XXXVIU INTEODUCTION. 



case, too, we should find that the apparent distinction Ibetween 

 general and local abundance almost entirely disappeared, the two 

 groups of facts on which it depends gradually becoming merged into 

 one. 



The value of botanical districts is well exemplified in Mr. Watson's 

 Cyhele Britannica, in which the whole of Great Britain is so treated. 

 To this comprehensive work, and especially to the fourth volume, 

 we refer our readers for full explanations of the philosophy of plant- 

 distribution. 



We have divided the county of Middlesex into seven districts, and 

 the principle followed has been to adhere strictly to the natural 

 drainage, i. e. to make the land drained by oach stream (when not too 

 small) a separate division, which, when necessary, is divided again. 

 It results from this, that the districts are irregular in shape and 

 not very equal in size, and it may be asked what advantages are 

 gained by such divisions when the county might be easily cut up 

 into neai-ly equal spaces with regular and easily-defined boundaries. 

 To this there are several answers : — (1.) It has been frequently shown 

 by geogi-aphical botanists, that hills or watersheds separate floras, which 

 are formed naturally by river basins. (2.) The system was followed 

 out for the primary divisions in the Cyhele Britannica with the best 

 results. (3.) Inasmuch as a county is a tract of country with often 

 purely artificial boimdaries, it is very desirable that its flora should 

 be examined and recorded with reference to some fixed natural con- 

 formation applicable to all such tracts, so that, neighbouring counties 

 being similarly treated, their adjacent districts may fit together and 

 form complete river-basins. (4.) In particular, the Flora Hertford- 

 iensis is drawn up on the same plan, and with this, as the boundary 

 between the two counties is entirely artificial, it is important that 

 the present Flora should correspond. Of course, in so small a county 

 as Middlesex it is scarcely to be expected that results of any import- 

 ance can be obtained ; still it seems right to carry out what is believed 

 to be a sound principle to its furthest application. 



An account of the rivers and streams of the county has been already 

 given (see pp. xvii. — xxv.) and the districts into which, by their 

 means, it is divided are these : — 



I. Upper Colne. IV. Upper Brent. 



II. Lower Colne. V. Lower Brent. 



III. Cran. VI. Lea. 



VII. Metropolitan (drained by the Fleet, "West Bourne, Wall 

 Brook, &e.). 



