This shows the kindness of God to His creatures, even to us weak beings. His righteousness and justice as regards all animals are well known; for in the transient world there is among the various kinds of animals no individual being distinguished from the rest of the same species... all physical, psychical, and vital forces and organs that are possessed by one individual are found also in the other individuals. If any one is somehow different it is by accident, in consequence of some exception, and not by a natural property; it is also a rare occurrence.

Ch.12 - Guide for the Perplexed (c. 1190) - Part III