52 



The Naturalist. 



But the great end, the scientific purpose, our map must serve, is, 

 that a member referring to it maj not only be able to say of such 

 flora or fauna as he may have found that they occurred in such and such 

 a river-basin, near such and such a village, on such and such a 

 common, — ^but may he led from the shading, colouring, or other 

 indication marking the locality upon the map, to associate in his 

 mind the reasons why the plants he found occurred where he found them. 

 Can we suggest such a connection upon a map at all ? I think we can. 



11. Features governing Distribution. — What, then, we must 

 first ask, are the features which most potently govern or influence the 

 distribution of plants and animals over any tract such as our county ? 

 These it is we must express before everything else upon our map. In 

 two words, they are climate and soil ; but since we can hardly mark 

 these in their collective sense upon a map, we must analyse the 

 various influences bound up in the words, and see if we cannot 

 express these. 



Accumulated experience has shown that the climate of a tract 

 depends upon a complexity of circumstances. Briefly, these are — its 

 altitude above sea-level, determining its mean annual temperature ; 

 its aspect, its exposure to winds ; and the average amount of moisture 

 in the air, its humidity and rainfall. Plants and animals have their 

 likes and dislikes in regard to all of these ; they are the chief factors 

 in the sum of distribution. As we leave the sea and follow a river 

 upwards towards its source, the loftier level we reach the colder it is 

 upon the average of the year, the more exposed to wind it is, and the 

 more rain falls. The greatest rainfall actually happens not to be upon 

 the highest peaks, but in the little valleys below them ; this, however, 

 does not invalidate my general proposition. With these changes in 

 climate comes a change in flora and fauna. Some plants loving 

 warmth, a high annual temperature, and little moisture, gradually 

 disappear ; and taking their place upon the hill-slopes, in part, but 

 never wholly compensating their loss as to number of species, we find 

 others, montane in type, whose conditions of life are ampler. Now, 

 we can upon a map express these climatal conditions by adopting 

 certain zones or hells of altitude. In Yorkshire we have three easily 

 discriminated. The lowest one, the Mid- Agrarian, embracing all the 

 surface at a lower elevation than 300 yards, its upper line indicating 

 the limit at which wheat can be grown to ripen ; the middle one, the 

 Super- Agrarian, between 300 and 600 yards, its upper line the limit 

 of Fteris aquilina in Yorkshire ; and the highest one the Infer- Arctic, 

 ail that surface above 1800 feet. 



