32 Origin of the British Flora. 
? 
circumstance that most of them must fall on ground that is 
already occupied, we should continually have to record the 
introduction of new species. New plants are rarely intro- 
duced at the present day, merely because all the species 
occurring within a reasonable distance have already had 
their chance, and those that were suited to our climate 
established themselves long ago. The modern introduc- 
tions are mainly weeds of cultivation that cannot compete 
with the native plants on uncultivated ground, or are 
species from distant lands. 
As instances of how readily our native plants will 
occupy any tract newly made fit for them, I will mention 
two or three cases that have particularly struck my 
attention. When the new railway to Cromer was made, 
the turf and top soil were pared off for a long distance, but 
nothing more was done for several months. Next summer 
the route of the new line was marked by a scarlet ribbon, 
which could be seen stretching across the country, the 
newly bared sub-soil having been taken possession of by a 
profusion of poppies. A new embankment on the Bourne- 
mouth line near Brockenhurst, again, for several years was 
gay with corn-marigolds, which have since died down and 
mostly disappeared. A still more remarkable case is seen 
in the rapidity with which aquatic plants and animals 
spread to a newly dug pond. In fact, so continuous is 
this migration that we can get a fair idea how long a pond 
has been made, and has contained water, by the number of 
species of aquatic plants and mollusca that it yields. A 
medizeval fish-pond or moat contains a much more varied 
fauna and flora than is found in a newly dug dew-pond 
on the Chalk Downs, though it is surprising how many 
species find their way to these ponds.* 
* See Reid, ‘The Natural History of Isolated Ponds,’ Trans. 
Norfolk Nat. Soc., Vol. V., pp. 272-286 (1892). : 
