246 ESSAY ON CLASSIFICATION. 



into larger or smaller communities, as we observe them, 

 particularly among Polyps and Acalephs. These aggrega- 

 tions have not, as far as their form. is concerned, the same 

 importance as the form of the individual animals of which 

 they are composed, and therefore seldom afford trust- 

 worthy family characters. But this point may be more 

 appropriately considered in connection with the special 

 illustration of our Hydroids, to which the third volume 

 of my Contributions is to be devoted. 



I have stated above, that botanists have defined the 

 natural families of plants with greater precision than 

 zoologists those of animals ; I have further remarked also, 

 that most of them make no distinction between orders 

 and families. This may be the result of the peculiar 

 character of the vegetable kingdom, which is not built 

 upon such entirely different plans of structure as are 

 animals of different branches. On the contrary, it is 

 possible to trace among plants a certain gradation between 

 their higher and lower types more distinctly than among 

 animals, even though they do not, any more than animals, 

 constitute a simple series. It seems to me, nevertheless, 

 that if Cryptogams, Gymnosperrns, Monocotyledones, and 

 Dicotyledones can be considered as branches of the vege- 

 table kingdom, analogous to Kadiata, Mollusks, Articulata, 

 and Vertebrata among animals, such divisions as Fungi, 

 Algse, Lichens, Mosses, Hepaticse, and Ferns in the widest 

 sense, may be taken as classes. Diatomacese, Confervse, 

 and Fuci may then be considered as orders, Mosses and 

 Hepaticse as orders, and Equisetaceoe, Ferns proper, Hy- 

 dropterids, and Lycopodiacere as orders also, as they 

 exhibit different degrees of complication of structure, 

 while their natural subdivisions, which are more closely 

 allied in form or habitus, may be considered as families ; 



