DIFFICULTIES OF COLONIZATION 73 
fought in the great struggle for existence, and have 
established their right to the places which they occupy ; 
they will not readily give way to any newcomer whose 
seeds happen to be imported into their strongholds. 
Of course exceptions can be quoted, where plants 
accidentally or intentionally introduced by man into | 
new areas have not only maintained a foothold, but 
have spread remarkably. Note the case of the Sweet- 
brier (Rosa eglanteria) in New Zealand, of the 
Mexican Bryophyllum calycinum in many Tropical 
countries, of the American Monkey-flower (Mimulus 
Langsdorfu) in our own islands; but these are ad- 
mittedly exceptional. It is nearer the truth to say 
that the troubles of an immigrant only begin where 
dispersal ends; and that the chance of seeds carrying 
out a successful migration is much greater than the 
chances of their giving rise to a new colony when that 
migration is successfully accomplished. Every head 
of the Reed-mace liberates about a quarter of a million 
seeds of marvellous lightness; yet the Reed-mace does 
not increase in the country, nor is it a particularly 
abundant plant even in its chosen habitats. The Fox- 
gloves (Digitalis purpurea) in a wood shed, each plant, 
say a hundred thousand seeds; yet on an average only 
one of these attains maturity, otherwise the species 
would become more abundant in the area. This 
enormous destruction of seed is largely due to compe- 
tition. The reception which a plant receives in its 
new home is the thing that matters, and that may 
usually be summed up in the phrase “ House full.” 
Nevertheless, the present flora of Great Britain is 
in the long run the result of migration from surround- 
ing areas; so that ease of dispersal has undoubtedly 
