CHAPTER V. 
CONDITIONS ON WHICH DEPEND THE DISTRIBUTION 
OF VEGETATION. 
Tr a map of the world were coloured with tints represent- 
ing the number of varieties, or species, or genera, of 
different plants, growing on different localities, it would 
be found that the tints representing many of these would 
be deep in certain localities and apparently shaded away 
in others—as if there were certain marked centres, where 
the greatest number of the genera, or of the species of 
different plants occurs, and the number of these dimin- 
ished as we receded from these centres, ending, perhaps, 
in solitary representatives of them in some distant country. 
Similar, probably, would be the result of so colouring 
and shading a map in accordance with the numbers of the 
plants of any one species growing in different localities. 
And this might sometimes correspond with the tinting 
and shading on the other map; but more frequently, per- 
haps, it would be very different. In some localities where 
there is a great variety of species, there may be a 
comparatively small number of individual plants of some 
or all of these species; in others, a great number of the 
plants, but all of one or of few species; and in others a 
great number both of plants and of species. 
Professor Edward Forbes, a distinguished naturalist of 
great promise, who perished in all the leaves of his spring, 
considered that there were such centres as I have spoken 
of, and that in all probability these were the localities in 
which these several species of plants first appeared ; that 
thence they spread ; that in some cases thence they might 
probably have been traced continuously from the centres 
to the most remote regions in which they are found, 
