WHY DOGS TAKE PLEASURE IN SMELLING AT EACH OTHER.
This animal has a horror of the poor, because they eat poor food, and it loves the rich, because they have good living and especially meat. And the excrement of animals always retains some virtue of its origin as is shown by the faeces ...
Now dogs have so keen a smell, that they can discern by their nose the virtue remaining in these faeces, and if they find them in the streets, smell them and if they smell in them the virtue of meat or of other things, they take them, and if not, they leave them: And to return to the question, I say that if by means of this smell they know that dog to be well fed, they respect him, because they judge that he has a powerful and rich master; and if they discover no such smell with the virtue of meet, they judge that dog to be of small account and to have a poor and humble master, and therefore they bite that dog as they would his master.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.