The active life of Sir William Hooker divides itself naturally into two main periods, during which he held two of the most responsible official posts in the country, viz. the Regius Chair of Botany in Glasgow and the Directorship of the Royal Gardens at Kew. We may pass over with but brief notice the years from 1806 to 1820, which preceded his attainment of professorial rank. Notwithstanding that notable work was done by him in those years, the period was essentially preparatory and provisional, and can hardly be reckoned as an integral part of his official life. He was in point of fact an enthusiastic amateur, one of that class which has always been a brilliant ornament of the Botany of this country, and has contributed to its best work. He travelled, making successive tours in Scotland and the Isles, no slight undertaking in those days (1807, 1808). In 1809 he made his celebrated voyage to Iceland, described in his _Journal_, published in 1811. But his collections from Iceland were entirely lost by fire on the return voyage. His son remarks that the loss to science was probably greatest in respect of the Cryptogamic collections; this naturally followed from the fact that already he had taken a prominent place as a student of the lower forms, and the field for their study was more open than among the flowering plants of the island. It was among the Cryptogams that Sir William found the theme of his first great work, the _British Jungermanniae_, published in 1816. Nearly a century after its appearance it still stands notable not only for the beauty of the analytical plates, but as a foundation for reference. It must still be consulted by all who work critically upon the group, subdivided today, but comprehended then in the single genus _Jungermannia_. During this period he also produced the _Musci Exotici_, with figures of 176 new species from various quarters of the globe. Thus up to 1820 his chief successes lay in the sphere of Cryptogamic Botany.