I'm not quite sure how to summarize this. First, the goofy stuff -- one of the ant species discussed has the scientific name F. (for "Formica" -- I bet naming firms hire science majors as consultants all the time) flava. "Flava!"
The other thing that leaps out is that, in this four-page passage, Darwin waits until the last paragraph to note its effect on his theory:
By what steps the instinct of F. sanguinea [one of the slave-making species] originated I will not pretend to conjecture. But as ants which are not slave-makers will, as I have seen, carry off the pupæ of other species, if scattered near their nests, it is possible that such pupæ originally stored as food might become 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 this species to capture workers than to procreate them—the habit of collecting pupæ, originally for food, might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves.But why the three or four pages of observation beforehand -- more than seems necessary? Because Darwin thinks it's cool, that's why:
During the months of June and July, on three successive years, I watched for many hours several nests in Surrey and Sussex...One day I fortunately witnessed a migration of F. sanguinea from one nest to another, and it was a most interesting spectacle to behold the masters carefully carrying their slaves in their jaws instead of being carried by them, as in the case of F. rufescens.(I guess you have to be reared as a slave, otherwise it doesn't take.) I like that idea, Darwin, in his long beard, just lying on the ground, looking at ants, perhaps with his own servants to get him food and so forth.
Another day my attention was struck by about a score of the slave-makers haunting the same spot, and evidently not in search of food; they approached and were vigorously repulsed by an independent community of the slave-species (F. fusca); sometimes as many as three of these ants clinging to the legs of the slavemaking F. sanguinea. The latter ruthlessly killed their small opponents, and carried their dead bodies as food to their nest, twenty-nine yards distant; but they were prevented from getting any pupæ to rear as slaves. I then dug up a small parcel of the pupæ of F. fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly seized and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their late combat.