INTRODUCTION. Xvii 



fluence of such constitutional change as the bird may at any 

 time experience, and such a change is strongly demonstrated 

 at the season of pairing ; witness its effects in the high de- 

 gree of spirit frequently demonstrated, and in the superior 

 song generally called forth at this particular period. That 

 there is an invisible circulating fluid pervading the feather, 

 appears from the striking difference in elasticity and bril- 

 liancy of colour between the feather upon a bird whilst 

 alive, and upon the same bird in but a short time after 

 death. In water-birds, this principle of life in the feather 

 (if I may be allowed the expression), is singularly appa- 

 rent ; as the plumage that is impervious to water upon the 

 living bird, is almost immediately after death subject to 

 its effect. There can be no reason, then, why the feather 

 may not be influenced by the constitutional state of the 

 bird, and as that is in its highest degree of vigour imme- 

 diately previous to the season of propagation, why may not 

 such vigour be exhibited to the very extreme points of the 

 circulating medium, by a partial variation of colour, or an 

 increased lustre of tone in the former hue of the feather ? 

 It is not fanciful, for it has been established as a maxim in 

 Pathology, that the state of health may, in man, be ascer- 

 tained by the occasional flaccidity or crispness of the hair ; 

 and have we not repeatedly met with well authenticated in- 

 stances of great and sudden changes having taken place in 

 its colour, under strong mental affections, acting only, of 

 course, through the organic structure of the body? 



Changes of colour in the hair of many animals are seen, 

 and that also at particular seasons ; and (as Dr FLEMING 



