VII.] SPECIES AND SPACE. 159 



Nevertheless, when brought in merely to supplement 

 and accentuate considerations and arguments derived from 

 other sources, in that case difficulties connected with the 

 geographical distribution of animals are not without sig- 

 nificance, and are worthy of mention even though, by them- 

 selves, they constitute but feeble and more or less easily 

 explicable puzzles which could not alone suffice either to 

 sustain or to defeat any theory of specific organization. 



Many facts as to the present distribution of animal life 

 over the world are very readily explicable by the hypothe- 

 sis of slight elevations and depressions of larger and 

 smaller parts of its surface, but there are others the exist- 

 ence of which it is much more difficult so to explain. 



The distribution either of animals possessing the power 

 of flight, or of inhabitants of the ocean, is, of course, easily 

 to be accounted for ; the difficulty, if there is really any, 

 must mainly be with strictly terrestrial animals of mod- 

 erate or small powers of locomotion and with inhabit- 

 ants of fresh water. Mr. Darwin himself observes, 1 " In 

 regard to fish, I believe that the same species never occur 

 in the fresh waters of distant continents." Now, the au- 

 thor is enabled by the labors and through the kindness of 

 Dr. Giinther, to show that this belief cannot be maintained ; 

 he having been so obliging as to call attention to the fol- 

 lowing facts with regard to fish-distribution. These facts 

 show that though only one species which is absolutely and 

 exclusively an inhabitant of fresh water is as yet known to 

 be found in distant continents, yet that in several other 

 instances the same species is found in the fresh water of 

 distant continents, and that very often the same genus is 

 so distributed. 



- The genus Mastacembelus belongs to a family of fresh- 

 water Indian fishes. Eight species of this genus are de- 



1 "Origin of Species," 5th edit. 1869, p. 463. 



