Of the above species, 244 are absent from Middlesex, whilst oceurriiig in 

 one or more of the surrounding counties: of these, however, only 19 are 

 found in all four adjacent counties, and most of them are chalk plants. 



Of the 200 Middlesex species which are wanting in one or more of the 

 surrounding counties, only 11 are absent from all four. 



V. Comparison with M. Thur?naiin's ' Essai de Phytostatiqiie' (1849). — 

 M. Thurmann, from an examination of the Jura and adjacent districts of 

 France and Switzerland, arrived at the conclusion that the distribution of 

 plants is influenced chiefly by the mechanical properties of the subjacent 

 rocks, and but very slightly by their chemical composition. 



He divides rocks into two classes: ' dysgkogencs' and ' eiigeogenes,' pos- 

 sessing opposite physical properties. Dysgeogenous rocks, though very 



