xviii. 9 GALAPAGOS BIRDS 525 



nected by a land bridge with South America, it is much more probable 

 that their limited stock of plants and animals has arrived across the 

 sea. Of hundreds of species of land birds on the mainland, descendants 

 of only seven species are found in the Galapagos. The only land 

 mammals are a rat and a bat. The land reptiles include giant tortoises, 

 iguanas, a snake, one lizard, and one gecko. There are no amphi- 

 bians and only a limited number of land insects and molluscs. There 

 are large gaps in the flora; for instance, no conifers, palms, aroids, or 

 Liliaceae. This fragmentary flora and fauna strongly suggest that the 

 islands have been colonized by chance transportation across the sea, 

 and that, once arrived, the animals and plants have proceeded to settle 

 not only in the habitats they occupied on the mainland but also in 

 others, not filled, as in their homeland, by rivals. Thus the tortoises 

 and iguanas, arriving presumably by chance, have grown to large size, 

 to occupy the ecological position usually taken in other faunas by 

 mammalian herbivores. The composite plant Scalesia and the prickly 

 pear, Opimtia, have become tall trees in the Galapagos. We have here, 

 therefore, an example of the results of evolution over a relatively 

 limited period of time (perhaps less than 20 million years) from a 

 limited number of initial creatures, and this provides an excellent 

 opportunity for trying to discern the forces that have been at work. 

 There are thirteen larger islands in the Galapagos group, the largest 

 80 miles long. They are separated by distances of up to 100 miles and 

 several of the peculiar Galapagos animals have formed island races. 

 The land birds present perhaps the most interesting features of the 

 whole strange fauna. Besides two species of owls and a hawk they 

 consist of five passerine types and a cuckoo, all very close to others 

 found on the South American mainland, and a group of fourteen 

 species of finches, placed in a distinct subfamily, Geospizinae. These 

 finches are related to the family Fringillidae, which is represented in 

 South America, but they cannot be derived from any single species 

 now r existing there. Like the other animals in the islands the birds tend 

 to become differentiated into distinct island races, but this process has 

 gone to varying extents. The cuckoo, warbler, martin, and tyrant 

 flycatcher are similar in all the islands and it is significant that they are 

 all very close to species occurring in South America. Presumably they 

 are recent arrivals. The vermilion flycatcher, also a South American 

 species, has three island races. The mocking-bird, Nesomimus, is 

 placed in a genus distinct from that on the mainland and has different 

 races on each island, some of them being reckoned as separate species. 

 The extreme of island differentiation is shown by the finches, now 



