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of these climatic zones. The midagrarian, or with reference to the county 
only, the lower zone, comprises all that portion of the surface which does 
not attain an, elevation of about 300 yards. The inferarctic or upper zone 
includes those mountain summits which exceed about 600 yards ; and the 
superagrarian or middle zone, the intermediate space. 
If we wish to make more exact and accurate comparisons of the vertical 
distribution of species than can be supplied by tracing their zonal range, 
it is easy to estimate, with sufficient accuracy for practical purposes, the 
isotherms or lines of mean annual temperature, within which their 
localities are respectively circumscribed. The influence of latitude in 
causing a variation between the climate of different parts of the county 
is so slight as scarcely to deserve consideration. The mean annual tem- 
perature of the whole coast line may therefore be stated in round numbers 
at 49 degrees, though in reality, it is rather less. That of all inland 
situations which are not much above the coast level, including the whole 
of the great central vale, may be taken at 48 degrees For the remainder 
of the surface, we shall not fall far short of accuracy in adopting the rule 
first proposed by Dalton of deducting one degree of mean annual tem- 
perature for every hundred yards of elevation. The summit of Micklefell, the 
highest mountain in Yorkshire, attains an elevation of 2580 feet. So that 
according to the rule just stated, its average annual temperature will be 
rather less than 40 degrees ; and consequently the range embraced within 
the limits of the county, about nine degrees. 
Although several districts have been only very partially explored, it 
is not probable that the relative proportion which the Yorkshire flora 
bears to that of the whole of Britain will be materially increased as 
knowledge advances : seeing that most of the species have been found 
which might be expected to occur, from their distribution beyond the 
limits of the county. But although such is the case, we can only estimate 
very imperfectly from the data which have been accumulated, the manner 
in which each species is diffused over the surface, and the proportion 
which each bears to the general mass of the vegetation. 
In order to obtain a means of judging respecting the horizontal distri- 
bution over the island, of the species which make up the British flora, and 
reducing to accurate expression through the medium of a numerical for- 
mula the knowledge which has been and maybe obtained respecting each, 
