From John Cassian –
There are many kinds of friendship and companionship which unite men in very different ways in the bonds of love:
For some a previous recommendation makes to enter upon an intercourse first of acquaintance and afterwards even of friendship. In the case of others some bargain or an agreement to give and take something has joined them in the bonds of love. Others a similarity and union of business or science or art or study has united in the chain of friendship, by which even fierce souls become kindly disposed to each other, so that those, who in forests and mountains delight in robbery and revel in human bloodshed, embrace and cherish the partners of their crimes.
But there is another kind of love, where the union is from the instincts of nature and the laws of consanguinity, whereby those of the same tribe, wives and parents, and brothers and children are naturally preferred to others, a thing which we find is the case not only with mankind but with all birds and beasts. For at the prompting of a natural instinct they protect and defend their offspring and their young ones so that often they are not afraid to expose themselves to danger and death for their sakes.
Indeed those kinds of beasts and serpents and birds, which are cut off and separated from all others by their intolerable ferocity or deadly poison, as basilisks, unicorns and vultures, though by their very look they are said to be dangerous to every one, yet among themselves they remain peaceful and harmless owing to community of origin andfellow-feeling. But we see that all these kinds of love of which we have spoken, as they are common both to the good and bad, and to beasts and serpents, certainly cannot last for ever. For often separation of place interrupts and breaks them off, as well as forgetfulness from lapse of time, and the transaction of affairs and business and words. For as they are generally due to different kinds of connexions either of gain, or desires, or kinship, or business, so when any occasion for separation intervenes they are broken off. (Conferences 16.2)