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 : — 
4 Desiring to ascertain whether F. sanguined could dis- 
tinguish the pupae of F. fused, which they habitually 
make into slaves, and which are an unwarlike species, 
from F. flavd , which they rarely capture, and never 
without a severe fight,’ he found 6 it was evident that 
they did at once distinguish them ; ’ for while 4 they 
eagerly and instantly seized the pupae of F. fused , 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 
