actually published until 1789, when all the 65 orders devised by him were included in the _Genera Plantarum secundum Ordines Naturales disposita_ of his famous nephew Antoine Laurent de Jussieu. Here at last was a fairly complete natural system, consisting of one hundred natural orders arranged in fifteen classes, within the three great subdivisions, _Acotyledones_, _Monocotyledones_, _Dicotyledones_, constituting the framework of that which is accepted at the present day. It has undergone many modifications, of which the first and most important were those effected by A. P. de Candolle (_Théorie Élémentaire_, 1813), who, while he improved upon Jussieu in various ways, made the unfortunate, but happily unsuccessful, attempt to substitute "_Endogenae_" for "_Monocotyledones_" and "_Exogenae_" for "_Dicotyledones_." The system has proved itself capable of expansion to accommodate all the new genera and natural orders that have since been established: it has justified itself as a natural classification in its susceptibility to development in precision as well as in extent, and in that it has survived the many experiments made upon it during the first century of its existence. The glory of this crowning achievement belongs to Jussieu: he was the capable man who appeared precisely at the psychological moment, and it is the men that so appear who have made, and will continue to make, all the great generalisations of science. Jussieu's achievement, like other great scientific achievements, would have been impossible without the labours and failures of his predecessors, of which some account has been given in this lecture. He himself attributed much of his success to the work of Tournefort, but it is clear that he owed at least as much to Ray: if he learned from the former the systematic importance of the gamopetalous and of the polypetalous corolla, he gleaned from the latter the value of the cotyledonary characters upon which are based his three primary subdivisions of the Vegetable Kingdom.