ORGANIC EVOLUTION — THE FACTORS 117 



living protoplasm, atropLy, and tend to disappear. 

 Similarly the more active structures in an adult limb 

 tend to disappear on withdrawal of stimulation. There- 

 fore, not only has the action of Natural Selection caused 

 the evolution in high animal organisms of complex and 

 heterogeneous structures, whereby they are brought into 

 harmony with a complex and heterogeneous environ- 

 ment, and not only has it endowed these structures 

 with the power of developing proportionately and 

 co-adaptively under appropriate stimulation ; but yet 

 more to increase the harmony with the environment, it 

 has endowed them with the power of retrogressing in 

 the absence of appropriate stimulation, whereby the 

 organism is relieved wholly or partly from useless and 

 burdensome structures. It should be observed also, 

 that any structure, say a bone, is itself compounded of 

 parts, which, like the structure as a whole, severally 

 vary under stimulation ; wherefore if the stimulation 

 under which the parts develop be not proportionately 

 the same as regards each of them as that under which 

 they normally develop, then their development will be 

 unequal, and the shape of the structure as a whole will 

 be abnormal ; for instance, if the muscles attached to 

 one part of a bone be exercised more in proportion than 

 the muscles attached to other parts of it, that part of 

 the bone to which they are attached, in response to the 

 extra stimulation, will develop more than the other 

 parts, and as a result the shape of the bone as a whole 

 will be abnormal ; similarly, if the muscles attached to 

 one part of a bone be exercised less in proportion than 

 the muscles attached to other parts of it, that part of 

 the bone to which they are attached, because less 

 stimulated, will develop less than the other parts, and 

 as a result, the shape of the Avhole bone will again be 

 abnormal. 



In higher species the structures of each individual 



