See how man is served by creatures possessing opposite qualities.

But seek out completely the creatures as related to one another, and seek them out again as related to man, and see that creatures which are not all useful to one another are all useful to man, and those which are thought to be strange (to one another) are all related to the service of man. For how is the bull like the horse in running? And (yet) the swiftness of the horse and the slowness of the ox are both useful to man. [P.108] And how is the winter like the summer in comparison? And (yet) the coldness of the one and the heat of the other are a source of help to man. And how are fierce things like gentle things? And (yet) they both do one common service. And, therefore, their histories are too long and their numbers are too great, and their kinds are too abundant that we should labour (?) to complete the comparison of them, but some tastes (i.e., specimens) of them are sufficient to convince concerning them all. [lxxxiii]