148 CLASS AVES. 



the males shine in the richest apparel at the epoch of pairing, a 

 phenomenon, without question, intimately connected with the 

 secretion of the seminal fluid, and more especially observable 

 under those burning skies. The intertropical birds, having 

 usually two broods every year, resume their nuptial dress, 

 when the sky becomes pure and serene. They then seek out 

 the females. But when the rainy season sets in, they lose 

 their beauteous plumage and sonorous voice, at the same time 

 with their sexual desires. Dull, and, as it were, ashamed of 

 their gray or brown dress, they then bury themselves under the 

 thick foliage, as if to escape, during this temporary degrada- 

 tion, the observation of those who admired them in the days of 

 their brilliancy and enjoyment. 



In the very cold countries, a different system of moulting is 

 observed in various birds and quadrupeds in winter. The 

 covering, which accompanies the slumber of the sexual organs, 

 is peculiarly proper to secure the animal from cold. Thus the 

 lepus variabilis, the ermine, many other mammalia, and a 

 croAvd of northern birds, of palmipedes, and grallse, which in 

 summer have the plumage brown, or shaded to various depths, 

 moult in autumn their hairs and feathers, and change them for 

 white, or pale tints, for winter. This whiteness is caused by 

 the inaction of the rete mucosum, and its colouring matter, 

 from the constriction of cold. An effect altogether similar can 

 be produced on sparrows, by depluming them, and rubbing them 

 over with spirits of wine. The feathers that spring afterwards 

 remain white, because the spirits of wine prevents the develope- 

 ment of the subcutaneous colouring matter. These white ani- 

 mals resume in spring, with their sexual desires, their coloured 

 hairs or feathers. The pen-feathers of the wings and tail do 

 not usually moult at this time, but only the smaller feathers. 



The philosophy of moulting in birds (to which we must 

 confine ourselves here, though the principle is the same in all 

 species) may be explained in a few words. In the feather of 

 the bird, at the extremity of the tube, a blood-vessel penetrates, 



