species 

 which had seized them — if it were more advantageous to the spe- 

 cies to capture workers than to procreate them — the habit of 

 collecting pupae, originally for food, might by natural selection be 

 strengthened and rendered permanent for the very different 

 purpose of raising slaves. When the instinct was once acquired, 

 if carried out to a much less extent even than in our British 

 F. sanguinea, which, as we have seen, is less aided by its slaves 

 than the same species in Switzerland, natural selection might 

 increase and modify the instinct, always supposing such modifi- 

 cation to be of use to the species, until an ant was found as 

 abjectly dependent on its slave as is the Formica rufescens. 



Ants do not appear to be the only animals of which 

 ants make slaves ; for there seems to be at least one case 



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