304 INTRODUCTION TO EVOLUTION 



tion to a particular environment are not without danger to the species. Thus 

 GuUck (1932) wrote concerning these birds: 



Adaptation has become completely and narrowly specialized for feeding upon 

 the nectar, seeds, and insects of their Hawaii. And by the same token, all but a 

 few of them are now ready to pay the penalty of over-specialization in a re- 

 stricted environment. Such as are not exterminated by enemy pests like the 

 mongoose [introduced by man], that they know not how to evade, are doomed 

 to disappear whenever cattle destroy the particular environmental combination 

 on which they depend. The Hawaiian woodlands, alive with song in 1850, are 

 today already largely silent, except to some degree on the single island of 

 Hawaii, where the destructive forces seem to have moved a little less rapidly. 



And Lack, referring to the world's largest museum collection of Hawaiian 

 birds, commented somewhat sadly, 'The drawers of the Rothschild collec- 

 tion contain more representatives of some of the Hawaiian sicklebills than 

 are alive in the islands today." 



Conclusion 



We have dwelt at some length on the inhabitants of oceanic islands be- 

 cause they afford examples of evolutionary change occurring within rela- 

 tively recent times and under conditions still largely observable. The islands 

 themselves are geologically young; hence any observed evolution of their 

 inhabitants must have occurred within a relatively short span of time. Thus, 

 basing his conclusion on the opinion that the Hawaiian Islands are of Plio- 

 cene and later age, Amadon (1947) estimated that about 5,000,000 years 

 were available for the evolution of the drepanid birds. Although by human 

 standards this is a very long time, it is but a small portion of geologic time, 

 or even of that part of geologic time which has elapsed since the first birds 

 appeared (p. 187). And the Hawaiian Islands are among the older oceanic 

 islands. 



Owing to their isolation, oceanic islands develop disharmonic floras and 

 faunas. Taking advantage of environmental niches left vacant in such dis- 

 harmonic faunas, animals reaching larger and older archipelagos early in 

 their history underwent adaptive radiation quite unlike that possible to 

 their relatives on continents. Hence oceanic islands have become the set- 

 tings for some of the most vivid examples of evolution-in-action available 

 to us. 



References and Suggested Readings 



Amadon, D. "Ecology and the evolution of some Hawaiian birds," Evolution, 

 1 (1947), 63-68. 



