Vin. LATITUDE. 453 



to under the tj'pes of distribution, and may be dismissed 

 for the present. 



8. Relation of Plants to Latitude. 



In tracing along the earth from north to south, or in 

 the reversed direction, within any given range of longi- 

 tude, far wider floral changes may be observed than 

 those alluded to in the preceding section. A successive 

 appearance and disappearance again occurs ; but it is 

 effected more rapidly and more completely. A change of 

 less than seventy degrees in latitude, from equator to 

 arctic circle, or from polar to tropical lands, — one-fifth 

 round the globe in latitude, — may be said to give almost 

 a complete change of species, on the low plains and 

 shores, and not far from a complete change of genera ; 

 with very wide changes also in the numerical values and 

 proi^ortions of orders. These floral changes are much 

 greater than those which correspond with a longitudinal 

 change amounting to one-fifth of the circumference of the 

 globe. 



Within the small area of Britain, as before stated and 

 shown, the floral changes in connexion with latitude also 

 much exceed those in connexion with longitude ; the 

 usual excess in one direction being probably augmented 

 in Britain through the elongated form and irregular out- 

 line of the island. In the ' summary of distribution ' and 

 ' census of orders,' pages 175 and 362, three latitudinal 

 divisions are made, with the pm'pose of giving a middle 

 and two extremes ; the triple comparison better sufficing 

 to show a gradation of difference ; the comparison be- 

 tween two divisions 'serving only to show a single dif- 

 ference. But it was stated that no such three-fold 

 division could be suitably made in connexion with longi- 



