498 POPULAR SCIENCE MONTHLY 



which Lake Bonneville was once drained. Other species are left locally 

 isolated, but in one species only (Agosia adobe) a small minnow of the 

 clay bottoms can be shown to have undergone any alteration. But 

 with the tiger beetle (Cicindelce) a large number of species have been 

 produced by isolation. 



From the Bay of Panama 374 species of fishes are recorded in the 

 recent monograph of Gilbert and Starks. Of these species, 204 are 

 recorded also from the Gulf of California, while perhaps 50 others are 

 represented in the more northern bay by closely related forms. 



Comparing the fish faunas separated by the isthmus, we find the 

 closest relation possible so far as families and genera are concerned. 

 In this respect the resemblance is far closer than that between Panama 

 and Chile, or Panama and Tahiti, or Panama and southern California. 

 On the Atlantic side, similar conditions maintain, although the number 

 of genera and species is far greater (about 1,200 species) in the West 

 Indies than at Panama. This fact accords with the much larger extent 

 of the West Indies, its varied groups of islands isolated by deep chan- 

 nels, and its near connection to the faunas of Brazil and the United 

 States. 



But it is also noteworthy that while the families of fishes are almost 

 identical on the two shores of the isthmus of Panama, and the great 

 majority of the genera also, yet the species are almost wholly different. 



Taking the enumeration of Gilbert and Starks, we find that out of 

 374 species, 43 are found apparently unchanged on both sides of the 

 isthmus; 265 are represented on the Atlantic side by closely related 

 species — in most cases the nearest known relative of the Pacific species 

 — while 64 have no near analogue in the Atlantic. Of the last group, 

 some find their nearest relative to the northward or southward along 

 the coast, and still others in the islands of Polynesia. 



The almost unanimous opinion of recent students of the isthmus 

 faunas finds the latest expression in the following words of Gilbert and 

 Starks ('The Fishes of Panama Bay,' p. 205) : 



The ichthyological evidence is overwhelmingly in favor of a former open 

 communication between the two oceans, which must have become closed at a 

 period sufficiently remote from the present to have permitted the specific dif- 

 ferentiation of a very large majority of the forms involved. That this differen- 

 tiation progressed at widely varying rates in different instances becomes at once 

 apparent. A small minority (43) of the species 2 remain wholly unchanged so 

 far as we have been able to determine that point. A larger number have become 

 distinguished from their representatives of the opposite coast by minute (but 

 not 'trivial') differences, which are wholly constant. From such representative 

 forms we pass by imperceptible gradation to species much more widely sepa- 

 rated, whose immediate relation in the past we can not confidently affirm. . . . 



It is obvious, however, that the striking resemblances between the two 

 faunas are shown as well by slightly divergent as well as by identical species, 



2 43, or 11 per cent, of the species found on the Pacific side; about 2.5 of 

 the combined fauna. 



