V, SUMMARY OF DISTRIBUTION. 231 



Middle or North Britain. And as many circumstances interfere with 

 exactness in the processes resorted to, for ascertaining the heights of 

 localities, it may be as well to consider these tbree stages of altitude as 

 slightly overlapping each other ; for instance, like imbricating leaves. 



4. Climatal Zones. — In the fourth column the ascending or climatal 

 zones are expressed by the six corresponding numerals, ' 1 2 3' fur those 

 of the agrarian region, and '4 6 6' for the zones of the arctic region. 

 The ascending zones were explained in volume first, pages 19 to 43 ; 

 and they will probably (space sufficing) be again adverted to in this pre- 

 sent volume. Tt will be recollected, that they are designed to illustrate 

 the distribution of plants in accordance with the joint influence of lati- 

 tude and elevation, more or less modified by longitude and local condi- 

 tions. In general, the altitudes may be said to become lower as the 

 latitudinal distance from the south coast of England increases. This is 

 represented by the oblique lines of the little diagram introduced in the 

 left-hand lower corner of the lithographic map inserted in volume third. 

 Comparatively few species have so wide a range of latitude and elevation 

 combined, as to place them in all the six zones. 



5. Census. — In the fifth column the numerals show in how many of 

 the 112 counties and vice-counties (volume third, pages 526 to 528) the 

 species have been respectively ascertained to occur. No doubt several of 

 them do occur in all those 112 sections of the surface; though incom- 

 plete records and knowledge lead to the numbers being always stated 

 below 100. In reckoning up these numbers it was deemed better to 

 reject many recorded localities, as being old and unconfirmed, or as being 

 improbable in themselves, and reported only on unsafe authority. It 

 may here be explained, while alluding to this matter, that the safety or 

 reliability of the authority is not exclusively a personal consideration, 

 but must in some degree depend also on tbe plant itself. Thus, any 

 tyro may be trusted for a record of Bellis perennis or Calluna vulgaris in 

 any county ; because these and various other such familiar species can 

 scarcely be mistaken and misnamed by any one who studies botany at 

 all. Such is not the case with respect to many other plants; and no 

 botanist pretends to an exact knowledge of all the critical and segregate 

 species even of one single country. To establish the facts, that Cyperm 

 fuscus occurs in Yorkshire, and Lychnis alpina in Cumberland, and 



truly wild, it required the exhibition of specimens in proof, and also an 

 examination of their localities by botanists of skill and credit. To 

 induce belief in the alleged wildness of Gentiana acaulis near Haverford 



