QUERIES IN LOCAL TOPOGRAPHICAL BOTANY. 123 
this of drought in the others, considering it is here in South Devon 
that the plant reaches its northern limit of extension. Thus may 
opposing climatal influences come into play, and work jointly to 
check any wide extension of a species. Hypericum linariifolium 
has been supposed to have a partiality for growing where slates 
adjoin granite. They do so in the neighbourhood of Calstock and 
near St. Just, two of its stations, but at its others in south-west 
England it is mostly on carboniferous slates that it appears. Thus 
it would seem that it is no chemical composition of rock that con- 
trols its distribution, though, from what has already been said, it is 
very probable that lithological conditions, acted on by climate, 
really do so. De Candolle states it as his opinion that the most 
general cause of the limitation of species is the relative dryness or 
humidity of different countries. Since the time, however, when 
his most interesting work, Geographie Botanique Raisomie, was 
written (1855), a cause of which he had no conception — namely, 
species warfare — has, mainly through the researches and ability of 
the great naturalist Charles Darwin, been proved to be a power of 
great force in determining the range of the various plants. The 
same most acute observer has also shown that insect agency acts to 
an extent not formerly dreamt of ; that by it fertility is often in- 
creased as well as reproduction insured. 
Among the Geraniacece we have a species, G. rotundtfolium, L., 
which seems to court the sunny side of the road as much as does 
the chilly invalid. On some of the warm banks by old lanes about 
Plymouth it abounds, and, considering the climate becomes gradu- 
ally warmer as we move hence in the direction of Penzance, surely 
we might expect to find it extending along the southern and warmer 
portions of Cornwall towards Land's End. But what are the facts 1 
It abounds about Stoke, appears across the Tamar about Cawsand, 
Torpoint, and near Saltash, occurs on a wall below Antony village, 
and has an outlying patch near Pentillie ; but is not known in the 
county of Cornwall westward of these spots. It is quite at home 
on the Plymouth limestone, yet it is also abundant and luxuriant 
in many spots on the adjoining slates, whence we may, I think, 
infer that it is the dry nature of the limestone soil, and not its 
calcareous properties apart from this, that make that rock so con- 
genial to it. Looking at this partiality for a dry soil, it may be the 
very damp atmosphere of the more westerly portions of Cornwall 
make the tract ill-suited to it, though I cannot believe that it is 
