The comparatively advanced state of differentiation of the vegetation into plant-associations or formations is no less surprising than the remarkably large number and the variety of species in the new flora of Krakatau. As Penzig showed, the development of plantassociations had already begun in 1897. On the beach the plants of the Pes-caprae formation predominated. In the interior of the island the vegetation presented the appearance of a grass-steppe. Ferns constituted the dominant plants on the slopes of the cone. Since 1897 the aspect of the vegetation has undergone considerable change; the strand-flora is divided into two formations. An outer zone of varying breadth, reaching in places to the tide-level, consists of low creeping grasses and herbaceous plants, bushes and shrubs; the typical Pes-caprae formation. Behind this rises the strandforest (Barringtonia formation), the composition of which is not yet equal in variety of species and in gloomy grandeur to the Barringtonia strand-forests on the coasts of Java and Sumatra, nor will it soon reach the same level. Neither of these strandformations is as yet closed. Grasses, Cyperaceae, Ferns and Composites spread from the grass-steppe of the interior through clearings in the forest to the lower carpet of Ipomaea and Spinifex, while in other directions groups of strand-plants have penetrated inland for a distance of 300—500 metres. The beautiful group of coconut palms, 400 metres from the shore-line, represented in the photograph (PL VI., fig. 7, p. 30), affords an example of the latter.

Dr Melchior Trcub, the distinguished and genial Director of the Botanical Institute at Buitenzorg in Java, rendered a great service to Science by initiating the study of the new Krakatau flora and thus paving the way for further investigations. The botanical explorations, made on three difterent occasions, of the islands which were entirely depleted of vegetation in 1883 have not only furnished results of general biological interest, but by demonstrating the successive stages in colonisation, they also afford one of the most important contributions towards the solution of the much discussed problem as to the source and history of the introduction of a flora into an island far removed from the mainland. The older literature concerned with this interesting problem in plant-geography, so far as regards direct observation, deals with the history of colonisation of recently formed coral islands, that is of flat land surfaces, and with the investigation of the means of dispersal of plants from older coral reefs and volcanic islands. On Krakatau, Treub had an opportunity of studying the more complex problem: how a volcanic island, which has lost the whole of its flora as the result of an eruption, acquires a new vegetation; how, in other words, an island of considerable height suddenly emerging from the sea becomes stocked with plants; and further by what successive stages the new floral elements appear on the island and by what external agencies the colonisation is effected.