But botanists could not rest content with merely naming natural groups; it was necessary to translate the indistinct feeling, which had suggested the groups of Linnaeus and Bernard de Jussieu, into the language of science by assigning clearly recognised marks; and this was from this time forward the task of systematists from Antoine Laurent de Jussieu and de Candolle to Endlicher and Lindley. But it cannot be denied, that later systematists repeatedly committed the fault of splitting up natural groups of affinity by artificial divisions and of bringing together the unlike, as Cesalpino and the botanists of the 17th century had done before them, though continued practice was always leading to a more perfect exhibition of natural affinities. But while natural relationship was thus becoming more and more the guiding idea in the minds of systematists, and the experience of centuries was enforcing the lesson, that predetermined grounds of classification could not do justice to natural affinities, the fact of affinity became itself more unintelligible and mysterious. It seemed impossible to give a clear and precise definition of the conception, the exhibition of which was felt to be the proper object of all efforts to discover the natural system, and which continued to be known by the name of affinity. A sense of this mystery is expressed in the sentence of Linnaeus: ‘It is not the character (the marks used to characterise the genus) which makes the genus, but the genus which makes the character;’ but the very man, who first distinctly recognised this difficulty in the natural system, helped to increase it by his doctrine of the constancy of species. This doctrine appears in Linnaeus in an unobtrusive form, rather as resulting from daily experience and liable to be modified by further investigation; but it became with his successors an article of faith, a dogma, which no botanist could even doubt without losing his scientific reputation; and thus during more than a hundred years the belief, that every organic form owes its existence to a separate act of creation and is therefore absolutely distinct from all other forms, subsisted side by side with the fact of experience, that there is an intimate tie of relationship between these forms, which can only be imperfectly indicated by definite marks. Every systematist knew that this relationship was something more than mere resemblance perceivable by the senses, while thinking men saw the contradiction between the assumption of an absolute difference of origin in species (for that is what is meant by their constancy) and the fact of their affinity. Linnaeus in his later years made some strange attempts to explain away this contradiction; his successors adopted a way of their own; various scholastic notions from the 16th century still survived among the systematists, especially after Linnaeus had assumed the lead among them, and it was thought that the