294 THE WORLD OF LIFE CHAP. 



tion from single cells, which go through their almost infinitely 

 elaborate processes of cell-division and recomposition, till 

 the whole vast complex of the organic machinery the 

 whole body, limbs, sense, and reproductive organs are 

 built up in all their perfection of structure and co-ordination 

 of parts, such as characterises every living thing ! 



Let us now recur to the subject that has led to this 

 digression the feathers of a bird. We have seen that a 

 full-grown wing-feather may consist of more than a million 

 distinct parts the barbules, which give the feather its 

 essential character, whether as an organ of flight or a mere 

 covering and heat-preserver of the body. But these barbules 

 are themselves highly specialised bodies with definite forms 

 and surface-texture, attaching each one to its next lateral 

 barbule, and, by a kind of loose hook-and-eye formation, to 

 those of the succeeding barb. Each of these barbules must 

 therefore be built up of many thousands of cells (probably 

 many millions), differing considerably in form and powers of 

 cohesion, in order to produce 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, what is the constructive power which 

 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, 

 what is the nature of the power which determines that every 

 separate feather shall always " grow " into its exact shape ? 



