149 



Now it will be found that the character of each species of plant is more 

 or less affected, and its distribution limited by certain physical conditions 

 acting on and through those powers of adaptation or special vital endowments 

 which undoubtedly vary even in species of the same genus. This special con- 

 stitution, then, will be more oonspicuous in a limited area, neutralised by 

 other and more complex agencies incidental to a wider field of distribution ; 

 hence we find " the most striking varieties are found in those families which 

 are capable of the widest geographical extent." 



In speaking of the temperature, &c, of a range of hills sixteen miles in 

 length, we should require a very accurate measuie of the sums of summer 

 heat and the minima of the colder parts of the year, with hygrometric obser- 

 vations, extended also over many years, to deduce any very definite rules to 

 guide us in the vegetable topography of such a surface ; but we can approxi- 

 mate, for doubtless each plant has its special zero. We find, accordingly, 

 classified to hand, certain montane species, then next, others locally ascend- 

 ing, and a third division spreading throughout or descending to the plains 

 below ; though in some cases their head quarters may be on the heights and 

 slopes. I repeat, on this head we can approximate merely, for as yet we have 

 no precise physical formula determined on jointly by meteorologists and 

 naturalists to express the effect of moisture on organic life. The main tem- 

 perature of the air, as we ascend from the vale to the hills, sinks at an 

 average rate of about one degree Fahrenheit for every hundred yards, the 

 lowering being in maxima than minima, and the ground mean temperature 

 falling less rapidly than the aerial mean. 



The proximity of large masses of land, and the distance from the sea, 

 are other factors which impress a certain character on the winds sweeping 

 over the table lands of these hills, contiibuting to make the harvests later 

 here than they are in other counties more favourably situated in this respect. 



Mr. Carr has kindly favoured me with the average rain-fall, collected 

 by himself, from observations in this neighbourhood, over a space of five years ; 

 and, looking at this return, I thick it may fairly be stated that the rain-fall 

 is not so much as we should a priori expect in a mountainous neighbourhood, 

 though, from the accounts of some visitors who do not consult their weather 

 glasses before coming here, Jupiter Pluvius would seem to reign supreme. 



The absence of any large body of water or morass, the arid nature of the 

 subsoil, and the fact of the vale of Church Stretton itself being at a consider- 

 able elevation above the sea level, are favourable to rapid radiation after rain 

 and a comparatively dry atmosphere. * 



The petrology of the Longmynd strata, or the mode in which their con- 

 stituent particles are combined, has, in some situations, a greater influence on 

 its vegetation than the mere lithology or class of rock, in the comparative 

 facility or resistance of abrasion occasioned by such differences in the inti- 

 mate structure. Where we find the siliceo-aluminous texture and hard con- 



