254 ESSAY ON CLASSIFICATION 



improvements and additions. Among the additions the indication 

 of the orders, which are introduced in brackets in the diagram above, 

 deserve to be particularly noticed. These changes relate chiefly to 

 the Mollusks and Polyps; the Tunicata and Bryozoa being removed 

 from the Polyps to the Mollusks. The Acalephs and Polypi, however, 

 are still considered as forming together one single class. 



The comparison instituted by Van Beneden between his classifi- 

 cation of the animal kingdom and that of the plants most generally 

 adopted now, leads me to call again attention to the necessity of 

 carefully scrutinizing anew the vegetable kingdom, with the view 

 of ascertaining how far the results I have arrived at concerning the 

 value of the different kinds of natural groups existing among animals 

 apply also to the plants. It would certainly be premature to assume 

 that because the branches of the animal kingdom are founded upon 

 different plans of structure the vegetable kingdom must necessarily 

 be built also upon different plans. There are probably not so many 

 different modes of development among plants as among animals; un- 

 less the reproduction by spores, by naked polyembryonic seeds, by 

 angiospermous monocotyledonous seeds, and by angiospermous dicot- 

 ylodonous seeds, connected with the structural differences exhibited 

 by the Acotyledones, Gymnospermes, Monocotyledones, and Dicot- 

 yledones, be considered as amounting to an indication of different 

 plans of structure. But even then these differences would not be so 

 marked as those which distinguish the four branches of the animal 

 kingdom. The limitation of classes and orders, which presents com- 

 paratively little difficulty in the animal kingdom, is least advanced 

 among plants, whilst botanists have thus far been much more ac- 

 curate than zoologists in characterizing families. This is no doubt 

 chiefly owing to the peculiarities of the two organic kingdoms. 



It must be further remarked that in the classification of Van Bene- 

 den the animals united under the name of Allocotyledones are built 

 upon such entirely different plans of structure, that their combina- 

 tion should of itself satisfy any unprejudiced observer that any prin- 

 ciple which unites them in that way cannot be true to nature. 



DIAGRAM OF THE DEVELOPMENT OF ANIMALS BY KOLLIKER 



KoLLiKER in his Entwickelungsgeschichte der Cephalopoden (Zurich, 1844), I, 175, has 

 submitted the following diagram of the development of the animal kingdom. 



