400 X. GENERAL REMARKS. 



single centres of inchoation, — before explained to mean, 

 natural creation or commencement, — those centres can 

 hardly have been for European orders or genera, as 

 groups, but only for species. Otherwise, it would seem 

 to be a warrantable inference, or very probable expecta- 

 tion, that the more circumscribed the space under view, 

 the more should its plants have belonged to the same 

 orders, and to the same genera, — the less diversity should 

 have been found in the flora. 



5. Numerical value of the Orders. — Phyto-geographers 

 usually appear to attach much importance to comparisons 

 between the numbers of species belonging to diiferent 

 orders, and to the per-centage proportions that the several 

 orders bear to the total flora. A table of such propor- 

 tions will assist in showing the leading peculiarities of 

 any single flora, and also the distinctive peculiarities be- 

 tween two floras, when these are sufficiently diversified to 

 alter the ordinal proportions. But the non-equivalence 

 of the orders, before j)articularly adverted to, pages 20 to 

 25, on which their inequality of numbers in any country 

 is partly consequent, renders even those proportions less 

 strictly comparable than they would be, if the orders 

 were more the equivalents of each other in structural and 

 physiognomical characters. The proportions of Gramina 

 are those of a numerous and well-distinguished group ; 

 and they represent the sum of many genera. The pro- 

 portions of Juncacece are those of a less distinct group ; 

 and they represent the sum of only few genera. The 

 proportions of Callitrichacece are simply generic, scarcely 

 entitled to be called ordinal at all ; representing one 

 genus only ; and that one with no decided or obvious 

 characters to stamp its distinctness, — say, for instance. 



