XII THE COLOURS OF BIRDS 257 



blue pigments usually are among animals, blue and 

 green tints are exceedingly common among birds' 

 sggs, while various shades of brown, red, and yellow 

 also occur. According to Professor Alfred Newton 

 {Dictionary of Birds, article " Eggs "), there is some 

 reason to believe that for a time the eggs increase 

 in brilliancy of colouring with each season until a 

 maximum is reached, after which the brilliancy may 

 again begin to decline. 



The pigments of the egg-shells of birds have been 

 investigated by several authors. The important 

 points upon which all agree are first, that the colour- 

 ing is due to definite pigments ; and second, that 

 these are derived directly or indirectly from haemo- 

 globin — results of much theoretic importance. The 

 interesting point is not that derivatives of haemoglobin 

 should be used in coloration, but why, if vivid and 

 beautiful colouring-matters do arise in this way, they 

 should not be employed in the coloration of the 

 feathers. It seems also generally admitted that even 

 the ingenuity of that highly esteemed person, the 

 field naturalist, is unequal to the task of explaining 

 the colours of all birds' eggs upon the hypothesis of 

 usefulness, so that from the theoretical point of view 

 these pigments are of quite special interest. 



Of pigments colouring eggs, Mr. H. C. Sorby 

 describes seven with the following names and pro- 

 perties : — 



I. Oorhodeine — a red-brown pigment of very common 

 occurrence and great permanence. 

 _ [blue pigments probably closely related, 



■ , <; , < of which the second only yields a 



3. Banded oocyan | ^^^^^^ spectrum. 



S 



