292 INTRODUCTION TO EVOLUTION 



"Darwin's Finches ' 



No group of Galapagos animals is of more interest to students of evolu- 

 tion than are the birds, partly because of the role played by these birds in 

 influencing the thinking of Darwin. He was particularly impressed by the 

 varied adaptations exhibited by the unique finches of the archipelago. In 

 commemoration of this fact, the most recent investigator of the Galapagos 

 finches. Dr. David Lack (1947), has had the happy inspiration to term 

 them "Darwin's finches." To his book of that title we are indebted for 

 much of the following material on Galapagos birds. 



As is true of other animals, the affinities of the birds on the archipelago 

 are with American forms. There is, however, great variation in the degree 

 of similarity between the island forms and their continental relatives. Thus, 

 the cuckoo of the islands is identical with a South American species, and 

 the single warbler is very similar to one living in Ecuador. The martin is 

 regarded as belonging to a separate subspecies of a species mainly inhabit- 

 ing the continent. The tyrant flycatcher is a distinct species, but closely 

 related to an American species. The mockingbird difl'ers so much from 

 American mockingbirds that it is placed in a separate genus from the 

 latter. Furthermore, the Galapagos mockingbird has become differentiated 

 into nine island forms, "two of which are sufficiently distinctive to be 

 treated as separate species, the other seven being treated as subspecies of 

 a third species" (Lack, personal communication). Similarly, the vermilion 

 flycatcher has difl'erentiated into three island races. Finally, Darwin's 

 finches are so different from any existing American finch that, as stated 

 by Lack, "there is considerable doubt as to their nearest mainland rela- 

 tive." 



Thus we see all degrees of similarity between island and mainland spe- 

 cies: from identity of characteristics to widely differing traits. How can 

 these facts be explained? 



In the first place, how does it happen that Galapagos birds resemble 

 American ones at all? Surely this resemblance must mean that the Galapa- 

 gos birds are the more or less modified descendants of American species. 

 If the birds had been especially created to live on the Galapagos Islands it 

 is difficult to see why they should have been created to resemble birds 

 living on the neighboring continent, rather than to resemble birds created 

 to live on other islands, e.g., other islands in the Pacific, or the Cape Verde 

 Islands near the coast of Africa. The Cape Verde Islands resemble the 

 Galapagos Islands in many respects, yet, in the words of Darwin ( 1839), 

 "the aboriginal inhabitants of the two groups are totally unlike; those of 



