II. ORDERS AND GENERA. 25 



may fairly be considered to represent the whole series, 

 with reference to the principles on which they are insti- 

 tuted. They plainly show that the same term ' Order ' is 

 used to designate groups which are in no sense on an 

 equality with each other. The ordinal groups of syste- 

 matic botany are as much non-equivalents in their dis- 

 tinctive structure or characters, as they are unequal in 

 numbers and diffusion. They are groups of twenty or 

 fifty gi-ades, co-equal with genera at one end of the scale, 

 while far above genera at the other end of the series. 

 Filices and Pinguicidacece, or Gramma and CallitricliacecB, 

 are surely groups which ought not to be placed on a par 

 by the same designation of " natural order." It would 

 be a juster view of nature, to put Filices and Carices, or 

 Gramina and Rzibi, on the same level of rank. 



Similar remarks might be extended to genera. Al- 

 though genera have been formed on a more uniform plan 

 than orders, for reasons before adverted to, even these 

 gi'oups are not co-equal or co-equivalents. The species 

 of the great and diversified Linnean genera Erica and 

 Euphorbia (the view must here stretch beyond Britain) 

 are surely kept together on some principle very different 

 from that which has been practically acted upon in the 

 subdivisions of genera among the CrucifercB and Umbel- 

 liferce. And if we compare together Ranunculus aquatilis, 

 R. acris, and R. Ficaria, — or Saxifraga iimbrosa, S. op- 

 positifolia, and S. hypnoides, we see differences between 

 these species that are stronger than those ti'aceable be- 

 tween the genera Brassica and Sinapis, or Carum and 

 Bunium, or Crepis and Hieracium, or Aster and Erigeron. 



In comparing the vegetation of different countries, or 

 of different climates, Phyto-geographers occasionally seem 

 to attach much importance to the number of orders or 



VOL. IV. E 



