422 X. GENERAL REMARKS. 



There remains yet another aspect meriting notice, 

 under which the ordinal grouping of plants may he 

 viewed in relation to a vegetal census. A glance at the 

 fourth column in the collective census, pages 359 to 361, 

 suffices to see that the species of some of the orders tend 

 to prevail in the vegetation, taken one with another, much 

 more than do those of other orders. The averages for some 

 orders are high, for others they are low, when compared 

 with the mean average of 41 for the whole. It is inte- 

 resting to seek an answer to the question, which of the 

 orders thus tend to prevail in the vegetation of this 

 island, hy the wide diffusion and frequent occurrence of 

 their individual species, whether few or many ? In the 

 ' ordinal census of the vegetation,' as printed on page 419, 

 numbers and frequency of species are taken together. 

 Suppose, for the sake of an illustration hy even figures, 

 one order including 20 species, with 50 comital repeti- 

 tions for each ; — another order including only 10 species, 

 with 100 repetitions for each. The aggregate census 

 would be 1000 for each of the orders ; and this figure 

 would correctly represent the total numerical value of the 

 orders in both cases. On the one side, the doubled 

 number of species would compensate for their lesser fre- 

 quency. On the other side, the doubled frequency would 

 in turn compensate for their scantier numbers. But this 

 ordinal equality would fail to express the special distinc- 

 tion between the orders ; namely, that the species of one 

 order were comparatively rare, and those of the other 

 order were comparatively common. The fourth average 

 or column referred to was added to the third or census 

 column, with a view of showing this difference ; and the 

 citation here of a few examples may render it more clear. 



The order Plantaginacece includes six species. One of 

 these is an inconspicuous aquatic ; a second of them is a 



