ORIGIN OF THE BRITISH FLORA 215 
reason Ireland contains fewer species than Great Britain, 
just as Great Britain contains fewer species than France 
or Germany. 
Native Plants and Aliens.—Plants which grow wild, and 
which we have every reason to believe have always 
formed part of our flora since the advent of man, are 
called indigenous, or native. Those which have been 
introduced since otherwise than by natural means— 
e.g., birds or wind—are aliens. Now, there are aliens and 
aliens. Some may be quite recent intruders, others came 
very early when the first cultivators disturbed the soil ; 
some have competed successfully for a long period of time 
with the native flora, and have spread far and wide over 
the land ; others spring up, flourish for a time, and then 
die out ; others are mere casual strays, seen here and 
there, and seldom in the same place two years together. 
The following various kinds of aliens have been distin- 
guished : 
1. Denizens.—Plants which we know or have reason 
to suspect are foreigners, but which have so successfully 
established themselves in natural or wild and closed 
habitats among the native flowers that they now form a 
constituent part of the flora—e.g., Hlodea, Impatiens fulva, 
Claytoma perfoliata and C. alsinoides, Crocus, cyclamen, 
snowdrop, and probably the gooseberry and red and 
black currants. 
2. Colonists.—These are generally more recent immi- 
grants, and do not, as a rule, compete on equal terms 
with the native vegetation. They are found in artificial 
habitats—that is, on ground disturbed by man, where 
there are always bare spots open to settlement. Most 
of the commoner weeds of cultivation are colonists in 
this sense (see below). 
3. Casuals are aliens which appear here and there, but 
never secure a permanent foothold even in artificial 
habitats. They soon die out, either because they fail to 
produce efficient seed, or because the conditions are against 
them in the struggle for existence. Some are weeds 
brought with seed from other lands, some arrive with 
merchandise, some are relics and escapes from cultiva- 
tion, others mere garden throw-outs. 
Weeds of Cultivation.—This is a term applied to those 
plants, generally annuals, which are found on cultivated 
