306 COMMON PLANTS. 
of Ling, Heath, ctc. are said to be generally distributed, because 
they always spring up wherever the débris of any gritty rocks 
constitutes a large portion of the surface soil. Yet these plants 
are absent for miles: between London and Birmingham, by rail, 
there is scarcely so much as a heather cowe to be seen. Between 
London and Southampton or London and Portsmouth there are 
miles of country entirely covered with these plants. There are 
_ but few exceptions to the following law, viz. that the number of 
individual plants of any assumed species will be in a direct ratio 
to the horizontal area and elevation of the species; or, as it may 
be stated otherwise, if the horizontal and vertical range of a species 
be great, the number of individuals will also be great. This law 
is not universal; there are exceptions, but ewceptio probai re- 
gulam. We appeal to the statistics and range of the plants quoted - 
above, as conclusive proof of the prevalence of the above law. 
The almost entire list of what we call common things may be 
cited as a proof that the extent of distribution and the number 
of individuals correspond. All gregarious plants may be called 
common : where they exist at all, they suffer little other vegeta- 
tion to grow among them. The Beech-tree, when growing in 
masses, as on the hills of Buckinghamshire and Oxfordshire, with 
its thick shade destroys most plants except its own progeny. The 
Grasses and Sedges and Brambles are examples of the same ex- 
clusive tendencies of other species. All these may be called com- 
mon, because wherever they grow they appear, as the geologists 
say, in great force. Number and extent, reiteration and fre- 
quency, appear to be the characteristics of common plants. There 
are however many plants that will not bear the test applicable to 
the above, and yet are not usually considered in any other light 
than that of common plants. Barbarea vulgaris, Alliaria officinalis, 
Trifolium medium, Saxifraga granulata, and many others are ex- 
amples. These are not so common as some common plants are, 
yet they are not so scarce as to entitle them to pass muster among 
the more recherchés of our botanical treasures. As it is obviously 
necessary to adopt some principle in drawing up such a list as is 
here subjoined, and as we wish the list to be tested by the obser- 
vation and experience of all who are competent to do this, we now 
state the principles by which we have been guided. First we take 
the Floras of the Southern Counties of England; the Devon- 
shire Flora of Jones and Kingston, with the supplementary lists 
