CHAP, xiii THE GALAPAGOS ISLANDS 289 



separation at various epochs by arms of the sea uniting the 

 two oceans across what is now Central America (the last 

 separation being of recent date, as shown by the consider- 

 able number of identical species of fishes on both sides of 

 the isthmus), and the influence of the glacial epoch in 

 driving the temperate American flora southward along the 

 mountain plateaus.^ At the time when the two oceans 

 were united a portion of the Gulf Stream may have been 

 diverted into the Pacific, giving rise to a current, some 

 part of which would almost certainly have reached the 

 Galapagos, and this may have helped to bring about that 

 singular assemblage of West Indian and Mexican plants 

 now found there. And as we now believe that the dura- 

 tion of the last glacial epoch in its successive phases was 

 much longer than the time which has elapsed since it 

 finally passed away, while throughout the Miocene epoch 

 the snow-line would often be lowered during periods of 

 high excentricity, we are enabled to comprehend the 

 nature of the causes which may have led to the islands 

 being stocked with those north tropical or mountain types 

 which are so characteristic a feature of that portion of the 

 Galapagos flora which consists of peculiar species. 



On the whole, the flora agrees with the fauna in in- 

 dicating a moderately remote origin, great isolation, and 

 chano'es of conditions affordino; facilities for the introduc- 

 tion of organisms from various parts of the American 

 coast, and even from the West Indian Islands and Gulf of 

 Mexico. As in the case of the birds, the several islands 

 differ considerably in their native plants, many species 

 being limited to one or two islands only, while others 

 extend to several. This is, of course, what might be ex- 

 pected on any theory of their origin ; because, even if the 

 whole of the islands had once been united and afterwards 

 separated, long continued isolation would often lead to the 

 differentiation of species, while the varied conditions to be 

 found upon islands differing in size and altitude as well as 

 in luxuriance of vegetation, would often lead to the ex- 

 tinction of a species on one island and its preservation on 

 another. If the several islands had been equally well 

 ^ Geofjrapliical Distribuiioii of Animals, Vol. IL p. 81. 



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