totality, i.e. lose part of their full development, especially as regards the most active and changeable tissues, those in which under stimulation chemical and vital changes most actively occur, the muscles. Such an atrophic change, since it results in a condition different from that which normally obtains, we term an acquired variation; but it is a question whether the full normal development of the adult limb is not in itself an acquired variation, for, unlike the horns of deer, which reach their full development without direct stimulation, the adult limb can reach its full normal development only by the aid of such stimulation acting on the inborn power to vary; and since such development can be attained only as a reaction to the direct action of the environment, it is difficult to understand why the "abnormal development" of the blacksmith's arm, due to and maintained by a greater amount of stimulation, should be termed an acquired variation, when the "normal" development of the muscles of an ordinary arm, due to and maintained by a lesser amount of stimulation, should not be so termed. I think on consideration it will be apparent that both the "normal" development and the more than "normal" development are really acquired variations produced by the direct action of the environment on the immense power that high multicellular organisms have of varying in such a manner as to put themselves in harmony with it.
Given a sufficient supply of food, low multicellular organisms are apparently able to reach their full development in the absence of other stimuli, but higher in the scale it becomes apparent that organisms are less and less so able. As the environment becomes more and more complex and heterogeneous, so the action of Natural Selection has developed more and more the power of individual organisms to vary in correspondence along certain lines. Not only are such special structures (e.g.
