CHARACTERS OF ANIMALS. 125 



shells and corals, arc filled with wax, or even metal, and 

 the external parts then abstracted by acids. 



In many animals of small size, it is impossible to obtain 

 satisfactory information concerning their internal structure. 

 The relations of such, therefore, with other animals, can 

 only be guessed at, under the guidance of analogy. Since 

 the examination of the external appearances of animals has 

 been conducted with care, and extended to the minuter 

 distinctions of form, naturalists are now better qualified, 

 by a knowledge of the degree of resemblance, to determine 

 what value is to be attached to external characters, as in- 

 difications of internal structure. Formerly, a very few 

 points of resemblance were considered sufficient to warrant 

 the inference of similarity in internal structure, and justify 

 the insertion of the species thus compared, in the same 

 genus. Such hasty combinations are frequent with Lin- 

 naeus, especially among the mollusca. From a slight re- 

 semblance in the form of the aperture of the shell, he in- 

 cluded in the same genus, animals which breathe by means 

 of gills and lungs, which are oviparous and ovoviparous 1 , 

 which have the sexes separate or united in the same indivi- 

 dual. The repeated exposure of such incongruous confor- 

 mations has led naturalists, before inferring a resemblance 

 of internal structure, to be convinced that there is a very 

 strong resemblance externally ; and even with all this cau- 

 tion, it may be questioned if a resemblance of external form 

 warrants the conclusion of a similarity of internal structure. 

 Yet, perhaps, nine-tenths of the species of animals which 

 are known, rest their claims to their present place in the 

 system, to the adoption of such a rule. Every year, how- 

 ever, demonstrates its fallacy, and produces corresponding 

 revolutions in the systems of arrangement. 



In the case of small animals, the external characters and 

 internal structure of which cannot be investigated by any of 



