Now as Linnaeus practically applied the idea of species with a sagacity which has never been surpassed and rarely equaled and indeed may be said to have fixed its received meaning in natural history, it may well be inferred that in the phrase above cited he did not so much undertake to frame a logical definition, as to set forth the idea which, in his opinion, lay at the foundation of species; on which basis A.L. Jussieu did construct a logical definition--"Nunc rectius definitur perennis individuorum similium successio continuata generatione renascentium." The fundamental idea of species, we would still maintain, is that of a chain of which genetically-connected individuals are the links. That, in the practical recognition of species, the essential characteristic has to be inferred, is no great objection--the general fact that like engenders like being an induction from a vast number of instances, and the only assumption being that of the uniformity of Nature. The idea of gravitation, that of the atomic constitution of matter, and the like, equally have to be verified inferentially. If we still hold to the idea of Linnaeus, and of Agassiz, that existing species were created independently and essentially all at once at the beginning of the present era, we could not better the propositions of Linnaeus and of Jussieu. If; on the other hand, the time has come in which we may accept, with De Candolle, their successive origination, at the commencement of the present era or before, and even by derivation from other forms, then the "in principio" of Linnaeus will refer to that time, whenever it was, and his proposition be as sound and wise as ever. In his "Geographie Botanique" (ii., 1068-1077) De Candolle discusses this subject at length, and in the same interest. Remarking that of the two great facts of species, viz., likeness among the individuals, and genealogical connection, zoologists have generally preferred the latter,[IV-4] while botanists have been divided in opinion, he pronounces for the former as the essential thing, in the following argumentative statement: