INTRODUCTION TO THE FLORA. 239 



of 400 yards is reached, such stations as are denominated by the 

 terms paludal, viatical, agrestal, sylvestral and septal exist no 

 longer, and that ericetal and uliginal greatly preponderate. We 

 see that many plants stop short of our higher altitudes, and yet 

 ascend elsewhere to much more boreal stations than we anywhere 

 have them. For instance, with us there are no trees, either wild 

 or planted, above 600 yards ; but in the Scotch Highlands the 

 Juniper ascends to 900 yards, the Rowan almost as high, the 

 Scotch Fir, Birch, Raspberry, Hazel, Gale, Aspen, Rosa vilhsa, 

 J?, spinosissima, and several Willows, to 500 yards and upwards, 

 500 yards under the more northern latitude being more than 

 equivalent to 600 yards in Yorkshire. But in spite of this it 

 cannot be doubted that the lowering of the number of species 

 as we ascend has a close connection with the lowering of the 

 sums of temperature. 



The hifluenceof Huinidity upon the Distributioji of Species. — The 

 distribution of aerial humidity and of the rainfall over the surface 

 of our field of study does not appear to exercise any considerable 

 direct influence upon the topography of its flora. The paludal 

 and lacustral plants which we have are naturally nearly all of 

 them restricted to the valleys and the lower levels of the slopes. 

 No doubt the greater humidity of the moorlands has something 

 to do with the restriction to their vicinity of the characteristically 

 Montane species, both the Flowering Plants and Ferns, and the 

 Mosses; and tends also to bring aljout a greater frequency and 

 luxuriance of many other damp- and shade-loving plants. The 

 average number of characteristically Hygrophilous plants in the 

 floras of the drainage districts is 39 for those of the West 

 against 45 for those of the East. Of five inland species of the 

 Atlantic type of distribution, three are confined to the eastern, 

 two to the western districts. Taking the flora of the eastern and 

 western sub-provinces as a whole, the most conspicuous and 

 essential difference between them is expressed when we say that 

 the West has a number of plants of the higher hills which the 

 East has not, and the East has a number of plants of the sea- 

 July T889. 



