EVOLUTION OF THE COLORS OF BIRDS. 161 



not so. If the laws of growth have any sort of univer- 

 sal validity, and I am firmly convinced that they have, 

 the line of movement of a mob of nren along the lines 

 of least and greatest resistance would typify any form of 

 involuntary movement along the same lines. The ques- 

 tion simply is to determine which are the real points of 

 greatest and least resistance. The sequence of color 

 assumption here exemplified may be infinitely varied in 

 detail, but will, I think, be found tolerably uniform in 

 general. Different forms of feathers may offer different 

 degrees of resistance in the various parts of their struc- 

 ture, but I think it will be found true in the main that 

 the pigment will pass down the shaft most readily, while 

 there can be no possible exception to the obstruction at 

 the tip. The channel of pigmentation may be diverted 

 along the sides, however, and thus not give rise to any 

 terminal spot or band. 



I have thus far assumed that there was some force 

 analogous to the blind terror of a mob which urged the 

 pigment cells away from the source of danger, the body. 

 This is probably not strictly true, however. It is more 

 likely that it is the warm sunlight which, with its genial 

 touch, causes them to crowd forward; to leave the prison 

 in which they were born, and press on joyfully into the 

 free air of the outer world. Capillarity or a variety of 

 undetermined forces might also tend to draw them out- 

 ward. The number which were thus enticed to the sur- 

 face would depend simply upon the amount of surplus 

 energy, or w^hatever else it may be, which results in the 

 formation of pigment in the system of the bird. As a 

 species grows older and more vigorous, and more com- 

 pletely adapted to its environment, the sum total of pig- 

 ment would increase, and accordingly, if no other ten- 

 dency were at w^ork in a contrary direction, there would 

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