410 



EVOLUTION AND GENETICS 



other than resulted in the general, generic wing reduction and leg 

 increase, and causes of relatively less importance, whether initiated 

 as internal variations or in response to external conditions we 

 know not. But however these were initiated and preserved, we 

 have a splendid example of evolution in the three planes of space. 



Fig. 209. — The geographical distribution of song sparrows. Each number 

 indicates the habitat of a distinct subspecies. (From Woodruff after Chap- 

 man.) 



Hence the facts so annoying to the taxonomist of a mocker in 

 Tower Island, far in the northeast of the archipelago, which has 

 a black moustache like the birds in Chatham, seventy miles to 

 the southeast, a black back like those in Bindloe, thirty miles due 

 west, a large beak corresponding to the beaks of the Abingdon 

 birds, forty-five miles northwest, and yet which in all characters 



