20 
the author of the “ Cybele ” has considered the surface of Britain as sub- 
divided into eighteen botanical provinces,* usually formed by grouping to- 
gether several counties which comprise the basin of a river, or possess some 
other physical peculiarity in common. Amongst the counties of Britain, 
Yorkshire, alone, is considered sufficiently extensive in area, and natural 
in its boundaries, to form a province of itself ; and is called the tenth or 
Humber province. For the sake of attaining greater accuracy, more 
recently he has mapped out the island into thirty-eight sub-provinces ; f of 
these, Yorkshire contains two, East Humber and West Humber, which are 
bounded from each other by a line drawn from the southern bend of the 
Tees to the mouth of the Humber along the course of the Wiske, the 
Swale, and the Ouse. These again he has further sub-divided into vice- 
counties, the total number of which is 112. The East Humber sub- 
province contains two of these vice-counties, South-east Yorkshire and 
North-east Yorkshire, the boundary between which is the same as that 
between the east and north ridings. The West Humber sub-province 
contains three vice-counties, South-west Yorkshire, Mid-west Yorkshire, 
and North-west Yorkshire. The boundary between the two former of 
these extends from the Ouse, along the course of the Aire, to the neigh- 
bourhood of Skipton, and then across to Lancashire: that between the 
two latter is a line drawn along the summit of drainage between the TJre 
and the Swale. 
But if our special object be to estimate, not the distribution of the flora 
of the whole island, but of that of the county only : if we wish to trace 
each species throughout the extent of its surface more accurately than 
could be done by ascertaining in how many of the sub-provinces it occurs; 
and to obtain a closer idea of the area of each, and the proportion which 
it bears to the general mass of the vegetation, it will be needful to descend 
a step lower into detail. With slight exception, not only the county, but 
each of the five sub-provinces is bounded by natural limits. They may 
therefore be readily subdivided into natural botanical districts, each of 
which will comprise the whole or some definite portion of the basin of a 
river, and can therefore be very easily recognised in practice. The object 
of the present sketch is to point out how the geological structure and 
physical aspect of the county are mirrored and reflected in its vegetation, 
* Vide “ Cybele Britannica,” vol. i. pp. 9 — 19. + Ibid. vol. iii. pp. 524 — 528. 
