work were the Genera Plantarum of Linnaeus (1737—1764) and of Jussieu (1789), to which may be added that of Endlicher (1836—1840). But all of these were written while the number of known genera and species was smaller. The difficulty of the task of Bentham and Hooker was greatly enhanced by their wider knowledge. But their Genera Plantarum is on that account a nearer approach to finality. Hitherto its supremacy has not been challenged.
The fourth of the great systematic works of Hooker mentioned above was the Index Kewensis. It was produced upon the plan and under the supervision of Sir Joseph by Dr Daydon Jackson and a staff of clerks. The publication began in 1893, and successive supplements to its four quarto volumes are still appearing at intervals. The expense was borne by Charles Darwin. The scheme originated in the difficulty he had found in the accurate naming of plants. For "synonyms" have frequently been given by different writers to the same species, and this had led to endless confusion. The object of the Index was to provide an authoritative list of all the names that have been used, with reference to the author of each and to its place of publication. The habitat of the plant was also to be given. The correct name in use according to certain well-recognised rules of nomenclature was to be indicated by type different from that of the synonyms superseded by it. The only predecessor of such an Index was Steudel's Nomenclator Botanicus, a book greatly prized by Darwin, though long out of date. He wished at first to produce a modern edition of Steudel's Nomenclator. This idea was, however, amended, and it was resolved to construct a new list of genera and species, founded upon Bentham and Hooker's Genera Plantarum. Sir Joseph Hooker was asked by Mr Darwin to take into consideration the extent and scope of the proposed work, and to suggest the best means of having it executed. He undertook the task, and it was he who laid out the lines to be followed. After years of labour by Dr Daydon Jackson and his staff, the work was produced. But Sir Joseph read and narrowly criticised all the proofs. Imagine four large quarto volumes, containing in the aggregate 2500 pages, each page bearing three columns of close print, and each column
