42 



THE ZOOLOGIST. 



This is wholly inexplicable when one considers how important 

 a matter vertical distribution is, and in the British Isles in par- 

 ticular it is fraught with a considerable degree of scientific interest ; 

 for, as is well known to students of physical geography, vertical 

 distribution estimates the affinities existing between the fauna 

 and flora of disconnected and isolated mountain chains and far- 

 distant latitudes. As far as the British Isles are concerned, it 

 affords a very efficient indication of the affinities existing, on the 

 one hand, between the montanic molluscous forms inhabiting 

 all the higher altitudes in the mountain districts of the British 

 Archipelago and their equivalents or representatives occurring 

 at lower altitudes in more boreal latitudes — that is, in the Arctic 

 and sub-Arctic regions of Scandinavia, Kussia, Siberia, and 

 North America ; and, on the other hand, at higher elevations still 

 in the Alpine and sub-Alpine areas occupied by the Alps, Pyre- 

 nees, Carpathians, and other elevated mountain ranges of 

 Central and Southern Europe. Then, again, some of the low- 

 land and maritime forms occurring exclusively in the South of 

 England, Wales, and Ireland possess their nearest relatives and 

 allies in the more austral region of the Lusitanian Conchological 

 Province (in the South of France and in the Iberian Peninsula). 

 It is thus evident that vertical distribution provides a better 

 index concerning the extreme range of temperature and other 

 climatic phenomena which each species can endure than mere 

 geographical distribution is capable of accomplishing in any- 

 thing like the same space upon the horizontal isotherms. For 

 example, there are greater differences of temperature experienced 

 in ascending a hill only three thousand feet in elevation than 

 there exists between the Scilly Isles and the extreme north of 

 the Shetland Islands, which are distant from one another about 

 seven hundred miles. On the average it will be found that the 

 distance afforded by two degrees of latitude (i. e. one hundred 

 and thirty-nine miles) either in a boreal or austral direction is 

 capable of producing only a difference in the mean annual tem- 

 perature of about one degree Fahrenheit, which a trifling vertical 

 ascent or descent of a hundred yards will accomplish upon the 

 side of any hill. This is well exemplified in Dr. H. C. Watson's 

 well-known treatise entitled ' Cybele Britannica,' which deals 

 with the geographical and vertical distribution of the British 



