CONCLUSIONS. 209 



different faunas are no more to be ascribed to the influences 

 of climate, than their organization is to the influence of the 

 physical forces of nature. If it were so, we should necessa- 

 rily find all animals precisely similar, when placed under 

 the same circumstances. We shall find, by the study of the 

 different groups in detail, that certain species, though very 

 nearly alike, are nevertheless distinct in two different faunas. 

 Between the animals of the temperate zone of Europe, and 

 those of the United States, there is similarity but not iden- 

 tity ; and the particulars in which they differ, though ap- 

 parently trifling, are yet constant. 



444. Fully to appreciate the value of these differences, it 

 is often requisite to know all the species of a genus or of a 

 family. It is not uncommon to find, upon such an exam- 

 ination, that there is the closest resemblance between spe- 

 cies that dwell far apart from each other, while species of 

 the same genus, that live side by side, are widely different. 

 This may be illustrated by a single example. The Menopo- 

 ma, Siren, Amphiuma, Axolotl, and the Menobranchus, are 

 Batrachians which inhabit the rivers and lakes of the United 

 States and Mexico. They are very similar in external 

 form, yet differ in the fact that some of them have external 

 gills at the sides of the head, in which others are deficient ; 

 that some have five legs, while others are only provided 

 with two ; and also in having either two or four legs. 

 Hence we might be tempted to refer them to different types, 

 did we not know intermediate animals, completing the series, 

 namely, the Proteus and Megalobatrachus. Now, the for- 

 mer exists only in the subterranean lakes of Austria, and 

 the latter in Japan. The connection in this case is conse- 

 quently established by means of species which inhabit con- 

 tinents widely distant from each other. 



445. Neither the distribution of animals, therefore, any 

 more than their organization, can be the effect of external 



18* 



