318 THE WORLD OE LIFE 



cluce the exact strength, elasticity, and continuity of the whole 

 web. 



Now each feather " grows/' as we say, out of the skin, each 

 one from a small group of cells, which must be formed and 

 nourished by the blood, and is reproduced each year to replace 

 that which falls away at moulting time. But the same blood 

 supplies material for every other part of the body — builds 

 up and renews the muscles, the bones, the viscera, the skin, 

 the nerves, the brain. What, then, is the selective or directing 

 power which extracts from the blood at every point where 

 required the exact constituents to form here bone-cells, there 

 muscle-cells, there again feather-cells, each of which possesses 

 such totally distinct properties ? And when these cells, or 

 rather, perhaps, the complex molecules of which each kind of 

 cell is formed, are separated at its special point, w^hat is the 

 constructive power wdiich welds them together, as it were, in 

 one place into solid bone, in another into contractile muscle, in 

 another into the extremely light, strong, elastic material of the 

 feather — the most unique and marvellous product of life ? 

 Yet again, wdiat is the nature of the power which deteiToines 

 that every separate feather shall always " grow " into its exact 

 shape ? For no two feathers of the twenty or more which 

 form each wing, or those of the tail, or even of the thousands 

 on the whole body, are exactly alike (except as regards the 

 pairs on opposite sides of the body), and many of these are 

 modified in the strangest way for special purposes. Again, 

 what directive a2:encv determines the distribution of the col- 

 ouring matter (also conveyed by the blood) so that each feather 

 shall take its exact share in the production of the whole pattern 

 and colouring of the bird, which is immensely varied, yet 

 always symmetrical as a whole, and has always a purpose, 

 either of concealment, or recognition, or sexual attraction in 

 its proper time and place ? 



Xow, in none of the volumes on the physiology of animals 

 that I have consulted can I find any attempt whatever to 

 grapple Avith this fundamental question of the directive power 



