278 



ISLAND LIFE. 



[part II. 



various epochs by arms of the sea uniting the two oceans across 

 what is now Central America (the last separation being of recent 

 (late, as shown by the 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 ma^y 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 duration 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 loAvered during periods of high ex- 

 centricity, we are enabled to comprehend the nature of the 

 causes which may have led to the islands being stocked 

 with those northern or sub-alpine types which are so char- 

 acteristic a feature of that portion of the Galapagos flora 

 which consists of peculiar species. 



On the whole, the flora agrees with the fa.una in indicating 

 a moderately remote origin, great isolation, and changes of con- 

 ditions affording facilities for the introduction 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 expected 

 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 vegeta- 

 tion, would often lead to the extinction of a species on one island 

 and its preservation on another. If the several islands had been 

 equally well explored, it might be interesting to see whether, as 

 in the case of the Azores, the number of species diminished in 

 ^ GeograpJi/'cnl Dlstrihution of Anhnab, Vol. TI. p. 81. 



