6 THE VOYAGE OF H.M.S. CHALLENGER. 



of islands cannot be placed in the same category. Belative age, however, which, after all, 

 is the fundamental principle of Wallace's classification, is in a measure indicated by the 

 nature of the flora : vet we have no such striking voids in the floras of oceanic islands 

 generally as that caused in the faunas by the absence of land-mammalia and amphibia. 

 The indigenous vegetation 1 of oceanic islands presents physiognomical rather than struc- 

 tural characteristics, but even such peculiarities are repeated in continental situations ; 

 therefore insular vegetation of the most ancient type cannot be said to possess any absolute 

 peculiarities. Certain forms predominate, governed by physical conditions, but, as is 

 more fully explained in succeeding paragraphs, counterparts, or even exaggerations of them, 

 occur on continents. In short, all the characteristic features of insular floras are repro- 

 duced elsewhere, except the invariable poverty in species in relation to area ; and as this 

 is not due to the same causes operating in continental desert tracks, the cases are not 

 parallel. The general characters of remote insular floras are : relatively large ordinal and 

 generic representation ; preponderance of endemic species, often belonging to endemic 

 genera ; w T oody and often subarboreous habit of a large number of the species ; prevalence 

 of small, narrow leaves, and absence or great rarity of brilliantly coloured flowers ; but the 

 exceptions of different kinds are numerous, and each island or group of islands usually 

 presents some special feature. 



For phyto-geographical purposes Insular Floras may be divided into three categories, 

 based upon their endemic element: — 1. Vegetation comprising a large endemic element, 

 including distinct generic types, the nearest affinities of which are not always all to be 

 found in any one continent ; 2. Vegetation comprising a small, chiefly specific, endemic 

 element, the derivation of which is easily traced ; and 3. Vegetation comprising no 

 endemic element. To the first category belong St Helena, Juan Fernandez, the Sandwich, 

 Galapagos, and Seychelles groups ; to the second, the Bermudas, Azores, Ascension, 

 the islands in the southern jiart of the Indian Ocean, and the Admiralty Islands ; 

 to the third, the Keeling, and numerous other rising coral islands in the Indian and 

 Pacific Oceans. This classification is not altogether satisfactory, because, if extended to 

 islands generally, the second category must include insular floras in which there is no 

 endemic element ; such, for example, as Spitzbergen and Iceland ; but the difficulty is 

 more apparent than real, as the third category is intended to be limited to islands which 

 have become stocked with plants in comparatively recent times. 



Before proceeding to the discussion of the means by which these various islands 

 became, or probably became, endowed with flowering plants, we will give some details of 

 the vegetation of some islands and groups of islands not touched upon in the Beports ; 

 and also fuller particulars concerning the general distribution of certain natural Orders, 

 together with other evidence bearing upon the inquiry. 



1 We repeat here that any plant that has certainly or apparently reached an island independently of human 

 agency, no matter how recently, nor how sparsely it may be represented individually, is regarded as indigenous. 



