Ill] 



THE MALE HORMONES 



267 



sex-linked hypertrophy; and in the singular and striking plumage 

 of innumerable birds we may easily see how enhanced growth of a 

 tuft of feathers, perhaps exaggeration of a single plume, is at 

 the root of the whole matter. Among extreme instances we may 

 think of the immensely long first primary of the pennant-winged 

 nightjar; of the long feather over the eye in Pteridophora alberti, 



Fig. 81. A single pair of hypertrophied feathers in a bird-of-paradise, 

 Pteridophora alberti. 



.-I 



Fig. 82. Unequal growth in the three pairs of tail-feathers of a humming-bird 

 {Loddigesia). 1, rudimentafy: 2, short and stiff; 3, long and spathulate. 



or the Fix long plumes over or behind the eye in the six-shafted 

 bird-of-paradise; or among the humming-birds, of the long outer 

 rectrix in Lesbia, the second outer one in Aethusa, or of the extra- 

 ordinary inequalities of the tail-feathers of Loddigesia mirabilis, 

 some rudimentary, some short and straight and stiff, and other two 

 immensely elongated, curved and spathulate. The sexual, hormones 

 have a potent influence on the plumage of a>bird ; they serve, somehow, 

 to orientate and regulate the rate of growth from one feather-tract 

 to another, and from one end to another, even from one side to the 

 other, of a single feather. An extreme case is the occasional pheno- 



