210 NOTES ON ALIEN AND CASUAL PLANTS.
Denizen. A species at present maintaining its habitats, as
if a native, without the aid of man, yet liable to some suspicion
of having been originally introduced ; e.g., Impatiens noli-tangere.
Colonist. A weed of cultivated land or the vicinity of houses
and seldom found except where the ground has been adapted for
its production by the operations of man, with some tendency to
appear also on shores, landslips, etc.; e.g., Lychnis githago,
Papaver and Adonis annua.
Alien. A species now more or less established, but either
presumed or certainly known to have been originally introduced
by human agency ; e.g., Mimulus langsdorffii, Camelina sativa,
Hesperis matronalis.
It will be observed that all these definitions deal with estab-
lished species and that I have not quoted a definition of the
term "Casual." So far as I can gather, Watson uses this term in
practically the same sense as "Colonist." There is no term to
designate such plants as are accidentally introduced into a
district as escapes from cultivation, or otherwise, and of a fugi-
tive nature and not persisting, and it seems to me that the term
"Casual" might be very aptly applied to such species, which
are mostly annual or biennial and of which the. Evening Primrose
is a type. It is at once obvious that, with the exception of the
first, all these definitions apply to plants which have been
either probably or certainly introduced by human agency,
the difference between them being only in degree, not in kind,
and it is the very existence of these degrees that constitutes the
difficulty in determining the status of a given plant. Suffice
it to say, however, that, broadly speaking, a plant which, while
not actually cultivated by man, clings to him and his works,
so to speak, or, in other words, is rarely found growing spon-
taneously (i.e. wild) anywhere but on or near cultivated land or
in the immediate vicinity of houses, is to be suspected as of alien
origin. At the same time it is to be borne in mind that a plant
known to be of alien origin, may be found growing in this country
apparently wild, and in a perfectly natural and suitable habitat,
while, on the other hand, another species growing under similar
conditions, but in an unsuitable habitat, judging from its geo-
graphical range in other countries and the kinds of locality in
which it is there found, would be presumed to be of alien origin,
although it might be impossible to discover the means by which