4th. After what we have said, discussion is useless with those naturalists who only base the distinction of species upon external characters. They forget all the experiments made from Buffon to the two Geoffroys, from Koelreuter to M. Naudin; they forget the innumerable observations made in our orchards, gardens and stables. To refuse to abandon morphological considerations, and to neglect the data of physiology and the lessons of filiation, is clearly going further back than Ray and Tournefort, and all discussion becomes impossible. 5th. Some of our opponents allow that things are now what we think them to be. “But,” say they, “it is possible that at some other time it was different.” What answer can be given to those who base their arguments upon _possibilities_? Is modern science composed of possibilities? 6th. Naturalists have often been reproached with multiplying the definitions of species. From the variety of terms employed by them in expressing ideas, it has been inferred that they were not agreed as to the ideas themselves. We may easily convince ourselves of their mistake, if we give these definitions a careful reconsideration. We shall see that their several authors have only endeavoured to express with greater clearness and precision, the double idea resulting from the facts of resemblance and filiation. In reality, divergencies only begin where experiment and observation cease. It is this which caused Isidore Geoffroy, however interested he might be in discussions of this nature, to remark—“Such are Species and Races, not only for one of the schools into which naturalists are divided, but for all.” 7th. It has been asserted that the distinction of species and race rests upon a syllogistic circle; that naturalists decided _à priori_ upon calling all those groups incapable of intercrossing, _species_, and all those amongst which crossing was possible, _races_. To appeal to the difference of the phenomena presented by the hybrids and mongrels is therefore only solving the question by the question.—This is an historical error. Naturalists came into contact with species, races and varieties, before they gave names to them. It was by experiment and observation that they learnt to distinguish them. _Knowledge of facts_ preceded _terminology_.