10 INTRODUCTORY EXPLANATIONS. 



counties, others from three counties; and so reckoning 

 downwards, until we got to the maps in which only a single 

 coimty was marked for some of the plants. 



In thus projecting the distribution of species on maps, by 

 way of illustrating facts as they exist in natm*e, those two 

 circumstances of distribution are brought under view, which 

 have usually received some attention from the describers of 

 plants ; — who, by the by, frequently appear to attach no 

 other idea to the words " geographical distribution of 

 plants," than the two cu'cumstances here alluded to. These 

 are, first, the extent of geogi'aphical sm'face, over which a 

 species is spread ; and, secondly, the gi-eater or less fi-e- 

 quency of the species within that space. The first is 

 shortly and conveniently expressed by the teim Area of a 

 species. And the word Census, almost as well naturalised 

 among us, may be used to indicate the floral population of 

 the island, or of any part of it. But between the human 

 and the floral census of a country, there is this important 

 difference, that in the former we count individuals, while 

 in the latter we can count up only species or habitats. 

 Thus, too, the floral census has itself a two-fold meaning 

 or apphcation. We may, census-like, reckon the number 

 of species in a coimtry, in an order, or in a genus ; and 

 thus institute numerical comparisons between countries, 

 orders, or genera. Or, we may reckon up the number of 

 habitats for species, &c., and then make comparisons 

 between their relative fi^equency. 



To return to om- supposed maps of species. Any other 

 sections of Britain, larger or smaller than counties, might 

 be taken for the like purpose of illustrating the areas and 

 census of species ; and they would equally tend to show a 

 local, a partial, or a general distribution of the plants. The 

 more numerous were the sections taken, the less numerous 



