EVOLUTION. 25 



Degeneration and Reversion. 



But the evolution of forms is not always an ad- 

 vance, backward steps are often taken, and there is 

 retrogression as well as progress. Yet these facts 

 endorse the Evolution theory that organisms alter to 

 suit their surrounding conditions ; if the forms de- 

 generate, it is because the assumed form is more suit- 

 able to the changed circumstances. Some forms are 

 found to exist unchanged from very early ages. The 

 fossil lampshells in the chalk resemble almost exactly 

 those living in our own seas, the reason being that 

 the conditions of their marine life are unaltered. 

 The king-crab of the West Indies, the nautilus, and 

 some fishes correspond closely to-day to their fossil 

 ancestors ; they have not been forced in the struggle 

 for existence to make use of changed forms. 



Darwin points out two great facts in nature. 

 First, that there is a tendency to vary and change ; 

 and second, that when a change of surroundings oc- 

 curs, any variation of form that is of advantage in 

 sustaining existence in the changed conditions is 

 perpetuated by the survival of the possessor of the 

 improved organs, while less favored ones perish. 

 Thus variation and natural selection account for the 

 progressive changes. But if, on the other hand, the 

 variations are of no advantage in the struggle for life, 

 they do not supplant the parent form ; or if earlier 

 conditions are restored, the same tendency of nature 

 leads to a reversion to the previous form of organs. 



