304 EVOLUTION AND ANIMAL LIFE 



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 

 cut 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 latter 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 expression in the following words of Gilbert' 

 and Starks {" 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 differentiation of a very large majority of 

 the forms involved. That this differentiation progressed at widely 

 varying rates in different instances, becomes at once apparent. A 

 small minority (43) of the species (11 per cent of the species found on 

 the Pacific side; about 2.5 of the combined fauna) 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 separated, whose 

 immediate relation in the past we cannot 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, and the evidence in favor of interoceanic connection 

 is not weakened by an increase in the one list at the expense of the 

 other. All evidence concurs in fixing the date of that connection at 

 some time prior to the Pleistocene, probably in the early Miocene. 

 When geological data shall be adequate definitely to determine that 

 date, it will give us the best known measure of the rate of evolution in 

 fishes." 



From this discussion, it is probable that even in isolation 

 some species change very slowly, that with similar conditions 



