CONDITIONS ON WHICH DISTRIBUTION DEPEND. 55 
tions of plants, but much remains intricate and unex- 
plained. ‘ 
‘Tn considering this subject, there are two essentially 
different. points which have to be distinguished. 
‘The heath plants occur on dry, sunny, sandy plains; 
they extend from the Cape of Good Hope, through Africa, 
Europe, and Northern Asia, to the extreme limits of vege- 
tation in Scandinavian Siberia ; these plants are distributed 
on this great region in such a manner that South Africa 
has innumerable distinct species, of which, however, never 
more than a few individuals grow side by side; and that 
then, towards the north, the number of species suddenly 
diminishes in an important degree, while the number of 
individuals increases—the common heather, Calluna 
vulgaris, overspreads whole countries in millions of single 
individuals. 
‘In the first place, we readily see that only the first 
determination, that of the occurrence, relates necessarily to 
each individual; while, on the contrary, the range of 
extenstons and the mode of distribution indicate courses 
which have scarcely any importance in reference to the 
single individual, but very great in relation to the larger 
groups of plants which we call species. genus, &c. 
‘From this it follows that the former only—the occur- 
rence of, plants—is related wholly, while the other two are 
related but partly, to conditions explicable by physical 
influence ; yet we must, at first keep more to that arrange- 
ment, since it is strictly logical, which will remain fixed 
for inealculably long time, while of course the last only 
holds good for the existing condition of science; and when 
we review the various influences upon which the life and 
healthy vegetation of a plant are, according to our present 
physiological knowledge, dependent, we quickly find that 
only a small number of physical forces are as yet detected 
by us in their action on the organism; and that, on the 
other hand, a proportionately large number baffle our 
endeavours after a more accurate comprehension of their 
action, although we may safely assert that the life of the 
