414 TEE ZOOLOGIST. 



The old Blackcaps have a much finer song than the younger 

 birds, and in addition have much finer plumage ; the feathers on 

 the belly are a much purer white, the back is more olive-green, 

 and the eyelid is much whiter. The Wood-Warbler shows very 

 great variation, but here the colours are so delicate that a 

 description is almost impossible ; the different shades of green, 

 however, are much more intensified in the older birds. The 

 Stonechat affords an excellent illustration, especially as several 

 pairs are frequently close together, and allow of comparison. 

 A.s a rule the feathers of the back are edged with black ; some- 

 times, in birds of the first year, they can hardly be said to show 

 any black at all. In the old birds, however, the back may be 

 said to be as black as the head, and in addition the rufous colour 

 of the breast is much deeper, and the belly a much purer white. 

 The Brambling in winter is another good illustration, because 

 the different grades of colouring are so marked ; the old males 

 have the head and back a much deeper black, and the breast a 

 more intense red. The old male Yellowhammers are marked 

 with a much more intense yellow, especially on the head. In 

 many cases also the feathers are longer, consequently the form 

 is more perfect. A development of the same kind takes place in 

 the song of many birds ; the Blackbird is most noticeable, and 

 the attempts of the younger birds can be easily picked out, such 

 attempts being often very feeble. Here, then, we have firm 

 ground on which a theory can be based, i. e. that the tendency 

 for the colours to become more intensified, and the plumage and 

 song more perfect, is proportionate to the age of the indi- 

 vidual. Such a tendency could not exist without motive on 

 the one hand, and support on the other ; this support is, I 

 believe, supplied by aesthetic sense. If, as a bird matures, it 

 becomes more beautiful, what reason, it may be asked, can there 

 be for any display ? What necessity for any other power ? I 

 wish, however, to show that such beauty is not solely dependent 

 on age, but must necessarily be subservient to some other 

 power. Those who have studied sexual selection amongst any 

 species will understand how difficult it is to happen to be present 

 when the final selection takes place, and therefore to place on 

 record specific cases as proof of the most perfect males being 

 chosen is a most difficult task. I can, however, in the case of 



