INTRODUCTORY EXPLANATIONS. 



41 



zones to be accurately traced within it ; but the external 

 shadings will show the three agrarian zones on the coast- 

 line. 



It is probable that six such zones are quite as many as 

 can be satisfactorily distinguished when attention must be 

 directed to the whole surface of Britain. Even with six 

 only, it would be useless to attempt very great precision 

 in assigning the species to their respective zones. We 

 must disregard the occasional and slight tresspass of a 

 species into a zone above or beneath those to which it is 

 otherwise restricted by its natural adaptation to their cli- 

 mate. Such trespasses (if the expression be allowable) are 

 usually found in connexion with some local peculiarity by 

 which the true climate is varied, or its influence on the 

 plants modified. For example, the cool spray of a water- 

 fall, or the efflux of a cold spring, will support the species 

 of a colder climate than is natural to the latitude and ele- 

 vation. And, on the contrary, the plants of a warmer cli- 

 mate will grow on the south face of rocks which are raised 

 above the altitude where the same species cease to exist 

 in more bleak situations. These apparent exceptions come 

 inconveniently in the way of precise lines on botanico-geo- 



G 



