LITHOLOGY. _ 69 



the moorlands of both lithological types, the difference for them 

 as a class, being certainly, as has been already said, a greater 

 degree of frequency and luxuriance in the eugeogenoas tracts ; 

 but not such a restriction as we have seen there is in the case 

 of the plants of the dry-loving category. 



To sum up then the bearings of the subjacent rocks upon the 

 topography of our North Yorkshire vegetation as tested by a 

 comparison of the distribution of species within our limits and 

 in the country respecting which M. Thurmann treats we may say : 



1. As compared with the flora of Central Europe the flora 

 of North Yorkshire is one of a predominantly damp-loving stamp. 



2. The species which in Central Europe are restricted to 

 dysgeogenous tracts only occur in North Yorkshire in small 

 number and are there restricted lithologically in a similar manner. 



3. The species which in Central Europe are restricted to 

 eugeogenous tracts are many of them plants of North Yorkshire 

 also : and under the more boreal and more humid climate grow 

 abundantly and cover wide areas of surface, without keeping up 

 any clearly-marked role of lithological restriction. 



And this shows us clearly that the nature of the subjacent 

 rock both may and does interfere ,to modify the influence of 

 atmospheric climate upon plant-topography, and it points out 

 also in what direction the interference operates. A more porous 

 and more humid soil evidently to some extent compensates for 

 a drier climate. In proportion as the climate is damper the 

 characteristically dry-loving species are more and more rigidly 

 restricted to dry-soiled tracts of country. This is the rule and 

 in botanico-geographical considerations it is evidently worth 

 bearing in mind : but to what extent it has operated in deter- 

 mining which species we should have and which we should not 

 have either in North Yorkshire or in Britain as a whole ; to 

 what extent it has for instance operated in the restriction to the 

 area which they occupy in our country of the plants of Mr. 

 Watson's Germanic type of distribution we can but guess vaguely. 



April 1888. 



