II. FLORAL AREAS. 383 



rule of a decreasing number of species northward, rela- 

 tively to area. The average or mean number of species, 

 to ten miles of space, decreases from south to north in 

 each trio ; but at the intermediate step for the provinces 

 there is a considerable excess. This is at once explained, 

 on observing that the average area for those seven pro- 

 vinces is much under the averages for the other two 

 groups. The narrowness of the island in its middle lati- 

 tudes, from about 53 to 56, causes the provinces 8 to 14 

 to be of smaller size one with another ; some of th^em 

 being thus more equivalent to the subprovinces of the 

 other two divisions. 



The relation of species to area is most clearly brought 

 into view in the latter portion of the table, under the 

 head of * series.' Commencing with total Britain, smaller 

 and smaller areas are there taken in succession ; each 

 one being a part of that which immediately precedes it. 

 In this manner, the comparison of a rapidly decreasing 

 area, with its less rapidly decreasing flora, is made suc- 

 cessively through portions of the same area and flora ; 

 much more correctly, therefore, than would be the case 

 if two different areas of unequal size were compared ; — 

 say, for example, a hundred miles round Paiis, with ten 

 miles round London or Edinburgh. It should be ob- 

 served that the single mile of North Surrey, at the end of 

 the list, is remarkable for the varietj' of its plants, arising 

 from diversitj'' of surface. This was desired for the par- 

 ticular comparison ; as a square mile of uniform surface 

 would have afforded too unequal a comparison with ten 

 miles of diversified surface, in other respects than the 

 one essential difference of extent. 



Further, it has been shown by the ' collective census 

 of orders' on pages 359 to 301, that the species of some 

 orders average a much higher census than those of other 



