M. Jourdain, under the head "Espèce," in his _Dictionnaire des Termes des Sciences Naturelles_, after citing a long list of definitions from leading authors, concludes with the following remarks, which, as the question now stands before the world, places the term species just where it should be:-- "It is evident that we can, among organized bodies, regard as a _species_ only such a collection of beings as resemble each other more than they resemble others, and which, by a consent more or less unanimous, it is agreed to designate by a common name; for a _species_ is but a simple _abstraction of the mind_, and not a group, exactly determined by nature herself, as ancient as she is, and of which she has irrevocably traced the limits. It is in the definition of species that we recognize how far the influence of ideas adopted without examination in youth is powerful in obscuring the most simple ideas of general physics." Although not written with the expectation of publication, I will take the liberty of publishing the following private letter just received from Prof. Leidy. He has not appeared at all in this controversy before the public, and we may safely say that no one can be better qualified than he is to express an opinion on this question of species. "With all the contention about the question of what constitutes a _species_, there appears to be almost no difficulty, comparatively, in its practical recognition. Species of plants and animals are daily determined, and the characters which are given to distinguish them are viewed by the great body of naturalists as sufficient. All the definitions, however, which have been given for a species, are objectionable. Morton says: 'A species is a primordial organic form.' But how shall we distinguish the latter? How can it be proved that any existing forms primordially were distinct? In my attempted definition, I think, I fail, for I only direct how species are discovered. "According to the practical determination of a species by naturalists, in a late number of the _Proceedings_ of our Academy (vol. vii. p. 201), I observe: 'A species is a mere convenient word with which naturalists empirically designate groups of organized beings possessing characters of comparative constancy, as far as historic experience has guided them in giving due weight to such constancy.'