392 



NATURE 



\Fcb. 24. 1 88 1 



rivedfromtheNorthAmerican continent, whenceeveryyear, of the Galapagos however and that of the nearest part of 



especially during the autumnal storms, numbers of birds South America a remarkable difference obtains. As 



are blown out to sea. Most of these no doubt perish, but usual, no indigenous mammalia or amphibia occur in 



some succeed in reaching Bermuda. Hence from this I these islands ; Isut a few species of reptiles abound — land- 



const.-int introduction of fresh individuals there has been 

 no development even of any distinct variety in the avian 

 fauna. The land-shells include twenty species, of which 

 at least four, or about a fourth of the whole, are peculiar. 

 The proportion of peculiar land-shells among the Azores 

 is about a half of the whole number of resident species. It 

 is obvious that these organisms have comparatively feelile 

 and uncertain means of transport as compared with birds. 

 They may be carried only at widely separated and irre- 

 gular intervals, enclosed in drift-wood from some other 

 islan I or continent. Hence the conditions for their 

 gradual change under the new circumstances of their 

 insular home are exceptionally favourable. The flora of 

 Bermuda contains a majority of tropical and West Indian 

 plants, and includes a number of species identical with 



those in the Southern States of the American Union. 

 The origin of this vegetation is thus easily traced, first, 

 to the operation of marine currents, whereby plants of 

 the West Indian Islands have been actually observed to 

 be washed ashore on Bermuda and to germinate there ; 

 ne.xt, of cyclones by which fine seeds transported in the 

 higher parts of the atmosphere may doubtless be easily 

 carried from the .American continent ; and thirdh', ot 

 birds, which among their feathers and in the mud adher ing 

 to their feet are known to transport living seeds to 

 enormous distances. 



The Galapagos, though less distant from the west side 

 of the American continent than the Bermudas are from 

 the east side, rise nevertheless out of a profoundly deep 

 ocean. The whole group of seventeen islands ranges 

 over an area of 300 miles in length by 200 in breadth, being 

 of volcanic origin, and still containing 



n the western 

 islands numerous active volcanoes, between the fauna | Undoubtedly subsidence was one, probably the prircipal 



tortoises, lizards, and snakes, that find their nearest allies 

 on the American continent, whence doubtless their 

 ancestors at some remote period were derived. Cut of a 

 total of fifty-seven species of birds no fewer than thirty- 

 eight are peculiar. In particular the land-birds number 

 thirtv-one species, which are all, with but one e.xception, 

 confined to the Galapagos, and more than half of them 

 are so peculiar as to be ranked in distinct genera, though 

 all are undoubtedly allied to birds inhabiting Tropical 

 America. Mr. Wallace points out that every gradation 

 can be traced, from perfect identity with continental 

 species to marked generic divergence, and that "this 

 diversity bear; a distinct relation to the probabilities 

 of and facilities for migration to the islands." A species 

 which is widely diffused and essentially migratory will, 

 by frequent arrival of fresh individuals 

 from the parent stock and intercrossing, 

 continue unchanged, while others, in 

 proportion to the rarity of their re- 

 introduction, will be subject to all the 

 variation which change of habitat and 

 prolonged isolation may induce. The 

 flora of these islands includes 174 

 peculiar flowering-plants, and 158 com- 

 mon to other regions. Among the 

 latter occur forms found both in North 

 and South America, with some that 

 range into the West Indies. Sir Joseph 

 Hooker has observed that the peculiar 

 plants of the Galapagos are allied to 

 torms now found in temperate America, 

 or in the high Andes, while the non- 

 peculiar species are such as live in 

 tropical latitudes near the sea-level. 

 These facts in zoological and botanical 

 distribution the author seeks to explain 

 by the meteorological conditions and 

 geological history of the region. The 

 Galapagos Islands lie in a tract of 

 almost perpetual calms. The storms 

 that annually transport a fresh immi- 

 gration of birds and seeds to the Ber- 

 mudas are there unknown ; conse- 

 quently the fauna and flora present a 

 far greater contrast to those of the 

 continent than is the case of Bermuda. 

 The presence of West Indian species 

 is regarded as pointing to the former 

 submergence of the Isthmus of Panama 

 and the consequent drifting of those 

 forms from the north-east, perhaps by 

 a deflected branch of the Gulf Stream. 

 Again, the affinity of a portion of the Galapagos flora 

 to plants of northern or sub-alpine types is looked upon 

 as an indication of that ancient southward migration of 

 northern forms consequent upon the extension of the 

 snow and ice of the Glacial Period. 



As e.xamples of Continental Islands the author describes 

 the British Isles, Borneo, Java, Japan, Formosa, and the 

 Madagascar group. The difference between the plants 

 and animals of continental islands and those of the 

 neighbouring continents varies extensively, one main 

 effective element in the case being the length of time 

 during which insular relations have been established. 

 Taking Brita-n as perhaps the most typical illustration of 

 a large and recently separated continental island (Fig. 2), 

 Mr. Wallace points out how many are the proofs of com- 

 paratively recent subsidence, which he regards as the 

 cause of the severance of Britain from the continent. 



