194 ZOOLOGY. 



(Fig. 132), which live on fishes.and the smaller Crustacea of fresh 

 \\ at rrs, known to zoologists by the name of Cyclopes (Fig. 134), 

 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, lor when young they differ so little from each other 

 that it would be often difficult to distinguish them (Figs. 133 and 

 135). Finally, to fulfil the first of the two conditions pointed 

 out above, it becomes necessary to overcome other difficulties 

 depending on the multiplicity of the relations of each animal 

 with those surrounding it, and of the diversity of the transitions 

 by which nature passes from one type to another. By reason of 

 these circumstances, it is also impossible to arrange animals in 

 a single linear series, without violating at every instant their 

 respective affinities, and we are obliged to disperse them into 

 several parallel lines, or lines branching out from each other. 

 367. The second condition in the establishment of a 

 natural classification, is an exact relation between the suc- 

 cessive divisions of the animal kingdom, and the importance 

 of the modification of structure serving as the basis to these 

 sections. 



The characters which distinguish animals from each other 

 are far from having the same value : some are of seemingly 

 little or no physiological importance, seeing that their varia- 

 tions do hot draw after them differences in the rest of their 

 economy; others never vary without coinciding with pro- 

 found modifications in the whole of their organization ; hence 

 they are called dominating, since they seem in some measure 

 to regulate these modifications. It is evident, then, that divi- 

 sions of an inferior rank can alone be based on subordinate 

 characters, while those of a higher rank ought to be founded 

 on those called dominating. To arrive, then, at a natural 

 classification of animals, it is above all necessary to know the 

 structure, functions, and mode of development of these beings; 

 next, to inquire into the dominating characters of the organi- 

 zation of each. This we arrive at sometimes by physiological 

 considerations, at other times by anatomy only. Fixity is an 

 index of an organic domination, whilst the characters which 

 vary from one small group to another, are generally but of 

 little interest. The nature and the degree of development of 

 the faculties, of which the organ thus modified is the instru- 

 ment, enables us also to judge, to a certain point, of the 

 zoological value of a modification of structure. 'But in other 



