VERTICAL DISTPJBrTION OF BRITISH LEPIDOPTERA. 3 



occurring in the British Isles, on similar lines to those laid 

 down by Mr. H. C. Watson, in his excellent work on the geogra- 

 phical distribution of the flowering plants, well known to British 

 botanists as the ' Cybele Britannica.' The author, however, does 

 not treat Avhatever of their vertical distribution (which is no 

 doubt owing to the paucity of data bearing upon the subject), 

 nor has anyone else attempted to do so since, that I am 

 aware of. 



Now vertical distribution is, in my opinion, in every particular 

 as important as geographical distribution, for it estimates the 

 affinities existing, on the one hand, between the lowland species 

 occurring in this country and their relatives in more elevated 

 areas in the South of Europe; and, on the other hand, between 

 the montanic forms found in these isles and their representatives 

 occurring at still higher altitudes in the alpine regions of the 

 central and southern parts of the Continent, as well as at lower 

 levels in arctic and subarctic latitudes. 



Vertical distribution, besides, gives a better index as to the 

 range of temperature and other climatic phenomena which each 

 species can endure than mere geographical distribution is capable 

 of doing in anything like the same area. For instance, there are 

 greater differences of temperature found in ascending a hill only 

 some 3000 feet in altitude, than there exists between the Scilly 

 Isles and the extreme north of the Shetland Isles, which are 

 distant from one another about 700 miles. On the average, the 

 distance afforded by two degrees of latitude (i. e., 139 miles), 

 either in a northerly or southerly direction, is capable of pro- 

 ducing only a difference in the mean annual temperature of 

 about one degree Fahrenheit, which a trifling ascent or descent 

 of a hundred yards will do upon the side of any hill. This is well 

 exemplified in Mr. H. C.Watson's celebrated 'Cybele Britannica' 

 previously alluded to. 



Although Mr. Watson's geographical divisions (employed by 

 Mr. Jenner-Fust) are in the main probably the most natural 

 provinces which could possibly have been decided upon, his 

 vertical divisions or zones do not seem to be so well suited for 

 studying the ascending limits of the Lepidoptera in this country 

 as some which I shall suggest. 



The brothers Speyer, in their great work on the distribution 

 of the Swiss and German Lepidoptera, define five vertical or 

 ascending zones as follows : — 



1. The Lowland zone. This includes all the valley region up 

 to the limit to which the walnut and sweet chestnut will grow. 

 In the southern alps it terminates at about 3000 feet above the 

 level of the sea ; in the northern limestone alps it is somewhat 

 500 feet lower ; and 1000 feet lower still in Central Germany (in 

 the region of the Schwarz Wald, or famous Black Forest). 



2. The Hill zone, which is the next in the order of ascending, 



b2 



