CHAP. XIII THE GALAPAGOS ISLANDS 289 



Mexican affinities of the Galapagos species, so that we may 

 consider this to be a thoroughly well-established fact. 



The most prevalent families of plants in the Galapagos 

 are the Compositse (40 sp.), GramineaB (32 sp.), Legumi- 

 nos8G (30 sp.), and Euphorbiacese (29 sp.). Of the Com- 

 positae most of the species, except such as are common 

 .weeds or shore plants, are peculiar, but there are only 

 two peculiar genera, allied to Mexican forms and not 

 very distinct; while the genus Lipochseta, represented 

 here by a single species, is only found elsewhere in the 

 Sandwich Islands though it has American affinities. 



Origin of the Galapagos Flora. — These facts are ex- 

 plained by the past history of the American continent, its 

 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 

 changes of conditions affording facilities for the introduc- 

 tion of organisms from various parts of the American 

 ^ Geograjphical Distribution of Animals, YoL IL p. 81. 



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