CHAPTER V. 



CONDITIONS ON WHICH DEPEND THE DISTRIBUTION 

 OF VEGETATION. 



IF 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, 



