CONDITIONS ON WHICH DISTRIBUTION DEPEND. 55 



tions of plants, but much remains intricate and unex- 

 plained. 



'In considering this subject, there are two essentially 

 different points which have to be distinguished. 



c 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 

 vulgartSj 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 

 extensions 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 incalculably 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 



