ZOOLOGICAL CLASSIFICATIONS. 203 



violently separating animals from their natural affinities, and 

 grouping together those which have none. 



364. Hy the natural method, the divisions and subdi- 

 visions of the animal kingdom are founded on the whole of 

 the characters furnished by each animal, arranged according 

 to their degree of respective importance ; thus, in knowing 

 the place which the animal occupies, we also know the re- 

 markable traits of its organization, and the manner in which 

 its principal functions are exercised. 



365. The rules to be observed in arriving at a natural 

 classification of the animal kingdom are of extreme simplicity, 

 but often there is much difficulty in the application. They 

 may be reduced to two, for the object of the zoologist in 

 establishing such a classification is, 



1st. To arrange animals into natural series, according to 

 the degree of respective affinities, that is to say, to distribute 

 them in such a manner that the distance from, or proxi- 

 mity to, a species, is the measure of the resemblance or 

 dissemblance. 



2nd. To divide and subdivide this series according to the 

 principle of subordination of characters, that is to say, by 

 reason of the importance of the differences which these animals 

 present between them. 



366. To be satisfied, for example, of the affinity which 

 exists between the cat and the tiger, it is not necessary to 

 study the anatomy of these animals, for the external forms 

 translate, as it were, the character of the internal. But in a 

 great number of instances the examination of the internal 

 structure becomes necessary, in order to avoid important 

 errors. Thus, for a long time, the relations which exist 

 between the Iernsea3, parasitical animals with strange forms 

 (Fig. 155), which live on fishes, and the smaller Crustacea of 

 fresh waters, known to zoologists by the name of Cyclopes 

 (Fig. 157), were not understood ; because in their adult state 

 these two animals do not resemble each other, but since 

 naturalists have studied their development they have become 

 convinced of their relationship, for when young they differ so 

 little from each other that it would be often difficult to dis- 

 tinguish them. (Figs. 156 and 158). Finally, to fulfil the 

 first of the two conditions pointed out above, it becomes neces- 

 sary to overcome other difficulties depending on the multi- 

 plicity of the relations of each animal with those surrounding 

 it, and of the diversity of the transitions by which nature 



