PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. Clll 
termed, for the sake of convenience, the “native” flora. Whether 
this native flora continued to receive accretions after the land con¬ 
nection with the Continent had been broken it is impossible to say, 
but it is not altogether improbable. Whether, however, that was the 
case or not, there is now in the flora a not inconsiderable number of 
plants the method of whose entrance into this country is not quite 
certain. These plants have been classified as “denizens” and 
“colonists,” terms first applied to them by H. C. Watson. A denizen 
is a species which is suspected to have been introduced by man, but 
which maintains its habitat. A colonist is a species which occurs 
only in ground adapted by man for its growth and continuous main¬ 
tenance. In addition to these we have “aliens,” or plants which 
have presumably been introduced by human agency; “natives” 
being the species which have not been so introduced. But, 
however nice these names look upon paper, their correct applica¬ 
tion is, as Sir J. D. Hooker remarks, exceedingly difficult in many 
cases. 
Plants have various contrivances by which the area they inhabit 
is extended. The seeds of some merely fall to the ground, and 
in these the progression by that means must necessarily be slow. 
Others have tall stems, which, when shaken by the wind, help to 
scatter the seed to a greater distance. Still others have their seeds 
or seed vessels so constructed that they may be carried to a con¬ 
siderable distance by the wind. In addition to these methods, there 
are other contrivances by which animals are utilised for the dis¬ 
persion of seeds. Migrating birds may carry seeds in their stomachs 
or in hardened mud adhering to their feet. Many seed vessels have 
hooks by which they can cling to the fur of mammals or to the 
feathers of birds. To birds a narrow sea presents no obstacles, and 
hence in that way the immigration of plants may still be going on, 
and species so introduced (if there are such) at the present day can 
scarcely be considered to be less native than if they had been 
brought thousands of years ago. The deduction to be drawn from 
this is, I think, that a plant which suddenly makes its appearance in 
a district in which it was not known to occur previously must not 
necessarily be looked upon as an alien introduced by human agency. 
I have been speaking of the British flora and Britain, but with the 
object of attempting to show that what concerned these in remote 
ages may have a more modern application to the Perthshire flora 
and to Perthshire. The various contrivances for the dispersion of 
plants, to which we have alluded above, clearly indicate that the 
extension of the area of plants is for the benefit of the species. 
Perthshire, is not cut off by the sea from the rest of Britain, therefore 
the methods by which the British flora reached Britain, when there 
was a land connection, may have always been, and still be, operative 
as regards Perthshire in relation to the rest of Britain, provided that 
climatic conditions are favourable, and that there is room. Conse¬ 
quently, if we find in Perthshire a species, for example, which is of 
common distribution farther south in Britain, but whose area of 
occurrence is separated from Perthshire by a more dr less wide 
interval, we must not, therefore, necessarily look upon it as intro- 
