12 Origin of the British Flora. 
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. ponticum) 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 Rusczs to fruit so freely in Hampshire that I counted 
upwards of forty ripe berries on each of several plants. 
