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By Root 572 0

romping and racing together, playfully nipping each other's legs, rearing up on their hindlegs in mock battles. The old horse was always gentle with the youngster, enduring with admirable patience a surprising amount of nonsense. This friendship continued unbroken until the colt passed his fourth year and reached maturity. Then the young stallion turned against the old horse five times his age, trying to drive him from the pasture and the mares, viciously biting and kicking him if he resisted. It became necessary to keep the stallion and the gelding in separate fields, in order to preserve the peace of the farm. This altered relationship was caused wholly by changes in the stallion; the older horse remained

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the same and gave the younger no provocation. Which was more central to the stallion's nature, his early friendliness to the old horse or the enmity that replaced it? Or did his nature undergo a radical change as he matured, so that both the friendly and the belligerent attitudes were equally expressive of his inmost disposition at the time when each prevailed?

Among birds we witness many examples of these opposite modes of behavior. Some species that, through much of the year, associate in compact flocks lose their sociability at the approach of the nesting season, when each male establishes himself on a separate patch of land and will fight any of his erstwhile companions who dares to intrude upon it. And there is a limit to the gregariousness even of species that not only flock when not engaged in reproduction but also nest in crowded colonies on cliffs or islands or in treetops. Breeding penguins, albatrosses, and gulls repulse with their bills neighbors that press too close to their nests, with the result that each pair maintains a small unoccupied space around its nest, and this ensures a rather uniform distribution of breeding pairs over the available area. Swallows perching on wires seek one another's company yet resist too close crowding, so that neighboring individuals are separated by a distance that is determined by how far each can peck without budging from its chosen spot. Birds of many kinds feed the young of other parents, of their own or even of alien species (Skutch 1987). Yet some parent birds, including several kinds of jays and toucans, are not above snatching the young from nests of other species, perhaps bringing the mangled corpses to feed their own carefully attended families. Could we say that the social or the antagonistic instincts, the helpful or tyrannical attitudes toward neighbors, are more central to the character of a bird? Or are both equally expressive of its inmost nature?

Similar contrasts are found in the behavior of insects. Ants are among the most social of animals, dwelling in populous colonies that rival human cities in their teeming inhabitants and complex organization. The workers tenderly nurse and feed the helpless larvae, caress one another with their antennae, and pass food from mouth to mouth. Yet not only do they battle fiercely with ants of

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Chinstrap Penguins, Pygoscelis antarctica

other species; they are almost equally hostile to other colonies of their own kind, and they may devour the offspring of vanquished rivals. Like humans at an early stage of moral development, they have one pattern of behavior for their family or clan and another for all outsiders. The two modes of behavior lie at opposite poles: treatment of members of the clan often evinces lack of respect for privacy and individuality that would be intolerable to cultured people; treatment of those beyond the narrow pale of immediate kinship reveals brutality that outrages finer feelings. Which of these two modes of behavior, that toward one's own tribe or that toward outsiders, is more central to the nature of ants and of men?

In trying to answer this question, we shall follow three lines of approach. First we shall see what light the study of animal behavior can shed upon our problem. Next we shall ask whether our understanding of evolution can help