5 z8 EVOLUTION OF BIRDS xvm. 9 



of colour or size. It remains to be shown whether these are themselves 

 significant or connected with other significant factors. 



The striking feature is that so many distinct races should appear, 

 even in birds, which could easily cross the distances, mostly less than 



o 



Culpepper 



Wenman° 



75% 



Cocos 



O 100A 



m 



(J3 Abingdon 

 Blnd/oeC^J, 



Tower 



207, 



Narborough 

 A Ibemarle 



10 20 30 40 50 



» I ' L I 



Scale in land miles 



IndeFatigable 



Chatham 



Charles 



o\ 



Hood cC5> 



Fig. 314. Percentages of endemic forms of Darwin's finches on each island, showing 

 the effects of isolation. (From Lack.) 



50 miles, between the islands. Evidently the birds tend to remain at 

 home, and there is no doubt that degree of geographical separation is 

 a main factor in producing the races. Thus the central islands of the 

 group, lying close together, have no endemic subspecies, whereas the 

 forms in the outlying islands are mostly distinct (Fig. 314). It is 

 evident that isolation is tending to break up the population into a 

 number of distinct units and it is remarkable that, in spite of the large 

 amount of variation, intermediates are rare between species and even 



