2o8 THE STORY OF THE BIRDS 



the amount of variation becomes apparent. The 

 larger the number of individuals we examine, 

 the more shall we be impressed with the differ- 

 ences they present. Confining ourselves to 

 external structure we shall find that the bill, 

 the legs and feet, the wings and the tail, are 

 all subject to individual variation, amounting 

 in the two extremes to an astonishing degree 

 of divergence. Colour is no less variable than 

 structure. As we may view each individual of 

 a large series of birds little or no difference 

 in tint might be detected, but by placing the 

 series — say of fifty or a hundred skins — side by 

 side the important variation in tint at once 

 becomes apparent. To a very considerable ex- 

 tent colour variation is correlated with geo- 

 graphical distribution, which in many cases 

 means that differences of tint are due to climatic 

 influences, as we have already seen. Then 

 again, structural variation is often correlated 

 with insulation, and often induced by it ; for we 

 often find the island representatives of a seden- 

 tary widely ranging continental species percep- 

 tibly larger or smaller than the others. It may 

 also be due to geographical distribution, as in 

 the case of the Crossbill, for instance. The dif- 

 ference in the size of the bill of the Common 



