146 ESSAY ON CLASSIFICATION. Part I. 



equally indiscriminate application of the results obtained, to characterizing classes. 

 Those who have not made a proper distinction between the plan of a structure 

 and the manner in which that plan is actually executed, have either overlooked 

 the importance of the great fundamental divisions of the animal kingdom, or they 

 have unduly multiplied the number of these primary divisions, basing their dis- 

 tinctions upon purely anatomical considerations, that is to say, not upon differences 

 in the character of the general plan of structure, but upon the material develop- 

 ment of that plan. Those, again, who have confounded the complication of the 

 structure with the ways and means by which life is maintained through any given 

 combination of systems of organs, have failed in estabhshing a proper difference 

 between class and ordinal characters, and have again and again raised orders to 

 the rank of classes. For we shall see presently, that natural orders must be based 

 upon the different degrees of complication of structure, exhibited within the limits 

 of the classes, while the classes themselves are characterized by the manner in 

 which the plan of the type is carried out, that is to say, by the various com- 

 binations of the systems of organs constituting the body of the representatives of 

 any of the great types of the animal kingdom ; or perhaps, still more distinctly, 

 the classes are characterized by the different ways in which life is maintained, and 

 the different means employed in estabhshing these ways. An example will suffice 

 to show that this distinction implies a marked difference between class and ordinal 

 characters. 



Let us compare the Polyps and Acalephs as two classes, without allowing our- 

 selves to be troubled by the different limits assigned to them by different authors. 

 Both are constructed upon the same plan, and belong, on that account, to the type 

 of Radiata. In establishing this fact, we do not consider the actual structure of 

 these animals, whether they have a nervous system or not, whether they have 

 organs of senses or not, whether their muscles are striated or smooth, whether 

 they have a solid frame or an entirely soft body, whether their alimentary cavity 

 has only one opening or two opposite openings, whether it has glandular annexes 

 or not, whether the digested food is distributed in the body one way or another, 

 whether the undigested materials are rejected through the mouth or not, whether 

 the sexes are distinct or not, whether they reproduce themselves only by eggs, or 

 by budding also, whether they are simple or not : all we need know, in order to 

 refer them to the branch of Radiata, is whether the plan of their structure exhibits 

 a general radiated arrangement or not. But, when we would distinguish Polypi, 

 Acalephs, and Echinodenns as classes, or rather, when we would ascertain what 

 are the classes among Radiata, and how many there are, we must inquire into the 

 manner in which this idea of radiation, which lies at the foundation of their plan 

 of structure, is actually expressed in all the animals exliibiting such a plan, and 



