102 The Scottish Naturalist. 



shire, and should be included in Perthshire (as indeed it is in 

 the " Flora" of that county). 



If we adopt this mode of recording the horizontal range of a 

 species, the areas into which the district is divided will be as 

 follows : — 



i.— North Forth. 



2. — Tay. 



3. — Dee. 



This is not only the more philosophic method of divison, as being 

 best calculated to illustrate the natural distribution of species, but 

 one that is in harmony with several works, treating of the fauna or 

 flora, that have already been published. 



I am rather inclined to think that it may be found desirable, 

 while adopting these main divisions, to sub-divide them again, 

 using the catchment basins of the tributary rivers as the sub-areas. 

 This is, however, a matter for consideration, and one which I will 

 not take up at present. 



Another matter for consideration is how far seawards is the 

 district of the Union to be extended. 



3. — The vertical range of a species. This is both an important 

 and an interesting feature of the distribution, and one that has sev- 

 eral aspects. As regards terrestial species, we must trace the 

 limits of the distribution up the hills in the case of lowland species, 

 and both upwards and downwards in the case of alpine species. 

 As regards aquatic and especially marine organisms, the limits of 

 their distribution djwn wards or upwards have also to be traced. 



The importance of recording the vertical range, so far at least 

 as regards terrestial plants and animals, is very considerable when 

 we remember that the Union includes in its district the most 

 alpine ranges of mountains in Britian, and that though it has not 

 the very highest point of land, yet that it possesses by far the 

 largest tract of very elevated country, by reason of which the dis- 

 tribution of species is greatly affected. The vertical range should 

 be recorded in feet, and as regards the upward range of lowland 

 species it is scarcely necessary to indicate it when it is under 1000 

 feet. But as regards the downward range of alpine species, this 

 should be recorded as low as it can be traced. 



4. — Habitat and abundance. By habitat is meant the kind of 

 place where a species occurs, as e.g., "sea-shore" "rocks" 

 " marshes," " fir-woods," &c. The indication of this is simple 



