EVOLUTION OF THE COLORS OF BIRDS. 67 



developed in extent along the lines of least resistance 

 and multiplied in forms on the same plane, or has de- 

 veloped in density along the lines of greatest resistance. 

 This may be called the Law of Phylogenic Extent and 

 Density. According to this view, living matter has 

 within it the potential possibility of indefinite growth 

 and modification. The growth is limited as Spencer has 

 shown by the mass outrunning the surface, while modi- 

 fication is limited (1) by the possibilities of environ- 

 ment to create variations, and (2) by the ability of the 

 organism to adapt itself to its environment. If it be 

 asked how an organism can adapt itself to an environ- 

 ment which is itself the cause of the variations, the re- 

 ply is by the selection of such environmental variations 

 as are adaptive, or conversely by the elimination of such 

 as are unadaptive. Living matter, then, like a gigantic 

 tree or bush, spreads out in all directions where the en- 

 vironment offers least resistance, and man was not an 

 accident or happy coincidence in the aimless wander- 

 ings of blind forces, but rather the inevitable outcome of 

 natural law. 



Metabolism, according to the Century Dictionary, is 

 " the sum of the chemical changes within the body, or 

 within any single cell of the body, by which protoplasm 

 is either renewed or changed to perform special func- 

 tions, or else disorganized and prepared for excretion." 

 Geddes and Thomson in their work on the Evolution 

 of Sex, have especially emphasized the importance of 

 the laws of metabolism in the development of species. 

 Metabolism should be clearly distinguished from bathm- 

 ism or growth force. The latter is a force analogous to 

 electricity, for instance, which conditions the growth of 

 an organism; while the former is an expression of the 

 changes continually taking place in protoplasm. There 

 are two phases of metabolism. When the protoplasm is 



