1. There was a time when animals did not exist, and also a time when they began to appear. Each species appeared as soon as the Earth acquired the conditions necessary to its existence; this we positively know. But how were the first individuals of each species formed? A first couple must have been formed. Many beings have sprung from them; but this first couple, whence did they spring? This is one of the mysteries of the beginning, about which one can form only hypothesis. If science cannot yet completely solve the problem, it can at least put us on the way to a solution.