CONCLUSIONS. 177 



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

 rently trifling, are yet perfectly 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 

 examination, that there is often the closest resemblance be- 

 tween species 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 Menopoma, Siren, Amphiuma, Axolotl, and the Meno- 

 branchus, are Batrachians which inhabit the rivers and lakes 

 of the United States and Mexico. They are very similar in 

 external form, yet differ in some of them having external 

 gills at the sides of the head, while others have them not; 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 former exists only 

 in the lakes of Austria, and the latter in Japan. The con- 

 nection in this case is consequently established by means 

 of species which inhabit distant continents. 



445. Neither the distribution of animals therefore, any more 

 than their organization, can be the effect of external influ- 

 ences. We must, on the contrary, see in it the realization 

 of a plan wisely designed, the work of a Supreme Intelli- 

 gence who created, at the beginning, each species of animal 

 at the place, and for the place, which it inhabits. To each 

 species has been assigned a limit which it has no disposition 

 to overpass so long as it remains in a wild state. Only 

 those animals which have been subjected to the yoke of 

 man, or whose subsistence is dependent on man's social 

 habits, are exceptions to this rule. 



446. As the human race has extended over the surface of 

 the earth, man has more or less modified the animal popu- 



