EXTERNAL FEATURES 33 



in the blood " go on as usual, but they are not expressed 

 externally when there are no new feathers to be coloured. 

 In the single tanager which was induced, by a sudden 

 temperature change, to moult in winter, the green winter 

 plumage appeared. In the others, which did not moult 

 till spring, scarlet feathers succeeded scarlet feathers without 

 the green potentiality ever finding expression. " We have 

 thus proof that the outward manifestation of the sequences 

 of plumage in these birds is not in any way predestined 

 through inheritance bringing about an unchangeable succes- 

 sion, in the case of the tanager, of scarlet-green, scarlet- 

 green, year after year." 



This is a particular and striking case of the general 

 biological theorem that every process of development — for 

 the moulting is a continuance of development — has an 

 intrinsic constitutional factor and an external environmental 

 factor. In the case of Beebe's birds, the modified food and 

 temperature brought about a bodily condition very different 

 from the thinness and fatigue usually associated with the 

 stress and cares of the breeding season. This was step one. 

 The unusual vigour gave a new lease of life to the feather 

 or obviated the onset of the moult. This was step two. 

 The deferring of the moult made it impossible for the 

 deeper pigmental change in the blood to find external 

 expression. "This was step three. What occurs normally is 

 a harmonised correlation of internal constitutional rhythms 

 and external seasonal periodicities. 



§ 5. Coloration 



Birds and Insects are probably the animals of intensest 

 Hfe, and they are the most brilliantly coloured. There is 

 some evidence of a physiological correlation here, that 

 bright colours have something to do with rapidity of meta- 

 bolism, that pigments may be sometimes the by-products 

 or waste-products of intense living. 



It is well known that animal coloration may be produced 

 in three ways — (a) by pigments, (b) by physical structure, 



D 



