12 Origin of the British Ftora, 



rule only about one plant in fifty produces any fruit, and 

 these are not only few in number, but, as they ripen in 

 November, an early winter may prevent them ripening 

 at all. The plant being perennial and hardy can survive, 

 but it has evidently reached its northern limit in Britain.* 

 The sycamore, maritime pine, and common rhododendron 

 (R. ponticiwi) are instances of plants undoubtedly intro- 

 duced, which seed and grow freely from seedings in the 

 South of England. That they were not till lately members 

 of our flora is evidently due to geographic, not to climatic 

 conditions. 



We cannot point to any British annuals which do not 

 seed freely in some part of the Islands, for the sufficient 

 reason that an annual which cannot seed well may be 

 entirely exterminated by a single exceptional season. 

 This points to a probable explanation of the curious 

 tendency noticed in the floras of small oceanic islands, 

 for genera ordinarily annual and herbaceous to be repre- 

 sented by perennial species. This may be explained in 

 the following way. In many annual plants a few in- 

 dividuals become biennial ; these in an island devastated 

 by an exceptional gale at flowering time, by a swarm of 

 locusts, or other adverse conditions, would be the only ones 

 to survive, and natural selection would thus tend to 

 perpetuate the biennial or perennial forms which so 

 characterise these islands. This change of annual into 

 perennial forms, however, in all probability has had little 

 effect on the British plants ; for the Islands, besides being 

 too large, are sufficiently close to the Continent to receive 

 occasional seeds or pollen of the same species, which by 

 intercrossing would tend to keep the species true. 



* The exceptionally warm and dry summer and autumn of 1898, 

 however, caused Ruscus to fruit so freely in Hampshire that I counted 

 upwards of forty ripe berries on each of several plants. 



