SOCIAL LIFE 303 



migration. The American bison, too, lived in great herds, and 

 the ground often thundered under their countless hoofs as they 

 stampeded from the northern feeding-grounds to the south. 



The causes of these animal migrations are, and always have 

 been, the old incentives, hunger and love ; and when human 

 tribes and peoples migrate it is the same want of sustenance 

 that drives them either to travel en masse, or to send off a 

 part of their surplus population, as Uhland has so magnificently 

 described in his Ver Sacrum. Yet there are other motives 

 as well, such as covetousness of the movable property of other 

 tribes and the appropriation of men themselves as labour 

 material. Thus arises war, the violent armed attack, which 

 even to-day occasionally initiates fighting amongst nations, and 

 in former days played a constant part in human affairs. " It is 

 no argument," says Darwin, 1 " against savage man being a social 

 animal, that the tribes inhabiting adjacent districts are almost 

 always at war with each other ; for the social instincts never 

 extend to all the individuals of the same species." Ancient 

 history describes whole nations conquered and taken captive 

 into slavery ; we know that up to Montezuma's reign in Mexico, 

 thousands were taken captive in war, some to be made slaves, 

 and some to be sacrificed. Many slave-dealers in Central 

 Africa used to organise regular men-hunts. We own, therefore, 

 with shame, that side by side with social instincts, the brute lies 

 dormant in man, and we scorn to console ourselves with the 

 thought that there are animals too that wage war and make 

 slave raids, namely, certain species of robber-ants : Polyergus 

 rufescens, Strongylognathus testaceus and Formica sanguinea. 



The workers of these robber-ants are not the progeny of the 

 common ancestress of the nest, but are stolen as larvae or 

 pupae from the nests of entirely different species, so that the 

 colony is a mixed one. Polyergus rufescens and Strongylo- 

 gnathus testaceus are distinguished by toothless jaws not 

 adapted to work ; the former steals the worker-brood of 

 Formica fusca and Auricularia, while Strongylognathus takes 

 the Tetramorium caespitum. The Formica rufescens, however, 

 takes the eggs and pupae of its two species of formica and 

 allows itself to share in the labours of the workers when they 



1 Darwin, loc. cit., i., p. 166. 



