A THEORY OF RETROGRESSION 53 



in any useful part, for instance in sight, hearing, or 

 digestion, it stands a great chance of being elimini- 

 nated. It follows that at any given time, in any 

 species of complex animal, while stringent selection 

 causes the evolution of a few parts, a lower degree 

 of stringency maintains, at a more or less fixed 

 standard, the other useful parts, which had been 

 previously evolved. 



What then happens to parts which were 

 formerly useful, and which, therefore, in a different 

 environment, underwent evolution under the in- 

 fluence of selection, but which a change in the 

 environment has rendered useless, and therefore 

 no longer subject to selection ? For example, 

 what happens to the eyes of cave-dwelling animals 

 that live in total darkness,^ or to the wings of a 

 species of bird {e.g. dodo, ostrich, cassowary, 

 apteryx) that has abandoned the practice of flight ? 

 Overwhelming evidence proves that such characters 

 undergo degeneration. Every complex animal 

 displays numerous " vestiges," dwindling remnants 

 of characters which were once useful, but which, 

 through a change in the environment, have ceased 

 to be so, and which have therefore long ceased to 

 be selected. We now reach the gist of our present 

 inquiry. What is the nature of this degenera- 



' Vide "The Present Evolution of Man," pp. 97-9, for a discussion 

 of the eyes of cave-dwellers. 



