DISPERSAL 



157 



that any large-scale migration of this sort may have two reasons, 

 either to get away from the centre of distribution in order 

 to prevent disaster through overcrowding, or to reach some- 

 where at the circumference and so extend the range of the 

 species. 



II. It is plain that an enormous wastage must occur 

 while the establishment after dispersal is taking place, and 

 that only a tiny fraction of the original emigrants will ever 

 succeed in establishing itself even temporarily. We cannot 

 do better than quote here the words of Wood-Jones,^°^^ who 

 had a peculiarly good opportunity of appreciating the factors in 

 dispersal and establishment of species, in connection with the 

 arrival of new animals and plants on the coral islands of Cocos- 

 Keeling. He says : *' Those creatures that are settled and 

 established are the elect, and they are appointed out of a 

 countless host of competitors, all of whom have had equal 

 adventure but have gone under in the struggle, through no 

 fault of their own. They are the actual colonists, the survivors 

 of a vast army of immigrants, every one of which was a potential 

 colonist." One particularly striking example was noted by 

 him ^^''^ : occasionally huge flights of dragon-flies would arrive 

 (belonging to the species Pantula flavescens^ Tramea rosen- 

 bergiij and Anax guttatus)^ and would live for some time 

 and feed ; but, owing to the absence of any permanent open 

 fresh water on the islands, they never succeeded in establish- 

 ing themselves permanently, although they actually laid eggs 

 in temporary pools, which were not suitable for their breed- 

 ing purposes. Another example of abortive colonisation on 

 a huge scale was encountered by the sledging parties of the 

 Oxford University Arctic Expedition whilst crossing the ice- 

 cap of North-East Land in the summer of 1924.^^ One day 

 in August, all three parties in different parts of the country 

 encountered vast swarms of aphids {Dilachnus picece), normally 

 found on the spruce {Picea ohovata) of Northern Europe, 

 together with large numbers of hover-flies of the species 

 Syrphiis ribesii. These insects had travelled on a strong gale 

 of wind for a distance of over eight hundred miles, and had 

 been blown in a broad belt across the island of North-East 



