120 ANNALS OF SCOTTISH NATURAL HISTORY 



climate, the chief requisite being moisture. In the case of 

 nearly all the Atlantic species this must be combined with 

 an equable temperature. This combination does not extend 

 far from the west coast, and the distance to which it extends 

 marks the point to which these species are prevalent. Next 

 to moisture in the atmosphere the most important requisite 

 is the physical condition of the soil. Where the nature of 

 the rocks is such that their disintegration does not form a 

 moist and retentive soil, it will be found that, even under 

 the otherwise most favourable conditions on the west coast, 

 Atlantic species are scarce or absent. This is very notice- 

 able in ravines running through sandstone and some volcanic 

 rocks. Nearly all species of hepatics are scarcer in such 

 ravines, but the Atlantic species, being the most delicate, are 

 affected in a more marked manner. 



The next point in treating of the distribution of these 

 species is that too much importance must not be given to 

 the watershed dividing Scotland west and east. Watersheds 

 in themselves have no relation to the distribution of Hepaticae, 

 whatever they may have in the case of phanerogams. The 

 north and east sides of the watershed, with their constantly 

 moist ground, have hepatics in plenty, while the dry sun- 

 exposed south and west sides have few. An instructive 

 instance can be seen where our main watershed divides 

 Argyllshire to the west and Perthshire to the east, at the 

 part of the latter county where its eastern watershed lies 

 nearest to the west coast. This is at Tyndrum and Ben 

 Laoigh. The west, or Argyllshire side, is dry and sun- 

 exposed, with little suitable ground for hepatics, while the 

 east and north sides, both belonging to the watershed of 

 the German Ocean, have abundance of hepatics, among 

 which are the characteristic Atlantic species Clasmatocolea 

 cuneifolia, Mastigopkora Woodsti, Scapania nimbosa, and 

 Cololejeunea microscopica, which have not been found else- 

 where except on our western watershed. It is the proximity 

 of the Atlantic, not the watershed, which is of consequence, 

 provided that there is moisture and shelter for these species. 

 It is only in places with such moisture and shelter that the 

 low ground species of the Atlantic type extend eastward 

 from the west coast. These conditions prevail a short 



