THE FACTORS OF DISTRIBUTION 67 



apparently such beetle-like seeds as those of Ricinus are 

 carried to some distance by birds. 



OCCASIONAL DISPERSAL. If, however, these usual 

 methods of dispersal commonly effect small steps in 

 migration, there are rarer possibilities that are not so 

 limited. Even so large a fruit as an acorn is carried by 

 rooks to a considerable distance, and the great pigeons 

 of the Moluccas swallow whole nutmegs and have 

 transported them from island to island. Migratory 

 birds, or birds blown out of their course by strong winds, 

 cover great distances, e.g. from North America to 

 Bermuda; or from Europe and Africa to Madeira, and 

 sometimes carry seeds either in their crops, or in pellets 

 of mud adhering to their feet. Freshwater fish swallow 

 seeds, and these might therefore be dispersed by fish- 

 eating birds. Whole plants, even large trees, are often 

 carried down by rivers, and may drift across considerable 

 stretches of ocean bearing their ripe but unopened fruits 

 and, perhaps, also epiphytes, or with seed-containing 

 soil clinging to their roots ( Fig. 4). In this case, and in that 

 of icebergs, which have commonly a considerable load 

 of earth, there need be no special structural adaptation 

 of fruit or seed to facilitate their dispersal. 



ECESIS. It is quite possible that one exceptional 

 introduction may lead to the establishment of a species 

 in a new area. Nevertheless, it is important to bear in 

 mind the fact that mere migration need not secure such 

 establishment. The climate and soil must be suitable, 

 and the new-comer may have to hold its own in com- 

 petition with species already " in possession." For this 

 establishment the term ecesis has been proposed. 



Those plants which are believed to have reached any 

 district without human action are termed native or 

 indigenous, but a species known only in one area is 

 termed endemic. Such a species may have originated 

 where it is now found, or it may have originated else- 

 where, having once had a wider area of distribution, of 

 which its present limited area is the only remainder. 



ALIENS. Besides the intentionally-introduced species 

 cultivated by man, whether for food, timber, fibre, 

 medicine, or other purposes, several grades have been 

 recognised among those unintentionally introduced, or 

 alien species, according to their degree of ecesis or 

 establishment. Temporary introductions, such as many 



