ORGANIC EVOLUTION — THE FACTORS 91 



totality, i. e. lose part of their full development, especi- 

 ally 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 

 fiill 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 develop- 

 ment " 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 ajiparent 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 develop- 

 ment 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. 



j^ ~- 



