"This is the secret of species. Having always before our eyes numbers of single groups of animals or vegetables, for one reason or other we hardly consider them as descendants of one only primitive pair; we call what we see a _species_; if there are differences observable among these groups, they are _the races of this species_. "Observe that, in my explanations, I have not given for a certainty the existence of one primitive source for rabbits and turkeys. I do not affirm the fact, as neither observation nor experience--the two guides we must follow in science--teaches anything in this regard. I simply say, all are as though descended from one only primitive pair. "In summing up the question of _species_ _and_ race, it is not difficult to understand nor to believe, when we know the savage type, and have historical authority which permits us to attach to this type the groups, more or less different, according to their domestication. But when we are ignorant of the savage type, and in want of historical authority, the question becomes extremely difficult at first, because the differences we find in one and the other, and above all, in the different groups, could hardly be considered other than such as characterize different species. "Happily, physiology comes then to our relief. We find in this science one of those grand and beautiful general laws, which holds and maintains the established order, and which we admire the more we study it. It is the law of _crossing_, which governs animals as well as vegetables, and is, consequently, applicable to man himself. "We understand by the term _crossing_, all unions effected between animals belonging to different species or to two different races. The result of the unions obeying these laws is, that if the animals of _different species_ unite, in the majority of cases the union is barren. "Thus, for example, it has been tried a million of times all over the world, to effect a union between rabbits and hares. It is said to have succeeded twice. "Much doubt is cast upon this operation by the testimony of a man of undoubted talent, habituated to experiments, who believed these unions to be possible. Though availing himself of all possible means of proof, he was not more fortunate than his predecessors, Buffon and the brothers Geoffrey St. Hilaire. Thus, the rabbit and the hare, though presenting a great conformity in appearance, cannot reproduce. Such is the general result of crossing two different _species_. {73}