GEOGRAPHICAL DISTRIBUTION 153 



agents of transportation of seeds for the new growth, 

 about 54 per cent, of a total of 157 species having seeds 

 adapted to dispersal by birds, only 21 per cent, adapted 

 to wind dispersal, and only 9 per cent, apparently brought 

 by currents of water. Of course, the distance from 

 Volcano Island to the nearest uninjured vegetation was 

 short, and the various agents of dispersal would, no doubt, 

 assume a different relative importance for greater dis- 

 tances, as they did, for example, in the case of Krakatoa, 

 noted above. 1 



116. Struggle for Existence a Factor. DeCandolle 

 early contended that it was not sufficient that one or 

 even a few seeds be carried to a country already well 

 covered with vegetation in order for the new arrivals to 

 become established, but that a very large number of 

 vigorous seeds must be introduced to insure success in 

 the struggle for existence with the native plants. Atten- 

 tion has been called in a preceding chapter (pp. 94-96), 

 however, to the enormous rate of propagation of plants 

 and animals, which proceeds in geometrical progression; 

 so that if we allow a sufficient time period, and postulate 

 a species suited to the climate and soil of its newly found 

 home, we may expect a large degree of success in its be- 



1 In contradiction to the above statements of fact and logical inference, 

 there should be noted here Warming's quotation (Botany of the Faeroes), 

 p. 676. London, 1901-1908) from the Danish Zoologist, H. Winge, of the 

 Zoological Museum, Copenhagen, who stated in a letter to Warming that 

 he had carefully examined thousands of migratory birds picked up dead at 

 Danish lighthouses, and had never found any seeds adhering to the feathers 

 beaks, or feet. Dried mud was found "fairly often," but there were ad- 

 hering to it no seeds large enough to be seen with the naked eye or the 

 hand-lens. Moreover the stomachs of migrating birds were always found 

 to be practically empty, indicating that migrating birds travel on empty 

 stomachs. See, however, p. 164, infra. 



