1 905- 1906.] Formation and Flora of a Shingle Islatid. 309 



by streams, and may therefore find the island more congenial 

 than the fields. 



In the case, then, of the 11 plants flourishing on the 

 island and absent from the fields and riparian strip, we 

 may conclude that diflBculty of access has been the chief 

 factor in determining that absence, inasmuch as it accounts 

 for 9 species ; while unsuitability of habitat accounts, at 

 most, for only 4 ; and deliberate eradication for only 1. 



The 53 plants groioing toell on the island, — It will be 

 interesting now to turn our attention to all those plants 

 which are growing well on the island, and to consider to 

 what causes their prosperity is due. The question resolves 

 itself into two parts : (1) the number of individual plants 

 of any species, which is determined more or less by the 

 plant's chances of reaching the island ; and (2) the vigour 

 of the plants, which is dependent on soil, light, moisture, &c., 

 and on certain qualifications inherent in the plant itself. 

 In the following list we have made general suggestions (1) 

 as to what facilities the various plants have for reaching the 

 island (columns 1, 2, 3, 4); and (2) suggestions as to the 

 possible causes of their vigorous condition when there 

 (columns 5, 6, 7, 8). 



