Nothing of the same sort takes place with mankind. This does not mean, however, that he cannot also be reduced to a state of domesticity.[215] Slaves, indeed, are nothing else; and all that is wanting in order to place them in comparison with animal domesticity, would be the history of a race of Ilotes, which has always been free from any mixture, and has continued so during a time equal to that which separates us from the first conquest of the dog, the sheep, and the ox, upon the high table-lands of Asia. Let us, then, leave all comparison[216] between man, free to come, to go, and to choose his own food,--and domestic animals. Let us return to those who live free, and say, once for all, that if we stop our progress with so many details concerning these comparisons, it is from a kind of respect for the character of certain learned men who have thus treated anthropological science. We believe very little in biology, or in demonstrations by _similarities_. Every animal, every organ, every anatomical element, has its own life, its own laws of birth, development, nutrition, and reproduction. At the commencement of science, everything is clear and easy, like the cellular theory, for instance, in the elements of anatomy; but every day the laws of life (we might say, the laws of nature) are multiplied and complicated; every morning the searcher after truth must expect to discover some phenomenon which will disturb the scientific belief of the night before. “Every evening,” said one of the masters of science, “our best prayer is to form afresh a synthesis of the sciences.”[217] Well, if modern anatomy has taught us that the initial phase of the development of the egg differs according to the animal,[218] even as nothing resembles less the development of certain bones of the face than that of their neighbours, how shall we dare to compare any animal with man?[219] Having said this, let us return to the influence of climate upon wild or free animals.