428 X. GENERAL REMABKS. 



species, not known in B, then the diversity would be 

 positive on both sides. 



It is conjectured that a distinction between positive 

 and negative diversity, akin to that here made, will 

 eventually prove to be more important in phyto-geogra- 

 phical investigations than has hitherto been made to 

 appear. Differences of this sort are mostly found to be 

 very slight when the floras of two small adjacent tracts 

 are compared together, especially if the tracts are very 

 similar in their geographical features. They become 

 more evident when the flora of one small tract is put in 

 comparison with that of a much larger adjacent tract ; 

 the diversity being then principally, if not exclusively, 

 negative in the small tract, positive in the large tract. 

 The diversity is never entirely negative on either side, 

 between two extensive tracts or countries however closely 

 adjacent to each other, or between any two tracts dis- 

 tantly separated, whether large or small; — and in the 

 latter cases, it may perhaps be correctly added, whether 

 the intervening space is terrestrial or oceanic. In such 

 instances, it is believed, the floral diversity will always 

 be found positive for both tracts. And if remote coun- 

 tries are contrasted against each other, the floral diver- 

 sity becomes sufficiently wide to out-balance the species- 

 identity, so usually existing in excess over the diversity 

 between neighbouring countries. 



Floral diversities are found all over the world ; varying 

 much in kind and quantity, but everywhere perceptible. 

 Frequently, the}'^ may be said to admit of a partial expla- 

 nation ; that is, in so far as the tracing out of some 

 seeming relations with present geographical and climatal 

 conditions can be deemed explanatory of the facts. The 

 antecedent pages of this volume, pai'ticularly those con- 

 taining the summary of distribution and census of orders, 



