VIII 



BIRDS IN WHITE 



jA LMOST every species of bird and beast throws 

 /^ off an occasional albinistic variation or sport, 



/ — ^ which tends to breed true. Such sports 

 •^ -^^are of two kinds — complete and incomplete 

 albinos. In the former, the organism is totally devoid of 

 external pigment, so that the eye looks red, there 

 being no colouring matter in the iris to mask the small 

 blood vessels in it. In the incomplete albinistic form 

 the iris retains the pigment, so that the eye colour is 

 normal. True albinos have very poor sight, hence when 

 such sports occur in a species in a state of nature they 

 soon perish in the struggle for existence. The white 

 varieties with pigmented eyes are not handicapped by 

 bad eyesight, but their whiteness makes them con- 

 spicuous to the creatures that prey upon them ; so 

 that, unless they are well able to defend themselves' 

 or unless they dwell in a region of everlasting snow, 

 they tend to be eliminated by natural selection. 



If protective colouring were as important to the 

 welfare of birds as Wallaceians and modern Darwin- 

 ians assert, all the birds of the Polar regions would be 

 white and not a single white species would be found in 



42 



