156 Permanence and Evolution. 



not bear investigation. We find, for instance 

 ("Origin of Species," p. 417), that "no two 

 marine faunas are more distinct, with hardly 

 a fish, shell, or crab in common, than those 

 of the eastern and western shores of South and 

 Central America. Yet these great faunas are 

 separated only by the narrow but impassable 

 Isthmus of Panama." Later investigations, 

 however, seem to prove (Wallace, " Geo- 

 graphical Distribution," vol. ii. p. 20) that there 

 are at least 35 species of shells absolutely 

 identical, whilst so many more are so close that 

 they may be only varieties. Nearly 70 others 

 are distinct but representative species. Of the 

 genera of marine mollusca, more than 40 are 

 common to the East and West Coast. Thirty 

 per cent, of known fish are also identical, not to 

 mention the affinities shown by Miocene corals. 

 On these grounds Wallace supposes that the 

 isthmus has been submerged in comparatively 

 recent times. Comment on this is superfluous. 



