80 ISLAXD LIFE part i 



rock offering crevices for concealment and hybernation. 

 The- second is a limited bird and mammalian fauna, in 

 which such species as are especially shell-eaters shall be 

 rare or absent. Both these conditions are found in certain 

 large islands, and pre-eminently in the Antilles, which 

 possess more species of land-shells than any single continent. 

 If we take the whole globe, more species of land-shells are 

 found on the islands than on the continents — a state of 

 things to which no approach is made in any other gToup of 

 animals whatever, but which is perhaps explained by the 

 considerations now suggested. 



The Dis]oersal of Plants.— Th.Q ways in which plants are 

 dispersed over the earth, and the special facilities they often 

 possess for migration have been pointed .out by eminent 

 botanists, and a considerable space might be occupied in 

 giving a summary of what has been written on the subject. 

 In the present work, however, it is only in tAvo or three 

 chapters that I discuss the origin of insular floras in any 

 detail ; and it will therefore be advisable to adduce any 

 special facts when they are required to support the argu- 

 ment in particular cases. A few general remarks only will 

 therefore be made here. 



Special Adaptahility of Seeds for Dispersal. — Plants pos- 

 sess many great advantages over animals as regards the 

 power of dispersal, since they are all propagated by seeds or 

 spores, which are hardier than the eggs of even insects, and 

 retain their vitality for a much longer time. Seeds may 

 lie dormant for many years and then vegetate, while they 

 endure extremes of heat, of cold, of drought, or of moisture 

 which would almost always be fatal to animal germs. 

 Among the causes of the dispersal of seeds De Candolle 

 enumerates the wind, rivers, ocean currents, icebergs, birds 

 and other animals, and human agency. Great numbers of 

 seeds are specially adapted for transport by one or other of 

 these agencies. Many are very light, and have winged 

 appendages, pappus, or down, which enable them to be 

 carried enormous distances. It is true, as De Candolle 

 remarks, that we have no actual proofs of their being so 

 carried ; but this is not surprising when we consider how 

 small and inconspicuous most seeds are. Supposing every 



