Konrad LORENZ, 1963; English translation by Marjorie LATZKE, 1966: On Aggression, London: Methuen.
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As we know from Chapter VIII, there are animals totally devoid of aggression which keep together for life in firmly united flocks. One would think that such animals would be predestined to develop permanent friendships and brotherly unions of individuals, and yet these characteristics are never found among such peaceable herd creatures; their association is always entirely anonymous. A personal bond, an individual friendship, is found only in animals with highly developed intra-specific aggression, in fact this bond is the firmer, the more aggressive the particular animal and species is. There are few more aggressive fish than cichlids, few more aggressive birds than geese. The proverbially most aggressive all of all mammals, Dante's bestia senza pace, the wolf, is the most faithful of friends. Some animals are alternately territorial and aggressive, and non-aggressive and social, according to the season, and in these species every personal bond is limited to the period of aggressiveness. Undoubtedly the personal bond developed at that phase of evolution when, in aggressive animals, the co-operation of two or more individuals was necessary for a species-preserving purpose, usually brood tending. Doubtless the personal bond, love, arose in many cases from intra-specific aggression, by way of ritualization of a redirected attack or threatening. Since these rites are tied up with the person of the partner, and since they later become a need as independent instinct actions, they make the presence of a partner an absolute necessity and make the partner itself the 'animal with home valency' -- having the same emotional value as the home. [p. 185 - 186].
Intra-specific aggression is millions of years older than personal friendship and love. During long epochs of the earth's history there have been animals that were certainly extraordinarily fierce and aggressive. Nearly all reptiles of the present day are aggressive and it is unlikely that those of antiquity were less so. But the personal bond is known only in teleost fishes, birds and mammals, that is in groups that did not appear before the early Tertiary period. Thus intra-specific aggression can certainly exist without its counterpart, love, but conversely there is no love without aggression. [p. 186]
A Behaviour mechanism that must be sharply differentiated from intra-specific aggression is hate, the ugly little brother of love. As opposed to ordinary aggression, it is directed towards one individual, just as love is, and probably hate presupposes the presence of love: one can really hate only where one has loved and, even if one denies it, still does. [p. 186]
It is superfluous to point out the analogies between the social behaviour patterns of many animals, particularly wild geese, and those of man. All the truisms in our proverbs seem to apply equally to geese. As good evolutionists and Darwinians we can and must draw important conclusions from this fact. We know that the youngest common ancestors of birds and mammals were very low reptiles of the Upper Devonian and Lower Carboniferous strata, which certainly had no highly developed social life and were scarcely more intelligent than frogs. Thus the similarities in the social behaviour patterns of the greylag goose and of man are not derived from a common ancestor but have arisen by so-called convergent adaptation. They do not owe their existence to change; this would be an improbability which could be calculated, but could only be expressed in astronomical figures. [p. 187]
If, in the greylag goose and in man, highly complex norms of behaviour, such as falling in love, strife for ranking order, jealousy, grieving, etc., are not only similar but down to the most absurd details the same, we can be sure that every one of these instincts has a very special survival value, in each case almost or quite the same in the greylag and in man. [p. 187.]