Power to Adjust Parts to Surroundings 231 



ment would be sideways. In all cases it is the same engine, the 

 same machinery, the same motive power; the difference consists 

 only in the way a small part of the machinery is set; and the 

 reader will please to observe that this set of the machinery is not 

 the cause of the movement of the engine, but merely determines 

 the direction thereof when the power, which is steam, is applied. 

 Now something of analogous kind, it is most probable, deter- 

 mines the direction of turning of the plant organs. The structure 

 and motive power in all of these parts is substantially the same, 

 but in each some portion of the machinery is differently set, so 

 that the application of the power, which is growth, causes turn- 

 ing in the distinctive direction, the stem towards light, leaf 

 across it, and root from it. Of course the machinery is not metallic 

 but protoplasmic, and in last analysis is probably of a chemical 

 nature, while, moreover, the set of the machinery is usually not 

 alterable at a touch, but is hereditarily fixed in each kind of 

 organ. And the subject may stand out yet more clearly if we 

 return for a moment to our sailor, who, in order to reach a cer- 

 tain eastern port, sets his steering gear to hold his good ship at one 

 angle to his compass, and in order to reach a western port holds 

 her at another. It is the same compass, ship, machinery, and 

 power; only the set of the steering gear is different. This is the 

 principle, I believe, which underlies the different kinds of responses 

 to any single uniformly-acting stimulus. 



Sixth, how the advantageous direction of response has become fixed 

 in each part. Or, in the simile of the preceding section, how did 

 the machinery become set so differently in leaf, stem, and root; 

 and especially, how did it become set in each of those organs in 

 the manner most advantageous for the performance of its particu- 

 lar function? Now it is perfectly plain that the power of a part 

 to respond advantageously to a stimulus, that is to say, the set 

 of its responding machinery, is an hereditary and adaptive 

 feature, and must therefore have arisen in precisely the same 

 manner as any other adaptive features, including those of visible 



