IV. VEGETATION OF BRITAIN. 413 



seen every year. The rarely found Menziesia ccerulea is 

 an unit of the British flora, equally with the abundant 

 Calluna vulgaris. Widely unequal, however, are these 

 two shrubs in the vegetation. The insignificant Limosella 

 aqiiatica, small in size and occupying only a small space 

 in each special locality, is florally an equal with Betula 

 alha or Vaccinium Myrtillus. By individual size, and by 

 mass of vegetation, the two latter are enormously supe- 

 rioi'. The genus Orobanche, with its eight or ten species, 

 counts as the equal of Hypericum or Viola in the flora of 

 this country. But in its vegetation single species of 

 Hypericum or Viola prevail more than do all the species 

 of Orobanche added together. 



No method has yet been devised for adequately ex- 

 pressing these vast differences between species, even 

 when viewed singly ; much less when grouped into 

 orders, &c. The terms usually employed — ' rare -scarce - 

 occasional - common - abundant ' — are quite inadequate ; 

 although, if mentally construed to mean ' units- tens - 

 hundreds - thousands - millions,' and qualified by the addi- 

 tion of ' rather ' and ' very ' for intermediates and 

 extremes, they may serve to express the facts intelligibly, 

 if somewhat too unprecisely. But when thus understood 

 and used, they belong properly to the census of species, 

 and are badly applicable to groups of any kind. It is 

 very desirable to have some method for showing the com- 

 parative prevalence of orders and of genera, which shall 

 bring into joint account the number of their species and 

 also the frequency and dimensions of their species. Very 

 difficult is it to make any fair approximation to such a 

 joint result ; and not easy, even if the most difficult item 

 in the account, that of size or dimensions, be left out of 

 the reckoning. 



On looking to the last section of the ' census of species,* 



