1. _Classification and Affinity._—The sixteenth-century herbalists and still earlier writers (see p. 17) recognised a property of _affinity_, by which plants were associated in natural groups. Bock ( 1546) tried to bring together all plants which are related (verwandt) to one another, but similarity of any kind was with him a proof of affinity; it did not shock him to place the dead nettles next to the stinging nettles. L'Obel gave names to several families of flowering plants which are still admitted as natural. Ray spoke of the _affinity_ (cognatio) between plants, and his affinity was a thing not to be violated for the sake of practical convenience or logical rules, but he was unable to explain what he meant by it. Linnæus tried to illustrate affinity between plants by contiguous provinces on a map, a better metaphor than the linear scale, for the scale can only express affinity on two sides, while the map can express affinity on many. His practical experience of classification taught him a truth, shocking at first sight to the logician[44]—viz., that the characters which serve for the definition of one genus may be useless for the definition of the next, and he laid it down that the characters do not make the genus, but the genus the characters. After Linnæus we find for a long time no advance in the philosophy of natural classification. Cuvier (1816) is even retrograde, for he sets aside the maxims of Linnæus, maintains that adaptive characters (characters closely related to the conditions of life) are relatively constant, and that large groups should be defined by characters drawn from organs of great physiological importance. These decisions of his are repudiated by later naturalists. The key to the affinity puzzle which had so long baffled thinking naturalists was at last supplied by Darwin, who explained that "the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, all true classification being genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike."[45] Natural groups, large or small, result from the long-continued operation of divergence, the survival of some, and the extinction of others; they are to be respected as facts; they are not created by definitions, which only serve to indicate and remind; any character, however trifling, will suffice, if only it is constant and distinctive.