MORPHOLOGICAL GROUrS. 23 



a genus. The generic groups thus constituted are susceptible 

 of being arranged in a similar manner into groups of succes- 

 sively higher order, which are known as families, orders, 

 classes, and the like. 



The method pursued in the classification of living forms is, 

 in fact, exactly the same as that followed by the maker of an 

 index in working out the heads indexed. In an alphabetical 

 arrangement, the classification may be truly termed a mor- 

 phological one, the object being to put into close relation all 

 those leading words which resemble one another in the 

 arrangement of their letters, that is, in their form, and to keep 

 apart those which differ in structure. Headings which begin 

 with the same word, but differ otherwise, might be compared 

 to genera with their species ; the groups of words with the 

 same first two syllables, to families ; those with identical first 

 syllables, to orders ; and those with the same initial letter, to 

 classes. But there is this difference between the index and 

 the Taxonomic arrangement of living forms, that in the for- 

 mer there is nothing but an arbitrary relation between the 

 various classes, while in the latter the classes are similarly 

 capable of coordination into larger and larger groups, until 

 all are comprehended under the common definition of living 

 beings. 



The differences between " artificial " and " natural " clas- 

 sifications are differences in degree, and not in kind. In each 

 case tlie classification depends upon likeness ; but in an artifi- 

 cial classification some prominent and easity-observed feature 

 is taken as the mark of resemblance or dissemblance ; while, in 

 a natural classification, the things classified are arranged ac- 

 cording to the totality of their morphological resemblances, 

 and the features which are taken as the marks of groups are 

 those which have been ascertained by observation to be the 

 indications of many likenesses or unlikenesses. And thus a 

 natural classification is a great deal more than a mere index. 

 It is a statement of the marks of similarity of organization ; 

 of the kinds of structure which, as a matter of experience, are 

 found universally associated together ; and, as such, it fur- 

 nishes the whole foundation for those indications by which 

 conclusions as to the nature of the whole of an animal are 

 drawn from a knowledge of some part of it. 



When a paleontologist argues from the characters of a 

 bone or a shell to the nature of the animal to which that bone 

 or shell belonged, he is guided by the empirical morphologi- 

 cal laws established by wide observation, that such a kind of 



