Browne: The Aquatic Coleoptera. of the Isle :of Man. 16x 
I have quoted only from one work but similar examples of 
Dr. Scharff’s evidence occur again and again in his books and, 
papers, and in every case they refer to the attempted acclimati- 
sation of new species im a country already fully stocked, and it 
seems to me that not only is this the important fact which 
keeps out invaders, but that it is upon the competition between 
species, the biological factor, that we rely when we recognise 
in the northern, southern, and eastern groups of the Britannic 
fauna and flora elements which reached the country at different 
times ; it is largely owing to this factor that we find a localisa- 
tion of species within a country. For example, various groups 
of plants are recognised according to the habitat they occupy ; 
there are some which occur on peaty soil, others in salt marshes, 
others again on mountains, and so on, yet most of the species 
from these very different habitats can be grown side by side in 
ordinary garden soil. Competition drives them, under natural 
conditions, each to that kind of environment where alone it 
can hold its own against invaders, to that kind of environment 
where climatic and edaphic factors may not be the most suitable 
to it, but are more suitable to it than to its competitors. 
Numerous examples can be brought forward to show that 
the relationship of a species to its physical environment is ~ 
controlled by its biological environment. For instance, we 
describe certain plants as calcicolous, and others as non-calci- 
colous, according as they naturally live upon soil rich or poor 
in lime, yet only those plants to which lime is a poison are 
permanently excluded from it.* Praeger + notes several cases 
in which non-calcicolous plants occur on limestone, and calci- 
colous species on other rocks. Warming { mentions that 
‘Alders attain their most luxuriant development on well 
drained soil. But they are usually expelled from this by“other 
competing trees. Only in swamps where they do not thrive so 
well, are they dominant. In like manner Calluna vulgaris 
flourishes upon rich soil better than on poor soil, but it is ex- 
cluded from the former by competing species,’ etc., etc. (pp. 
367-8). 
Mr. A. W. Stelfox tells me that certain land mollusca may 
be calcicoles in one place, and calciphobes, or indifferent in 
another. He mentions that Helix arbustorum, which is practi- 
cally confined to limestone in the north of Ireland is quite in- 
different in Britain, being common from the south of England 
to the north of Scotland, while ‘ Helix lapicida is generally 
looked upon in England as calciole, but it occurs in Norway in 
purely precambrian areas.’ Mr. Davy Dean in a paper on the 
5 GRE A ae iden Oi 2 ee ee 
* Schimper, ‘ Plant Geography,’ Eng. Transl., p. 100, 1903. 
+ ‘A Tourist’s Flora of the west of Ireland,’ pp. 12 and 108, Dublin, 
1909. 2 
* ‘cology .of Plants,’ Eng. Transl., Oxford, 1900, 
rgii April r. 
