living protoplasm, atrophy, and tend to disappear. Similarly the more active structures in an adult limb tend to disappear on withdrawal of stimulation. Therefore, 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 environment, 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 whole bone will again be abnormal.
In higher species the structures of each individual
