168 ESSAY ON CLASSIFICATION 



of association of individuals into larger or smaller communities as 

 we observe them, particularly among Polyps and Acalephs. These 

 aggregations 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 trustworthy family charac- 

 ters. But this point may be more appropriately considered in con- 

 nection with the special illustration of our Hydroids, to which my 

 next volume^^ 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 cer- 

 tain gradation between their higher and lower types more distinctly 

 than among animals, even though they do not, any more than ani- 

 mals, constitute a simple series. It seems to me, nevertheless, that if 

 Cryptogams, Gymnosperms, Monocotyledons, and Dicotyledons can 

 be considered as branches of the vegetable kingdom, analogous to 

 Radiata, Mollusks, Articulata, and Vertebrata among animals, such 

 divisions as Fungi, Algse, Lichens, Mosses, Hepaticae, and Ferns in 

 the widest sense may be taken as classes. Diatomaceae, Confervas, and 

 Fuci may then be considered as orders; Mosses and Hepaticse as or- 

 ders; Equisetaceas, Ferns proper, Hydropterids, and Lycopodiaceas 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; natural 

 families among plants having generally as distinct a port, as families 

 among animals have a distinct form. We need only remember the 

 Palms, the Coniferae, the Umbelliferas, the Compositas, the Legumi- 

 nosas, the Labiatas, etc., as satisfactory examples of this kind. 



21 [See Contributions ... Ill (I860).] 



