BAHAMA BIRD-LIFE 153 



than forty-four have become sufficiently changed from their 

 ancestral stock to be designated as new forms or species. In 

 only one instance, and that strange to say, is supplied by a 

 Swallow {Callichelidon cyaneoviridis), lias this differentia- 

 tion progressed far enough to be accorded generic rank. In 

 the remaining forty-three cases it is so slight that in almost 

 every instance it is possible to point with assurance to the 

 particular species from which it is believed the Bahaman 

 bird has been derived. 



Such an analysis shows us that Cuba has made the 

 largest contribution to Bahaman bird-life, while Hayti and 

 Florida have been drawn on to a lesser degree. ( )f evident- 

 ly fortuitous origin are some half-a-dozen birds apparently 

 derived from Mexican or Central American species. The 

 ancestors of these birds possibly owe their occurrence in the 

 Bahamas to the action of tropical storms. Blown to an 

 island, their opportunities for establishing themselves 

 would be far more favorable than on the more thickly popu- 

 lated mainland. 



As to the causes which have been potent in producing 

 these peculiar Bahaman forms little can be said. We may 

 assume that changed conditions of environment acting on 

 isolated species, have resulted in their evolution into new 

 species, presumably better adapted to new surroundings. 

 In the further division of a Bahaman species into two or 

 more races, each restricted to a single island, the case be- 

 comes more perplexing. We have not then different physio- 

 graphic or climatic conditions to the influences of which may 

 ascribe the changes observed. On the contrary, we find dif- 

 ferent forms of the same species inhabiting islands almost 

 within sight of each other, where all conditions of soil, cli- 

 mate, and flora are essentially similar. Perhaps we can as- 

 sume here that through the continued isolation of a compar- 

 atively small number of individuals, certain characters, due 

 originally purely to individual variation, have become per- 

 petuated and specific. Among a smaller number of birds 



