54 



The Naturalist, 



possible ; and it lias the disadvantage in tlie West-Riding of including 

 nearly every variety of climate and soil within the limits of each 

 drainage basin. The valleys of the Riding, being long and narroAV, 

 cut through the geological formations almost at right angles. The 

 only purpose river-basin or any other divisions not climatic or geologic 

 serve at all is the enabling a census of superficial distribution to be 

 made. When made, we can by it tell only whether any given species 

 is local or general or frequent, as it occurs in less than a third, or 

 more than two-thirds of the divisions adopted. It has no other use 

 than this — it tells us nothing as to why the plant is restricted to one 

 district or found in many, because it tells us nothing of the conditions 

 as to climate and lithology of such districts. Upon these depend 

 species' occurrences or absences, and in our county our river-basins- 

 are far from coinciding with geological conditions. Still it is not 

 quite correct to say that " the flora of two mountain streamlets arising 

 in the same range, and flowing in opposite directions into distant 

 oceans, would resemble each other far more closely than the flora of 

 a mountain torrent would that of the same stream when it had become 

 a slow navigable river." If the hill-range were lofty, and the streams 

 flowed east and west, or south-west and north-east, the flora would 

 very probably differ greatly (as happens in Yorkshire to the rivers Lune 

 and Wenning, running west) by reason of the new factor of climate^ 

 different exposure^ &c., introduced ; whilst as the mountain stream 

 enlarged and became a river, it would retain for a very long way down 

 a certain proportion of the floral characteristics it showed near its 

 source. Upon the banks of the main stream and larger streamlets, 

 long after they have entered upon the gritstone tracts in Airedale, 

 Wharfedale, and Nidderdale, do species of plants occur, even more 

 luxuriant in size than in their real home — telling the story of how 

 winter-torrents and floods of old, dislodged dormant roots or seeds 

 from their crevices high up among the hills, washed them down to 

 deposit them in the back-washed mud of river creek or alluvial drift 

 overlying a stratum whose detritvs alone would not support their life. 

 The gritstone of even triassic tracts are enriched with many species in 

 this way, whose homes are properly far away, and often thus the 

 natural flora is masked and confused by the adventitious one, as much 

 as the real geology is by the overlaid drift and gravel. 



Hill ridges, too, if lofty, even if looking N W and S E, do often 

 present an impassable barrier to plants. This is only not true of 

 grasses, compositse, and those plants whose berries and seeds are to 

 any extent the food of birds. 



