a INTEODUCTORY EXPLANATIONS. 



of distribution may appear in comparison and contrast, 

 as reciprocal illustrations of each other. 



The causes that now continue the existing distribution 

 of plants over the surface of the earth, or those that have 

 originally and gradually determined their distribution, 

 are too wide in their influence, to admit of being proj)erly 

 treated in a work devoted to the plants of one small 

 country, and to their distribution within that Hmited 

 sj)ace only. Should the Author have life and leisure to 

 carry out his present wishes, and enduring inclination 

 adequate to the task, he may perhaps write a ' British ^**- A^a^ 

 and Foreign Cyhele, ' for the purpose of tracing the dis- y^* 



tribution of British species over other parts of the earth, 

 and of showing the true relation borne by the flora of 

 Britain to the floras of neighbouring countries. The 

 causes or conditions of their distribution might then ap- 

 propriately find place and room in a work of that more 

 com]3rehensive, and necessarily less detailed, character. 

 His investigations have not hitherto led him to adopt the 

 current opinion (or, rather, mere guess) that the flora of 

 the British islands has been derived from the opposite 

 countries of the Continent, — at least, not to any greater 

 proportionate extent, than the floras of those countries 

 may be said to have been derived from Britain. Inter- 

 change has most likely taken place; Britain giving, as 

 well as receiving. 



It is not expected that the fourth volume of Cybele 

 Britannica can be published within two years from the 

 date of the present volume, if so early as only two years 

 after. That contemplated fourth volume would of course 

 be founded upon the facts detailed in the three earlier 

 volumes ; indeed, such a volume might now be made 

 by a connected and comparative re-arrangement of the 

 same details. But there is still much that bears upon 



