78 



ISLAND LIFE. 



[part I. 



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 two 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 argument in particular cases. A few general 

 remarks only will therefore be made here. 



Special adaptability of Seeds for dispersal. — Plants possess 

 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 

 year a million seeds were brought by the wind to the British 

 Isles from the Continent, this would be only ten to a square 

 mile, and the observation of a life-time might never detect 

 one ; yet a hundredth part of this number would serve in a few 

 centuries to stock an island like Britain with a great variety of 

 continental plants. 



When, however, we consider the enormous quantity of seeds 

 produced by plants, — that great numbers of these are more or 

 less adapted to be carried by the wind, — ^and that winds of great 

 violence and long duration occur in most parts of the world, we 

 are as sure that seeds must be carried to great distances as if 

 we had seen them so carried. Such storms carry leaves, hay, 

 dust, and many small objects to a great height in the air, while 



