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 

 mediaeval 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). 



